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Herbicides: How They Work and the Symptoms They Cause

Whether you are producing agricultural crops or tending a lawn or home garden, weed control will be important to your success. Weeds can be controlled mechanically, culturally, biologically, and chemically, and all these methods may be important in an integrated weed control program that is economical and friendly to the environment.

Chemical control with herbicides has been an important tool for managing weeds in crops and home landscapes for many years. Many of today’s herbicides are more effective and selective. These traits make them less harmful to the environment when they are used properly. Although herbicides are widely used, few people understand how they work to control undesirable plants.

Herbicide application

Generally speaking, herbicides are applied either preemergence or postemergence. That means they are applied either before or after weeds emerge from the soil and begin to grow. Preemergence herbicides kill weeds shortly after they germinate or emerge through the soil surface. Postemergence herbicides control weeds that are already growing and easily visible.

Some herbicides are applied to the soil and are taken up by seedling plant roots or shoots. They are said to have soil activity. Herbicides that are applied to plant foliage have foliar activity. Some herbicides have both. Herbicides with a high degree of soil activity usually are applied preemergence.

Selectivity

Selectivity is the process by which a herbicide controls or kills certain plants but leaves others unharmed. Selectivity may be as simple as controlling broadleaf plants but not grass plants. Many new herbicides have more sophisticated selectivity that differentiates between several broadleaf and/or several grass plants.

Herbicides with no selectivity, such as Roundup Ultra®, are called nonselective. These products kill all types of plants. Selectivity usually depends on the time or placement of the herbicide applied. Most herbicides can be harmful, even to normally tolerant plants, if the dose is too high.

Translocation

Some herbicides move (translocate) within the plant. Systemic herbicides translocate once they are taken up by the leaves, stems or roots. Herbicides that do not move after they enter the plant are called contact herbicides. Some products can be either contact or systemic herbicides, depending on the way they are applied.

Mode of Action

Mode of action refers to the effect a herbicide has on a plant. Herbicides work in many different ways. If we understand a herbicide’s mode of action, we will know what symptoms it produces at lethal or sublethal doses.

Other problems such as disease, nutrient deficiency, and insect damage may mimic the effects of herbicides. These other possibilities must be ruled out before herbicide injury is diagnosed. The following mode of action categories cover most of the herbicides used in Texas.

Growth Regulator Herbicides

These herbicides are widely used to control broadleaf weeds in grass crops such as wheat, corn, sorghum, forages, and turf grasses. One member of this group, 2, 4-D, was one of the first selective herbicides developed. Growth regulator herbicides upset the normal hormonal balance that regulates processes such as cell division, cell enlargement, protein synthesis, and respiration. That is why this group of herbicides is sometimes called the “hormone herbicides.” These herbicides are very versatile for weed control. They usually are applied to the foliage, but are also effective in the soil. Any herbicide that falls on the soil instead of the foliage can be percolated into the soil with rain or irrigation and will be taken up by weed roots.

Herbicides in this category

Phenoxy growth regulator herbicides have the least plant activity and soil residual activity; the carboxylic acids generally have the most. Broadleaf crops and turf grasses should not be planted into soils recently treated with these herbicides because they severely inhibit seedling emergence.

Symptoms

The most common symptoms for these herbicides are leaf and stem malformations. In broadleaf plants stems curl, twist, and droop, while leaves are cupped, crinkled, or have a “drawstring” appearance caused by irregular growth at the leaf edges. In cotton plants, points develop on leaf edges. Overapplication or application at the wrong stage of development in corn can cause leaf rolling and crinkling, brace root malformation, sterile flowers, and missing grain (blasting). Blasting and malformed seed heads are common symptoms of ill-timed applications in wheat.

Special considerations

Vapor from these products can easily drift to desirable plants, so they must be applied carefully. Equipment should be cleaned according to label instructions before it is used to treat susceptible crops with other herbicides.

Photosynthesis Inhibitors

Herbicides in this category inhibit photosynthesis, the process by which all green plants convert light energy from the sun into sugars (food). Photosynthesis inhibitors are broadleaf herbicides, but also control annual grasses to some extent.

