Can weed seeds germinate after a century of dormancy?
A new factsheet available for free download from the Weed Science Society of America (WSSA) dives into the topic of weed seed longevity, as well as how weed seeds travel, when and why they germinate, and ways they can be eliminated.
“Understanding weed seeds and their lifespan is critical for both farmers and backyard gardeners alike,” says WSSA member Greta Gramig, Ph.D., associate professor of weed science at North Dakota State University. “Seeds can remain viable in the soil for extended periods of time. That means if even a single weed is allowed to go to seed, you may be battling the aftermath for years to come.”
Here are a few of the weed seed facts in the WSSA fact sheet:
- Moth mullein seeds buried by a researcher in 1879 were still able to germinate more than 130 years later.
- Weed seeds can easily be spread and transported far from their original location. Some have found their way into the earth’s planetary boundary.
- Earthworms are known to collect weed seeds and move them into their burrows.
- Weed seeds that remain dormant in the soil will often germinate in response to changes in temperature, moisture, oxygen or light.
- Carabid beetles are voracious eaters and can consume large quantities of weed seeds that drop to the soil.
In addition to its fact sheet on weed seeds, WSSA offers a variety of other free fact sheets and educational materials online, including infographics and presentations on herbicide resistant weeds and their management.
A man named William James Beal started an experiment in 1879, which has become the longest-running test of seed dormancy in soil.
Beal buried twenty bottles containing sand and seeds from twenty-one common plants. He buried the bottles eighteen inches underground, uncorked to allow in oxygen, and upside down to prevent water from accumulating in the bottles and drowning the seeds. The experiment was designed to test how long these various seeds could survive under virtually natural conditions before they would lose their ability to produce a plant.
Seed dormancy is why some seeds wait for the correct season before they germinate. They’re programmed to wait for conditions suitable to the plant’s survival. Seed dormancy is also why weeds are never-ending nuisances. Weed seeds can lurk underground unseen, and then when you till soil to make way for a flower bed, the seeds awaken and before you know it, your flower bed is overrun with the pesky weeds.
Most of the seeds in Beal’s experiment were still viable after ten years, but as time passed, fewer and fewer of the seeds would germinate. The fifteenth of Beal’s twenty bottles was dug up in 2000, 120 years after it was buried, and two species were successfully germinated–moth mullein and a mallow called cheeses. These are also the same two species that grew from the fourteenth bottle in 1980.
The plant that holds the record for the longest dormancy is a lotus that was sprouted in 1995 from a seed that radiocarbon dating estimated to be a whopping 1300 years old. Now that’s one stubborn seed.
Seed Dormancy – Explained
In my previous post, Plant Seed Basics, I described the germination process in very general terms. A seed lands on the ground, absorbs water, and germinates. It is all so simple – or is it? In this post I will have a closer look at something called seed dormancy.
Seed dormancy – germinating Fritillaria seed, by Robert Pavlis. Notice the radicle growing through the seed and emerging from the top of the seed.
Seed Germination Process
Assume that a seed is from a plant that goes through a winter cold period – a time when it is too cold for the plant to grow properly. What happens to our seed?
The seed will be produced, land on soil, take up water and germinate. Just as things are getting going, winter sets in and kills the seedling. Any cold climate species that went through such a process would soon perish, so these plants must use a different process.
Seeds produced in cold climates do land on soil in the fall, and then might absorb water, but most don’t germinate. They remain dormant until certain environmental and physiological changes take place. Some seed just waits for several months in the hope that spring will be there when it germinates. Other seed requires a drop in temperature to turn on certain chemical changes before germination starts.
Seed dormancy is a term that describes these delays in germination. Understanding the cause of seed dormancy is frustrating when you are trying to grow a plant, but it is also what makes seed germination so interesting and challenging.
What about seeds produced in warm climates? They can also have a dormancy but it usually does not require fluctuation in temperature.
See Are Seeds Really Dormant, for a more in-depth description of seed dormancy.
Do All Seeds Have a Form of Dormancy?
Yes, but most have simple forms of dormancy that are short in duration. If you are starting out growing plants from seed you will find that most things you try to grow, will grow just fine with water and room temperatures. As you get more adventurous, you will find that quite a few of the less common and more interesting plants have a more complicated dormancy.
What Causes Dormancy?
Dormancy is both simple and complex. There are some fairly simple reasons why a seed remains dormant and they are easy to understand. At the same time the subject is complex for two reasons. Firstly, every seed has it’s own reason for dormancy and there have not been enough detailed studies for most seeds to understand them. The information that is available is mostly from amateurs and is anecdotal. Secondly, there can be more than one reason for a seed to be dormant. So what seems like a complicated system may actually be two or more simple dormancies, that just look very complicated.
Understanding the reasons for dormancy will help you understand the methods used to overcome dormancy – the subject matter for the next post in this series.
Almost all seeds need to absorb water as part of the germination process and it is usually one of the first steps needed to break seed dormancy. Seeds are like freeze dried food that needs to be re-hydrated in order to make a meal.
Once you start adding water to seed, they should never dry out again. Even if you do not see any external evidence of germination, internal chemical processes have started. Once they start they can’t be reversed. Drying too much after absorbing water, will kill the seed.
If you can’t see the above video, use this link: https://youtu.be/dhL57pqnHHQ
In many cases the seed that you collect is not fully developed. It looks ripe, and all external characteristics say it is fully developed, but internally the embryo is not fully grown. In this case the seed needs some more time in order for the embryo to complete it’s development. Then and only then will it germinate. You just have to wait.
