Handling Seeds That Need Chilling

Oh my, this is an important topic. As a seed seller, dealing with these seeds is a serious hassle, because so many people seem to ignore this consideration or try to cut corners and then complain when their seeds don’t sprout. As a seed-sower myself, though, I really appreciate these sorts of seeds, because they don’t need lights or heat mats or other special care if you plant them at the right time of year.

Providing Natural Chilling

While we normally think of seeds needing warmth to sprout, a good number of hardy plants require, or at least benefit from, a period of cool temperatures as part of their germination process. It makes a lot of sense: Otherwise, their seeds would germinate in late summer or fall, and the resulting seedlings might not have time to produce enough growth to survive their first winter.

Sowing seeds outdoors in fall or winter, in pots or milk jugs or in a nursery bed, is by far the easiest way to provide a natural chilling period. (That’s assuming you live in a climate with at least a few months of winter cold [around 40F or less]. If you live in an area with milder winters, you will need to provide an artificial chilling period instead, as described below.) I usually sow seeds that need chilling any time from mid-October through early January here in my southeastern Pennsylvania garden, to make sure they get about 3 months of appropriate chilling conditions. Many seeds don’t need quite that long (60 or even 30 days are enough for some), so I sometimes sow for outdoor chilling as late as early February, but I accept that I might not get the results I want if I sow that late. Depending on where you live, your ideal cutoff date could be weeks earlier or later. That’s something you need to figure out for yourself, based on your knowledge of your own climate and local growing conditions.

While you may choose to sow directly in an outdoor “nursery bed,” I find it easier to keep track of my seeds if I sow them in pots: not an individual pot for each seed, but one or two pots for each kind of seed. I often cover the surface of the growing medium with a layer of fine gravel (like small aquarium gravel) after sowing, to protect the seeds from getting washed out by heavy rain. Sometimes I sink the pots into my nursery bed, nearly up to the rim; other times, I put them in a plastic crate or nursery flat so I can easily move them around as needed. I have also used old fish tanks or plastic storage containers, and they are ok, but they need a solid lid (and then occasional watering) if they don’t have drainage holes, because it’s a Very Bad Thing if the pots end up sitting in water (the seeds are likely to rot). The advantage of a solid container with a lid is that it keeps the pots safer from mice, birds, slugs, and other critters, which can dig in the pots or eat the seeds before or just as they are sprouting.

For the past few years, I have have been doing more and more fall and winter seed-sowing in plastic milk jugs. There are so many advantages to this approach (including portability, little need for supplemental watering, and built-in pest protection) and very few disadvantages. I have written about this in more detail in Sowing in Milk Jugs (aka Winter Sowing).

Whether you use pots or milk jugs, set them in a place where they will get some light as well as some moisture, and where they will not get knocked over. A spot under a low-branched shrub, or under the edge of a deck or porch, can work well. Check them a couple times a month to make sure they are not too dry or wet, that their labels are intact, and that critters aren’t getting into them. Starting in early March here, I like to check outdoor seed containers every few days to catch the seeds as soon as they start sprouting, so I can move them to a brighter location. April brings the most sprouting activity, but it can continue well into June here. Some seeds may even need to go through an additional winter before sprouting, so don’t be too quick to give up if no seedlings appear the first spring or summer; leave the containers outdoor until the following summer.

Providing Artificial Chilling

If you live where outdoor winter conditions don’t provide enough cold, if you are sowing too late for your seeds to get enough natural chilling, if you don’t have a safe place for outdoor sowing, or if you simply enjoy keeping a really close eye on your seeds, providing an artificial chilling period may be your best, or only, option. Fortunately, a home refrigerator provides the ideal temperature conditions. (The disadvantage is that it’s a constant temperature, and some seeds germinate better in the alternating day/night temperatures they would get outdoors).

One option is to sow as you normally would in a pot, put the pot in a re-closable plastic bag or a regular plastic bag closed with a twist-tie, and tuck it into the back of a refrigerator shelf or in one of the bins. This approach works well with very small seeds, particularly, but can take up a lot of room if you’re dealing with more than a few kinds of seeds.

Placing seeds on a moistened paper towel or coffee filter, folding it up, sliding it into a plastic sandwich bag, and then placing it in your refrigerator saves a significant amount of space, and it gives you the opportunity to catch the seeds as soon as they start sprouting (some will germinate while they are still in cold conditions). The disadvantage is that you do have to remember to check them regularly, to make sure that they aren’t drying out, and you need to pick off and pot up the seeds soon after they sprout; otherwise, they can root into the paper towel or filter paper and be very difficult to remove. It is also very challenging to deal with tiny seeds if you try to germinate them this way.

A third way to handle seeds for an artificial chilling period is to mix them with a handful of moistened vermiculite or potting soil in a plastic bag, close it, and put it into the refrigerator. When you see the seeds start to sprout, you can pick them out and pot them up individually. Obviously, this works best with larger seeds (roughly pea-size or larger). If you try it with smaller seeds, you can spread the seed/medium mixture (kind of like icing) on top of moistened growing medium in a pot when germination is is starting, or when the chilling period is done.

If you are not sure how long your seeds need to be chilled and they haven’t started to sprout in the cold, leave them in your refrigerator for about 3 months. You could pot them up after that, if they have been on paper towels or filter paper or in loose growing medium; then set the pot(s) outside (NOT while still in a plastic bag) or keep them at room temperature under lights. Or, you could leave still-bagged seeds in a warm spot out of direct sun; check them frequently (every day for the first week or two, then every few days for the next few months), and pot them up as soon as they start to sprout.

On the whole, dealing with artificial chilling is a much higher-maintenance approach than natural chilling, and it requires careful attention for success. It’s fascinating and rewarding if you are really into growing out-of-the-ordinary seeds but may require too much time and care if you are a novice seed-starter. I don’t want to discourage you from trying if you are a beginner, because the only way to get experience is to attempt it, and it really is thrilling when it works out. But you should be aware that it IS a challenge, and if you’re not willing or able to deal with that, you’re much better off waiting to buy and sow the seeds at the right time for them to get natural chilling.