Herbicides such as Buctril®, Basagran® and Tough® are used as foliar, selective, postemergence products. Others such as atrazine, Bladex®, Caparol®, or Cotoran® are generally used as preemergence herbicides, but are sometimes used postemergence. Their selectivity when applied postemergence depends on the crop and application method.

Herbicides in this category

Several herbicides in this category are critical to cotton, corn, and rice production in Texas.

Symptoms

Symptoms depend on the product’s mobility within the plant. Herbicides in the triazine, triazone, uracil, and urea families move within the plant and exhibit these symptoms in older leaves first: yellowing between the leaf veins or in the veins, and yellowing of the leaf margins that eventually turn brown and die. Herbicides in the benzothiadiazole, nitrile, phenyl-pyridazine, and amide families are not mobile within the plant and affect only treated leaves. Symptoms include speckling, spotting, and yellowing or bronzing that may kill affected tissue.

Special Considerations

Soil pH higher than 7.2 can make injury from the triazine and triazone families more severe when used preemergence.

Pigment Inhibitors

Herbicides classified as pigment inhibitors destroy the green pigment (chlorophyll) in leaf tissue. Chlorophyll is necessary for photosynthesis; without it, plants die. These herbicides are often described as “bleaching herbicides” because they cause new leaves to appear yellow or white.

These herbicides are absorbed by roots and translocate to the shoot tissue where they inhibit the production of carotenoids—substances that protect the chlorophyll molecules that make plants green. Without carotenoids, chlorophyll is destroyed. These herbicides do not destroy carotenoids already formed, but prevent the formation of new ones.

Herbicides in this category

There are three families of herbicides that bleach plant tissue.

Symptoms

Injured leaves turn yellow or white, then often translucent. New growth is yellow to white with sometimes a hint of purple or pink. These symptoms can be found on cotyledons to the newest leaves of susceptible plants. Zorial® initially causes bleaching within veins; Command® initially causes bleaching between veins.

Special Considerations

In order to use Command® in cotton, an organophosphate insecticide (Thimet® or DiSyston®) must be used in-furrow first. If the insecticide is placed incorrectly or applied at the wrong rate, cotton may be injured. Some formulations of Command® are volatile and should be used with care. Consult the label for further precautions.

Seedling Growth Inhibitors

Some herbicides act on seedling weeds shortly after they germinate and before they emerge. These herbicides work beneath the soil so their effects are seldom seen. If overapplied, however, they may inhibit growth of weed or crop seedlings that do emerge through the soil surface. These herbicides can be divided into two groups—root inhibitors and shoot inhibitors.

Root inhibitors. These herbicides interrupt cell division, which stops root growth in seedling weeds. Plants die because they cannot take up enough water and nutrients to sustain growth. The root inhibitors are most effective on small-seeded broadleaf and grass weeds. Large-seeded weeds and crops generally survive normal dosages because their roots and shoots grow through the herbicide treated zone in the soil.

Shoot inhibitors. The seedling shoot growth inhibitors also act on newly germinated weed seeds. They are absorbed by the seedling shoots of grasses and roots of broadleaf plants, and they disrupt cell growth. They are most effective at controlling small-seeded grass and broadleaf weeds. Large-seeded crops and weeds are not usually affected. Once tolerant or susceptible plants emerge they can generally overcome the effects of the herbicide.

Symptoms

Root inhibitors. Injury to tolerant plants is caused by root damage. Grass crops may be stunted and have a purple discoloration because roots cannot take up enough phosphorus. Root systems appear stubby and thick, especially the lateral roots. Broadleaf plants may have swollen and cracked hypocotyls. If these herbicides are incorporated shallowly or applied preemergence, they sometimes cause callus tissue (tumors) to form on the plant stem at the soil surface. This weakens the stem and causes lodging. Dinitroaniline herbicides applied postemergence to broadleaf crops may cause stunting.

Shoot inhibitors. Symptoms caused by the shoot inhibitors are much different than those of the root inhibitors. Overapplication or extended periods of cool, wet weather shortly after planting may sometimes cause injury to tolerant crops such as corn or sorghum. Symptoms include leafing out underground, improper leaf unfurling (shepherd’s crooking), buggy whipping (tightly rolled leaves), and leaf crinkling. In broadleaf plants, the center vein (midrib) may draw in the leaf edge in a drawstring effect. Leaf puckering is also a symptom on broadleaf plants. There may also be stunting that persists until the soil warms enough to promote plant growth.

