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Germinating Seeds in Winter – Tips From the Nursery

Winter is here! Time to give up on germinating seeds and shift to knitting little jumpers for the seedlings we already have. Or perhaps I’ll organise an old-sock-drive, the ones with threadbare holes in the toe would do the job nicely; slip them over a pot and leave an escape hatch for any masochistic seedlings to grow out the top and brave the winter chills if they dare!

Honestly though – winter doesn’t spell the complete end to seed germination. There are probably quite a few seeds out there in the wild still glad for an opportunity with the good soil moisture, to have a go a life. But the rate of germination and growth for most species is very much reduced with the cool nights and mornings that herald winter.

In the nursery, there is still plenty of bench space to keep filling with more plants – the autumn rain gave the landcare CLS field crew a good opportunity to plant a few project sites that were waiting patiently for the summer dry spell to end. Plenty of other landholders have also been getting into autumn planting as well; the result is a considerable dent in our stock numbers for several popular species.

So, propagation goes on and this is where a certain bit of physics comes to the fore.

It’s not all that technical, but it does wonders for plant production. Inertia is in lay-terms the tendency of something to keep going, or doing, what it is already doing. Thermal inertia means something will keep being warm, until the rest of the universe convinces it to cool down.

If you have solar-heated hot water at home, you’re tapping into thermal inertia. Using the sun’s energy to warm water, then store it away for it to release that warmth later, when you turn on a tap. Thermal inertia is also what’s behind the ‘passive heating’ or ‘thermal mass’ designs in architecture.

If you’ve ever watched a snake or lizard warm themselves in the sun, you’ve witnessed nature being crafty with thermal inertia. The animal will circulate their blood near their skin, gather the warmth of the sun, then when they’ve warmed up enough, they can redirect most of their now-warm blood flow internally and retain that warmth for a decent period of the day (or night).

These same techniques come in handy for the nursery also. The days are still warm enough in a sunny spot to promote decent germination and growth. But the nights are longer.

The challenge really, is to catch and keep some of the day’s warmth to keep the night chills at bay.

I noticed this challenge being well met last week, as I moved the afternoon’s effort of seed-sowing to a new home – some spare space in one of the Biocontrol propagation ‘tunnels’. That particular area faces the sun for most of the day, with a dry gravel floor. As I entered, I could feel the warmth not only in the air but also in the gravel. Through the day, both the air and gravel had absorbed warmth from the sun. Gravel, having a far better thermal inertia than air, would be radiating its warmth back into the propagation area throughout the evening, probably enough to still keep the tunnel several degrees warmer than outside by sunrise the next day. The few degrees difference, spread across the night, will hopefully prompt the seeds and seedlings in that location to grow as fast as their comrades did outdoors back in March.

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Not all the nursery can be boosted along by such a method – or at least, not without considerable outlay in infrastructure. Trying to gather daily warmth to keep all our plant stocks growing faster through winter would also give us plants that were not ‘hardened’. We could nurse seedlings to a plantable size in a warm sheltered place, but they’d not fare well if taken out into the wilds! So to keep our stock suitably ready for field conditions, there has to be an element of exposure to the prevailing conditions, be that summer heat or winter frost. The slower growth, in the end, is better than growing a plant that doesn’t survive after planting.

Our plants have it a bit tougher than they need to though; a side-effect of growing them above ground in little containers. Naturally, a plant would have its green bits exposed to these chilly nights, but the roots would be in a more temperature-stable setting – mother earth. Nursery plants have the added challenge of their roots being in a small volume of earth up on a bench, where the night air can circulate around and cool both the shoots and roots. There are ways to alleviate this of course, but again it can mean added infrastructure/costs (heat-mats, solar-heated water circulating under benches, etc), or seasonal changes in procedure (e.g. placing stock on ground instead of benches). Each ‘solution’ comes with its own extra challenges. So for now at least, we’ll continue with our usual grow out process. If the threat of frosty mornings becomes a concern, we can make use of the thermal inertia of our water supply: our tanks will be warmer in the mornings than the air, so running irrigation in the coldest spell (usually a few hours either way from sunrise) can lift the temperature in and around our plants. Just as long as we don’t drown them in warmth!

Shane Litherland | Nursery Manager

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How to Start Seeds Indoors in the Winter

Step one of planting a wintertime garden is starting seeds in winter, and we’re here to take you step by step through the process. Colder temperatures mean you may be starting seeds indoors when you’re used to sowing them directly in the ground. You’ll probably also need to take some extra measures to get young plants acclimated to the outdoors through cold acclimation, also called hardening off, before moving them into their permanent places in the wintertime garden.

But there’s no reason to be daunted by the extra steps it takes to start seeds indoors in the winter. Read on to find out everything you need to know to get your wintertime garden off to a strong start, with answers to your questions as well as step-by-step instructions for starting seeds indoors.

Will seeds germinate in cold weather?

Because germination (the process by which seeds sprout) takes place underneath the surface of the soil, sunlight isn’t important—but warmth is another story. Seeds germinate the best in temperatures between 60 and 75 degrees Fahrenheit. Heated mats can add 10 degrees to the air temperature if you’re raising seeds in a room that doesn’t offer enough warmth for plants to germinate.

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(Note that seeds that rest on the surface of the soil may require light to germinate as well as heat. For example, begonias, impatiens, and petunias won’t sprout if they don’t get enough bright light.) When you aren’t sure of a plant’s specific needs, check the seed packet or the information provided online to determine what temperature the seeds should be kept in and how much light they need to sprout.