Embryo development might take place before water is absorbed, or after, depending on the type of seed.
Chemicals On The Seed Coat
Some seed has a natural chemical coating on the seed that prevents them from germinating. Most fruits surround the seed with fleshy fruit material, and this can contain chemicals that prevent germination. For example, tomatoes do this. It is important that all organic matter be washed off seeds to remove the chemical inhibitors. The best way to do this is with tap water.
Iris seeds are formed in a seed capsule without fleshy fruit material, but they do tend to have chemical inhibitors on the seed. Iris seeds are usually soaked in water for 24 hrs, then rinsed. This is repeated for a week to remove the chemical coating.
Some seed has a waxy outer coating which may contain chemicals that prevent germination or it might just be a physical barrier that prevents water from entering the seed. Waxy coatings can be removed with soapy water or you might need to wash them in alcohol.
Hard Seed Coat
Some seeds have a very hard seed coat (ie the outer layer of the seed). Think about the walnuts we eat. The hard corrugated outer shell is the seed coat and it prevents water from being absorbed by the seed.
In nature, physical abrasion by soil and rocks will eventually wear a hole in the tough seed coat and then water can enter. Birds also eat seeds and the acid in their stomach will help soften tough seed coats.
One of the best ways to handle tough seed coats is to use a form of scarification (pronounced ‘scar’, not ‘scare’). You can do this with some sand paper, nail clippers or even a file. The goal is to make part of the seed coat thin enough so that you see the seed inside.
When you use nail clippers or a file to scarify seed, it is important to stay away from the embryo side of the seed. On most seed you will see a small scar left at the point where it was attached to the mother plant. The embryo will be close to this scar, so nick the seed on the opposite side. If you don’t see a scar, stay away from any pointed end of the seed and nick the round part of the seed. The second half of the video below shows various seeds and their embryo which will help clarify this point.
A milder treatment works for some tough seed coats and this is done by dropping the seed in very hot water, and letting the seed sit there for 24 hours. The hot water helps break down the tough seed coat enough to absorb water.
For a detailed explanation of scarification, have a look at this video:
If you can’t see the above video, use this link: https://youtu.be/icB9HrqdQqU
Internal Chemical Inhibitors
In some cases the mother plant adds a chemical inhibitor into the seed as it is forming. The inhibitor is there to prevent the seed from germinating. As long as the amount of inhibitor is high enough, the seed will not germinate. Germination can only take place once the level of inhibitor goes below a critical level.
Why would a plant do this? Think of it from a plants point of view. You want your seeds to produce as many offspring as possible – that really is your only purpose in life. You don’t want seeds germinating just before winter because the tender seedling will die. An inhibitor can prevent this.
In some cases a single plant will produce seed with varying amounts of inhibitor. Some have a little bit of inhibitor and will germinate in a few months – next spring. Other seed from the same mother plant have more inhibitor and will not germinate for a year or even several years. This can be a big advantage for survival of the species since a single year of bad spring weather will not kill all of the seedlings.
What this means for the gardener is that such seed can germinate over longer periods of time – some each year for a couple of years.
The inhibitors in seed degrade over time. In some cases the best solution for this problem is to keep the seed moist and just wait. They might takes months or even years to germinate.
Temperature and Stratification
Seed from a temperate climate needs to control the time of year in which germination occurs. Some seed germinates in fall and usually stays under ground where it is better protected from the cold. Then in spring true leaves start to grow.
Other seed wants to stay dormant until the warm spring arrives. This seed may need a cold period (ie winter), before germination can start. Or the seed may need a warm period (mimicking late summer and fall), then a cold period (winter), followed by a warm period (spring). This final warm period triggers germination.
Giving the seed a cold spell to cause germination is called stratification.
Seed that requires stratification can be manipulated by the gardener by moving them from room temperature (warm) to a fridge (cold), then back to warm to germinate. Some seed will actually germinate in the cold.
Light and Dark
Light and or dark can also affect germination. Consider weed seeds. They get spread around and some will fall into holes, or be covered with too much soil as animals go digging in the soil. There is no point for seeds to germinate if they are too deep to grow. To solve this problem they have evolved to require light to germinate.
Weed seeds can stay dormant in soil for many years, and as soon as you disturb the soil to bring them to the surface, they germinate.
Other varieties of seed are the exact opposite. They want to be buried before they germinate. For them it is important that they remain dark in order to germinate, and if they get too much light, they stay dormant.
This might be the strangest form of dormancy. Cones from Scotch pine stay tightly closed for many years. The seeds inside can’t get water and so they don’t germinate. In a forest fire these cones pop open releasing the seeds which then germinate.
The Scotch pine is an example of a fruit that needs fire in order to release the seeds. There are also seeds that need to be exposed to smoke from a fire to germinate. This is common for some Australian seeds. Artificial heat and or smoke have been used to germinate these seeds.
Seed Dormancy – What Does It Mean To The Gardener?
Seed dormancy does not affect most vegetables and most common flowers. You can just go ahead and plant the seed and it will germinate.
Most other types of seed have a form of dormancy and in most cases we still do not fully understand which of the above factors are causing the dormancy. However, for most seed we do have information about steps that you can follow to overcome dormancy. So for example, if you are growing iris seed the instructions might say to wash the seed for several days, and then give them a cold period of three months, followed by 3 months of warm. You can follow this procedure and you should get germination.
In my next post I will have a look at various seed pre-treatments that you can use to break seed dormancy and get germination. I’ll also provide some tips on finding out which pre-treatment is the correct one for your seeds.
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