Herbicides in this category

These products are widely used in Texas row crops, turfgrass, and horticultural crops.

Special Considerations

The growth inhibitor herbicides have no postemergence activity; therefore, the timing of application is critical. Dinitroanilines have various requirements for incorporation into the soil. Consult the individual product label for specific recommendations. Grain sorghum treated with Concep® seed safener is tolerant to the acetamide herbicides.

Cell Membrane Disruptors and Organic Arsenicals

The cell membrane disruptor postemergence herbicides control both grasses and broadleaf weeds by destroying cell membranes and causing rapid desiccation of the plant. There are two types of cell membrane disruptor herbicides: the bipyridylium and the diphenylethers. The bipyridylium herbicides require thorough plant coverage to be effective, and they have no soil activity. The diphenylether herbicides act in a similar way but more slowly. Some of them are more selective between crops and weeds. The herbicides Goal and Reflex have significant soil activity.

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The organic arsenical herbicides DSMA and MSMA are often called contact herbicides, as are the cell membrane disruptors. However, their true mode of action is unknown. They are used to selectively control wide-leaved grasses such as crabgrass or dallisgrass in narrow-leaved grasses such as bermudagrass lawns. They are also very effective on cocklebur and common ragweed. These herbicides bind tightly to soil clay and organic matter, so they have no residual, preemergence activity.

Herbicides in this category

Symptoms

Plants rapidly turn yellow or pale and may look water soaked; then they dry up. The effects of the bipyridylium herbicides are rapid. Even small droplets that drift to nontarget vegetation cause specks of burned tissue. Roots of perennial weeds are seldom killed because these herbicides do not usually translocate to the roots.

The organic arsenicals accumulate in root and leaf tips and symptoms are first seen on leaf tips. They rapidly kill leaf and stem tissue. MSMA and DSMA are more effective on grass weeds than on broadleaf weeds, except for common ragweed and cocklebur. When applied over cotton to control grasses or cocklebur, they sometimes cause speckled leaf burn and red stems on the cotton plants; however, this has little effect on overall growth.

Special Considerations

Be careful to prevent drift during application so that non target plants are not harmed. Applying systemic herbicides shortly after cell membrane disruptors or organic arsenicals is not advised. Paraquat and diquat are generally considered to be nonselective and harmful to both grass and broadleaf vegetation. In peanuts, however, some selectivity can be achieved by using paraquat at the cracking stage. Another bipyridylium herbicide called Avenge® is used in wheat and barley for selective postemergence control of wild oat.

Lipid Synthesis Inhibitors

Lipid synthesis inhibitors are unique because they act only on annual and perennial grasses, not on broadleaf plants. With the exception of diclofop, these herbicides are applied postemergence and have little or no soil activity. Crop oil concentrate or some other type of adjuvant must be used to increase herbicide uptake into the leaf. To be most effective, these herbicides should be applied to actively growing grass weeds. If grass weeds are stressed and slow growing, these herbicides will be less effective.

These herbicides disrupt lipid biosynthesis in grass plants. All plants contain lipids, which are fatty acids essential for plants to function normally. Plant cells contain lipid membranes. Membranes help the plant cell regulate what moves in, what moves out, and what remains out. Because these herbicides prevent the plant from producing fatty acids, membranes cannot form. Leaves absorb these herbicides quickly and within an hour they can not be removed by rain.

Herbicides in this category

There are two families of herbicides that disrupt lipid biosynthesis.

Symptoms

Symptoms develop slowly on grass plants and may not appear for 7 to 14 days. Initial injury is seen where the newest leaves are developing. These regions usually turn pale or yellow and then die. The area at the base of new leaves quickly becomes mushy, has a rotted appearance, and new leaves in the affected area can be pulled easily from the rest of the plant. Reddish blue pigmentation may also be observed on the stem sheath, leaf margins, and/or leaf blade.

Amino Acid Synthesis Inhibitors

This new category of herbicides can be used at extremely low rates, controls both grasses and broadleaf plants, has soil and foliar activity, and is essentially non toxic to mammals and most non vegetative life forms.