How do you grow seeds indoors in the winter?

Gather clean containers, such as seed trays, peat pots, or flats. As an alternative, you may choose to go DIY and add drainage holes to recyclable plastic egg cartons.

Add equal parts vermiculite and perlite to soilless peat moss to create your own soil for seed starting. This combination allows the soil to retain enough water to keep seedlings hydrated while still permitting air to circulate, and it’s fine enough to let the tender young roots of your plants spread out, unlike standard potting soil, which just won’t do for starting seeds. You can also use Jiffy pellets or another brand of pre-formed seed starters to save time and hassle if you like. Make sure that any compost you use in creating your seed starting mix has been heated to 150 degrees Fahrenheit so that weed seeds and diseases are exterminated. You can add vermicompost, but keep it to 10 percent or less of the total soil mixture by volume.

Following the instructions on your seed packet or the guidelines given on the catalog or website, sow your seeds into the prepared containers. If you’re instructed to gently press seeds into the soil, the eraser end of a pencil makes a handy tool. Selecting the largest seeds in each packet to sow will get you the highest germination success rate. Sow no more than two or three seeds per cell to prevent overcrowding.

Use plastic cling wrap to cover up your containers and keep moisture locked in, creating something similar to the greenhouse effect. Be sure to add a few holes using a toothpick or other sharp tool so air can circulate and mold doesn’t develop.

Gently and carefully water your newly sown seeds. A watering can or pitcher may release too much water for new seeds with too much force. Spraying your seeds with mist from a spray can might be gentler on them, but it takes quite a long time to get plants as much water as they need.

A good middle ground for seeds you’re starting indoors is a turkey baster or large eyedropper. Another options is a pitcher with a narrow spout, which creates a gentler stream of water. These tools will give you the water output plants need without the water pressure you’d get from a watering can or the garden hose you use on more mature plants, which can displace soil—and the seeds you’ve just planted along with it.

Keep the soil where you’re starting seeds evenly moist but not waterlogged. Check the soil where you’ve started seeds at least once per day to ensure your seedlings are getting enough water. Many gardeners will find they need to water their seedling twice per day to keep them hydrated.

When plants are just getting started, warmth is more important than sunlight, so good places to set up shop include on top of the refrigerator or close to a radiator. Plants grow best in temperatures between 60 and 75 degrees Fahrenheit.

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Once you see evidence of sprouting, remove the plastic cling wrap and position plants where they’ll get 12 to 14 hours of bright sunlight per day. (Yes, that’s a lot, but young plants crave sunshine.) A south-facing window will offer more sunlight than windows that face in other direction. When plants have grown to one inch tall, thin them out by snipping the stem at the soil line to leave just one per cell for best results.

Once your seedlings have grown their second set of leaves, it’s time to transplant them. Carefully relocate young plants into individual containers filled with a potting soil that’s mixed with plenty of compost. Water transplanted seedlings thoroughly, and keep them out of bright light for a few days while they recover from the stress of being moved.

When should I start my seeds indoors?

For most annual vegetables, as a general rule, start seeds between four weeks and eight weeks before the last frost in your area is expected. However, this timeline can fluctuate based on your location and the specific plants you’re growing. The Old Farmer’s Almanac offers a planting calendar that will provide you with specifics on when to start your seeds if you key in your ZIP code.

Which seeds should I start indoors?

In the winter, many crops benefit from getting a head start indoors, especially for gardeners in chillier climates. The plants listed below are especially common for starting indoors. When in doubt, it’s best to go ahead and start seeds indoors in winter, as a head start won’t hurt your plants—but early exposure to winter’s chilly weather can have disastrous consequences for young plants that aren’t up to the challenge.

  • Beans
  • Broccoli
  • Brussels sprouts
  • Cabbage
  • Cauliflower
  • Celery
  • Cilantro
  • Eggplant
  • Kale
  • Lettuce
  • Moonflower
  • Parsley
  • Peppers
  • Petunias
  • Pumpkins
  • Spinach
  • Squash
  • Swiss chard
  • Tomatoes
  • Watermelons

How long does it take for seeds to sprout indoors?

Germination rates will vary when you start seeds indoors based on the room temperature where your plants are kept. As a general rule, however, most plants germinate within a week or two. A few plant types, such as chili pepper, tomato, or rosemary, can take closer to three weeks to sprout.

Do you need a grow light to start seeds indoors?

A few variables come into play to determine whether or not you’ll need to use a grow light when starting seeds indoors. In short, plants require six to eight hours of sunlight per day to promote healthy growth, and they’re happiest when they get closer to 12 or 14 hours of sun each day.

If you’re starting your garden late in the winter, that much light may not be available. Or you may find that the windows in your home where your seeds get their sunlight aren’t exposed to six to eight hours of sun per day. In these cases, a grow light is needed for plants to perform their best. Where installing grow lights is not possible, you may opt instead for fluorescent shop lights housing one warm bulb and one cool bulb.

When it’s time to relocate your plants into the garden for the winter, remember not to shock them by exposing them to the cold weather suddenly. Give plants a period of transition by using the hardening off process to gradually acclimate them to the cooler temperatures. You can learn more about hardening off and get step-by-step instructions to guide you through it in this article by Gardening Channel.