Amino acid synthesis inhibitors bind to a specific enzyme and prevent the development of amino acids essential to plant life. The enzyme to which they bind is abbreviated ALS or AHAS, so these herbicides are often called the AHAS/ALS herbicides.

Herbicides in this category

Symptoms

When these herbicides are applied preemergence, symptoms do not usually appear until the plants have emerged from the soil. Symptoms for grasses include stunting, purple coloration, and root systems that develop a “bottlebrush” appearance. On broadleaf plants, symptoms include red or purple leaf veins, yellowing of new leaf tissue, and sometimes blackened terminals.

Special Considerations

Herbicides in this category are very crop specific. The spray tank must be cleaned thoroughly before the sprayer is used on a potentially susceptible crop. It is very important that the susceptibility of future rotational crops be considered before herbicides in this group are applied. High soil pH increases the soil activity of sulfonylurea herbicides and the potential for rotational crop damage.

Other Herbicides That Inhibit Amino Acid Synthesis

The herbicides in this category also affect amino acid synthesis but in a different way than the previous group. These herbicides are nonselective and control a broad range of annual and perennial grasses, broadleaves and sedges. Roundup Ultra®, one of the most commonly used herbicides on the farm and around the home, is in this category.

Herbicides in this category

Herbicides in this category have not yet been classified by family. Instead, they are grouped by the active ingredient or common name.

Symptoms

Plants treated with glyphosate or sulfosate turn yellow in 5 to 7 days, then turn brown and die in 10 to 14 days. Glufosinate acts more quickly, in 3 to 5 days. An individual plant may have dead tissue, yellow tissue, and green tissue at the same time. Extremely low dosages of Roundup® cause leaf puckering.

Special Considerations

Because these herbicides are nonselective, it is very important to protect desirable plants from spray drift. These herbicides bind tightly to soil clay and organic matter and have no soil activity. For that reason they may be less effective when plants are dusty or when application water is dirty.

Download a printer-friendly version of this publication: Herbicides: How They Work and the Symptoms They Cause

Hermies, Pollen Sacs & Bananas

You may also see yellow “bananas” (stamens) growing around the pistils/hairs of the buds. A stamen normally grows inside a male pollen sac but sometimes appear directly on female buds, especially in times of stress. A stamen produces pollen and doesn’t even need to open up before it starts making seeds! Remove plants immediately if they start growing bananas (also referred to as “nanners”) or your entire grow room may get pollinated. No one wants to be surprised by seedy buds after harvest.

This highly stressed plant (from heat and too much light) grew a banana in a last desperate attempt to make seeds

Certain cannabis plants will herm even if healthy and unstressed. This is a genetic trait carried by some strains and particular plants. That’s part of why it’s important to grow seeds from a trustworthy breeder. For example, every clone of the following plant grew bananas in week 3 of the flowering stage, under no stress, in multiple different grow setups. When herming is part of a plant’s genes, there’s not much you can do.

These bananas weren’t triggered by stress. Herming is simply part of this plant’s genetics. The brown hairs are already pollinated.

Here’s a closeup of a herm banana

This poor hermie has male pollen sacs growing among the female flowers. It’s recommended to immediately toss any plant that shows both male and female flowers!

Male vs Female Cannabis Plants: Introduction

Cannabis Life Stages and Sex

Did you know there are “male” and “female” cannabis plants? Cannabis plants are “dioecious” plants, which means each plant shows a particular sex, just like humans and many animals. There will occasionally be plants that show mixed-sex and these plants are often referred to as hermaphrodites or “hermies”, which I will explain in much greater detail below.

The sex of a particular plant matters quite a bit to growers. That’s because only female cannabis plants produce buds. In fact, the “buds” that we smoke are actually the female flowers of the cannabis plant.

The highest quality bud is considered to be “sensimilla” and refers to female cannabis buds that have not been pollinated by a male cannabis plant. The word “sensimilla” actually comes from the Spanish phrase “sin semilla” which roughly translates to “without seeds.”

Regular marijuana seeds will usually be about 50% male, and 50% female. That means half of the seeds will be unusable as far as growing buds. Please note that some male cannabis plants (about 70% of male cannabis plants according to some estimates) may produce a small amount of useable THC via trichomes growing on the outside of the plant. Unless you’ve seen the males in your plant’s family tree, there is no way to know for sure if a particular male plant is going to produce THC/trichomes. Even if it does it will be a much, much lower amount than a female cannabis plant producing buds.

If you have a male plant and you are trying to grow bud, I strongly, strongly recommend throwing the male plant away immediately and starting another seed or focusing on your other plants. It’s a waste of time to grow male plants for THC or other cannabinoids – they don’t grow buds!

Here’s a picture of a male cannabis plant – no buds or trichomes, just pollen sacs!

Quick Tip: How do you make sure you only grow female plants so all your plants produce buds?

One way around the issue of having 50% male and 50% female plants is to purchase feminized seeds online. These seeds are available from all reputable online seedbanks, and the plants produced by these seeds are always female. Get your questions about buying seeds online answered! You can also make your own feminized seeds, but you have to start with two known female cannabis plants.

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It’s difficult to look at a young cannabis plant and know its sex

For the first part of your cannabis plant’s life, it will be in the first stage of growth known as the “Vegetative Stage.” In this stage, your plant will only grow leaves and stems, but no buds or flowers. Think of this as the time when your plant is gaining size to prepare for the second stage of life.

A young vegetative cannabis plant is unlikely to reveal its sex until it’s at least 3-6 weeks old, and sometimes even later than that.

Unfortunately, it’s difficult to determine the sex of a cannabis plant when it’s a young seedling. Although there is genetic testing that can be used on plants as young as 1 week old, most growers aren’t going to go that route. At a certain point, most strains will “reveal” their sex via pre-flowers at the joints. This can occur as young as 3 weeks old with male plants, and around 4-6 weeks old for female plants. This is normal and is just a sign that your plant is fully mature and ready to start flowering. Learn more about preflowers.

Another great option is to use clones. Cuttings (clones) taken from a female plant will always turn out to be female. Sometimes clones are showing preflowers by the time they’re rooted as a clone. We also know that when you breed two female plants together, you end up with feminized (all-female) seeds.

So, unless you start with a known female clone or feminized seeds, there’s no way to know what sex your plant will turn out until it actually starts showing signs of sex organs. This happens in the second stage of your plant’s life, known as the “flowering stage.” The first sex organs that appear are often called “pre-flowers.”


When do cannabis plants reveal their sex?

In addition to looking for preflowers, all cannabis plants reveal their sex when they reach their second stage of life, known as the “Flowering Stage”.

The first sign of sex almost always appears at the “V” where new growth tips form from a stem, like this….

See the little growths appearing at the “V” or “crotch” where the growth node meets the stem? These are the first sign of “pre-flowers“. In this case, we can see the pre-flowers are forming, but it could be tough to tell whether this plant is going to turn into a boy or a girl quite yet. (Note: It’s a boy)

Younger plants (that are less than 6 weeks old or haven’t shown preflowers yet) tend to take a little longer to switch into the flowering stage compared to older, more mature plants that have been vegetating for a while. Other than that, you can pretty much force a cannabis plant to start flowering no matter the age, even 2-3 weeks after the seed was germinated.

Read the full article about male vs female cannabis plants (and learn how to use cloning to identify the sex of young plants while they’re still in the vegetative stage – advanced only!): https://www.growweedeasy.com/marijuana-boy-girl

When Sex Isn’t as Certain… Avoid Accidental Pollination!

So now you know that most cannabis plants are normally considered to be either “male” and “female.” Yet sometimes you will run into plants that show both male and female characteristics, and these plants can accidentally self-pollinate, or pollinate your other female plants.

A cannabis plant that shows both male and female parts is often referred to by growers as a hermaphrodite or “hermie.” These can pollinate your plants and cause seedy buds.

There are a few different types of mixed-sex plants, and it’s important for a grower to understand some of the biggest differences so they make the best decision possible when faced with hermies.

Important: It’s not advisable to breed mixed-sex plants to create seeds because their offspring are more likely to display hermie characteristics.

Hermaphrodite Plants

Although growers will refer to all mixed-sex plants as “hermies,” there are technically two different kinds: hermaphrodite cannabis plants, and mixed-sex buds (like buds with nanners). The only reason I bother to differentiate between the two is that true hermaphrodite plants are more predictable.

With a “true” hermaphrodite plant, the male and female parts will grow on different parts of the plant. They won’t grow together in the same spot such as when nanners appear in the middle of buds.

Here is an example of a true hermaphrodite plant – notice how this hermie has both female pistils and fully formed male pollen sacs

What causes it? Stress can trigger this type of hermaphroditism, but unlike bananas, this particular type of mixed-sex plant seems to be a little bit more stable based on the plant’s genetics. It usually doesn’t take stress to cause these to appear. They’re more like a natural trait of the strain. A clone of a true hermaphrodite plant will often also turn into a hermaphrodite, and offspring will often show the same traits even under perfect environmental conditions.

It is recommended to never breed a plant that shows hermaphrodite traits since this is a highly inheritable genetic trait. A good “breeding stock” mother will not show signs of hermaphroditism even when subjected to stress.

What should the grower do? It is recommended that you remove hermaphrodite plants from your grow room or grow area as soon as possible to prevent accidental pollination of the buds.

If pollen from a pollen sac is allowed to make contact with your buds, those buds will stop focusing on making more buds and will turn all their “effort” into making seeds. No one wants seedy buds and reduced yields!

Unlike bananas, hermaphrodite plants tend to be more predictable. Though it’s not advisable, a grower who watches very closely can carefully pluck all pollen sacs before they’ve burst. However, this should only be done if it’s the only plant you have! Don’t do this if you have other female plants that can be pollinated!

Remember, while these pollen sacs can start appearing early, they may continue to appear throughout the flowering stage so stay vigilant!

The following type of hermaphrodite plant has mixed male and female parts, referred to in botany as “bisexual” flowers.

With mixed-sex buds you will see plants that grow a mix of pistils and pollen sacs together, like this…

Bananas (“Nanners”)

Another common type of mixed-sex buds is the type that produces “bananas” (sometimes called “nanners”) which grow from the middle of female buds.

Example of a “Banana” or “Nanner” growing among buds

Bananas are rarely round and they don’t look like a normal pollen sac. Instead, they’re often elongated and yellow, which is where they get the nickname “banana”. They may grow together in bunches that can look like a bunch of bananas. Occasionally they appear more lime green than yellow.

Sometimes a banana appears lime green instead of yellow

These can be a lot more difficult to control than actual pollen sacs, since they may start pollinating everything in the area as soon as they appear. A few bananas won’t do much damage, but if you have a big banana problem it may be best to harvest the plants immediately and cut your losses. Seeds take some time to develop, so if a plant starts herming right around harvest time, it’s less likely you’ll end up with seeds.

Just like real bananas, they can appear in bunches

What are they? Bananas are actually the exposed “male” parts of a pollen sac, called the “stamen” which would normally be surrounded by a sac to hold all the pollen until it bursts open. If you open up a fully formed male pollen sac, you will see what looks like bananas (stamens) inside.

But when bananas appear on your plants, they don’t need to “burst” in order to spread pollen, they will immediately start making pollen and often will seed the buds that are close by even if bananas are removed right away, and sometimes the pollen can drift to other plants and pollinate them as well, too.

This banana appeared a few days after the grower used the bud back building technique (cutting off the top tip of all buds to try to get them to grow more fat and round). Apparently, the plant felt attacked

This is what it looked like after being picked off

It’s possible that the pollen is sterile, and won’t pollinate bud successfully…but don’t rely on that happening!

The yellow bunches in this bud are bananas/stamens and will “try” to pollinate everything they can – they don’t have to wait for a pollen sac to burst. It’s possible that the pollen is sterile, but often you may find seeds.

If a female plant is allowed to go too long without being harvested or pollinated (allowed to go past the point of optimal harvest), she will sometime produce a bunch of bananas in her buds as a last-ditch attempt to self-pollinate and create seeds for the next year. This is sometimes known as rhodelization. This is not as destructive as other types of hermies since it only happens after plants are already past the point of optimal harvest.

What causes it? While genetics are ultimate the cause of whether a plant is capable of producing bananas and mixed-sex buds, environmental stress is often a big component in causing bananas to form. Luckily if you stick with high-quality genetics, you are much less likely to run into bananas even if you do accidentally stress your plants. Not all bananas are “fertile” and you may see them without ever getting seeds.

Male hermie banana growing among the beautiful buds


What type of stress can trigger bananas to form on cannabis buds?

Inconsistent Light Schedules & Light Leaks – When plants don’t get light at the same time each day, or if they’re exposed to light during their dark period (light leak). For photoperiod plants, this might be the largest contributor to hermies.

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Temperature – When temps get too high, hermies and nanners often appear. Cold night temps, or just large temperature swings in general, are also known to trigger bananas for some strains.

Too-Bright Light – Like too much heat, and/or light that is too bright can stress your plants and trigger hermies. This is most often caused by growers keeping their lights to close to their plants. You can light-burn your plants even when the temperature is under control.

Major Plant Problems – Major plant problems like nutrient deficiencies, root rot, pH problems, light-burn and nutrient burn can all trigger hermies to start growing.

Genetics – While stress plays a big role in the formation of bananas, the tendency to form them is genetic. This tendency is very common in the seeds of a plant that hermied. “Feminized” seeds, while always female, are much more likely to show the same herming traits as its parent. Growing seeds that were produced this way is naturally selecting to produce more buds that grow bananas. Only get feminized seeds from a trusted breeder.

What should the grower do? It is recommended that you remove plants showing bananas from your grow area immediately to prevent accidental pollination of buds. If the pollen being formed is allowed to make contact with your buds, those buds will stop focusing on making more buds and will turn all their “effort” into making seeds. If the plant self-pollinates, you will end up with a bunch of sub-par seeds that are likely to have the same problem.

Of the different types of “uncertain sex” cannabis plants, plants with mixed-sex buds (especially hermies with bananas) are the least predictable and this can make them more likely to cause unwanted pollination. This is partially because bananas may be hidden in the buds, and they don’t have a pollen sac that needs to burst to pollinate buds – it will start pollinating almost immediately.

A grower who watches very closely can carefully pluck all bananas, but they are unlikely to be successful and will probably end up with at least a few seeds.

Trying to salvage a plant that has started producing tons of bananas is NOT recommended, because it’s hard to get them all and you’ll end up with seeds. Even worse, once a plant gets started, bananas can appear in huge bunches overnight especially when the plant is stressed. Harvest the plant as soon as you can, before seeds get a chance to start forming.

How to Avoid Causing Hermies or Bananas

This section will explain what you can do as a grower to reduce your chances of running into hermies or bananas in your grow room…


1.) Avoid Inconsistent Flowering Light Periods & Light Leaks

Keep indoor lights on timer, and avoid changing the light schedule during the flowering stage if possible.

Prevent outdoor plants from being exposed to street lights, flood lights or other types of artificial lights during the night.

Respect the dark period – In the flowering stage it’s important to make sure all your plants (except auto-flowering strains) get at least 12 hours of uninterrupted darkness every night. Avoid shortened dark periods and light leaks!

No matter the strain, try to keep your plants on a consistent schedule throughout their lives, as this helps them set their circadian rhythms.

And for photoperiod plants in the flowering stage, do not interrupt the plant’s 12-hour dark period with light for any reason.

Why? During the dark period your plant is “counting” the hours until sunlight appears, and interrupting this process is one of the most common ways to stress the plant into producing bananas or hermies. It can also cause your plant to revert back to the vegetative stage.

Along with the point above, make sure you do not have any light leaks in your grow space, which could allow outside light to sneak in during the dark period. During the dark period your plants like complete darkness.

If anything ever happens with your timer or power that causes your plant to get too much light or darkness, it’s important to correct your timer as soon as possible. But don’t worry about it too much if it happens just for one day. It’s usually okay if it happens only once, but be careful not to let it happen again since messing up the light schedule can cause hermies. It’s better for a plant to get a too-long day than a too-short night period in the flowering stage. So, for example, it’s better for it to get an 18-hour day than a 6-hour night. Cannabis plants “count” the hours of the night period, so it’s most important to make sure the night period is at least 12-hours long (longer is better than shorter for night periods).


2.) Maintain Proper Temperature Through Flowering Stage

Maintain a comfortable temperature in the flowering stage, between 65-85°F (18-30°C).

Avoid big temperature swings – temps should be slightly cooler at night than during the day.

Always use “hand-test” to make sure it doesn’t feel too hot in the top canopy of buds under the light. Put one of your hands (palm facing down) under your grow lights where the top of your plants are located and wait 10 seconds – if it feels too hot for your hand it’s too hot for the plants! Be careful of cold drafts at night in cool climates during the winter.


3.) Don’t Give Your Plants Too Much Light (Light-Burn)

While generally more light is better for your cannabis plants, very high power brightness can light-burn your plants, which stresses plants, causes unwanted bleaching, and can trigger the plant to hermie on you.

With high power LED grow lights and big HID lights, make sure to always follow the manufacturer’s specifications as far as the minimum distance from the top of the plants!

Don’t keep your lights too close because even if the heat is under control, too much brightness can cause stress too.

Light-burn is only common with high power LEDs (3W chipsets and bigger, x-lens technology, COBs, etc.) and big HIDs or perhaps multiple smaller HIDs (usually with an intense cooling system).

It is more difficult to “light-burn” your plants with fluorescent lights, CFLs, smaller HIDs, etc. – with these lights, you only need to worry about heat.

High Power LEDs or HID Grow Lights Will Light-Burn Plants When Kept Too Close
(yes, even if the temperature is completely under control)


4.) Prevent Major Plant Problems

Major stress to the plant can cause the plant to react in unpredictable ways, including producing bananas and male pollen sacs. Major stresses include…

Nutrient Burn – given too much nutrients

Total leaf loss (usually as the result of overzealous defoliation or bugs)

Any other huge stress to the plant

Want to read about a real example?

The plant pictured to the right was subjected to cold temperatures and then grew directly into the grow light, putting it under a lot of stress.

From the grower Saberabre: “So I left this girl (or what I thought was a girl, notice the pistils at the bottom calyx) over the weekend and came back to the plant up in the light getting burned. Yikes! I’m not too sure what happened here but it got pretty cold the last few days. I think it’s a hermie…”

A few days before this pic, the plant was just showing a few white pistils and appeared to be female.

After the stress that it went through, the grower came back to a plant that was completely covered in male pollen sacs, with the first few white pistils being the only sign of this plant is female.


5.) Always Start with Trusted Genetics

Hermaphroditism and mixed-sex buds seem to be more common when growing plants from bagseed (seeds that you find) or seeds from an unprofessional breeder.

The reason is that seedy buds are worth less than sinsemilla (unseeded buds). Therefore, if you find a seed in your bud, it likely was the result of either bad growing practices (male plants weren’t removed in time) or due to some type of problem (plants were stressed and self-pollinated, which means the next generation is most likely to do so).

When you’re buying seeds from a trusted breeder, they go to great lengths to prevent unintended pollination, and they specifically select for plants that don’t ever show mixed-sex traits.

And remember… even if you do everything right, sometimes you will run into hermie plants – it’s just a fact of growing. Sometimes these things just happen, for example…

“I’ve always felt like seeded weed was not nearly as potent as sinsemilla and I do everything in my power to kill all males! Cannabis is so sneaky, though – last summer we had such wild weather that even a couple of clones turned out some male sex parts.”

Experienced outdoor grower

“Cannabis has both male and female plants. When both female and male flowers are in bloom, pollen from the male flower lands on the female flower, thereby fertilizing it. The male dies after producing and shedding all his pollen. Seeds form and grow within the female flowers. As the seeds are maturing, the female plant slowly dies. The mature seeds then fall to the ground and germinate naturally or are collected for planting the next spring.

“Unpollinated, female cannabis flowers continue to swell and produce more resin while waiting for male pollen to successfully complete their life cycle. After weeks of heavy flower and cannabinoid-laden resin production, THC production peaks out in the unfertilized, frustrated sinsemilla!”

Plant Symptoms

This page is part of our Plant Doctor series. You can use our tool to filter by symptom and help diagnose your plant.