What is winter sowing?
Winter sowing is an easy and economical seed starting method. In this method, recycled plastic containers are used to create mini-greenhouses for seeds to germinate and develop into robust seedlings. By using the winter sowing method, seedlings not only get an early start, but when the time comes for transplanting, they will already be acclimated to the outside elements and won’t require hardening off.
What winter sowing is not
For the purpose of this guide, when the term ‘winter sowing’ is used, it does not refer things like casting seeds on the ground in the winter or starting seeds in cold frames, tunnels or a framed greenhouse. While all of these alternative methods are great ways to start seeds, the method laid out here is exclusive to the process of creating miniature greenhouses from recycled plastic that work with all the elements that come with winter.
Sowing seeds when it’s still cold: when to start winter sowing
The best way to think about when to start winter sowing, is to think not in terms of sowing on a specific date, but within a window of time. There are two main windows for winter sowing.
First window
Group 1 seeds, followed by group 2 leading up to the second window
When the weather in your area becomes consistently cold — typically around winter solstice
Second window
Group 2 & 3 seeds
The final weeks leading up to your last frost date
The 3 categories of what to winter sow
Group 1 seeds - native plants and perennials that require cold stratification
Group 2 seeds - hardy annual flowers, cold weather crops, hardy herbs, native plants and perennials that don’t require long periods of cold stratification
Group 3 seeds - tender annuals: warm season crops, annual flowers, tender herbs
These categories are ordered based on timing of when to winter sow. Generally, if you winter sow the right groups of viable seeds in their corresponding windows, you should expect really great results.
In summary, finding the right window of time to winter sow is largely determined by these three things:
What type of seed you are sowing
When your winter becomes consistently cold
Your last frost date
Are there things you can’t winter sow?
Before diving into unpacking these windows for sowing, it’s helpful to keep in mind that there are some things that you should avoid starting using this method, but determining which things work best with this method and which don’t is not complicated.
If you’re new at gardening or this is your first (or even second) year winter sowing, it can feel overwhelming knowing when to start and what to winter sow. The good news is that, most likely, the things you are wanting to add to your annual and perennial gardens can be winter sown.
One of the simplest ways to determine if something can or can’t be winter sown is to consider how easy it is to find its seeds on the internet. If you’re running into issues finding the seed for what you’re wanting to grow, the reason might be because propagation via seed is not the ideal method for starting that plant—therefore, it most likely would not be an ideal candidate (or the most easy) for winter sowing. If you’ve found the seed you’re looking for, but it’s on the more expensive side, that’s also another indicator that there might be reasons it would not be a good candidate for winter sowing.
If you’ve easily found the seeds for what you’re wanting to grow, they’re most likely begging to be winter sown.
Things you will want to avoid starting with the winter sowing method:
asparagus crowns
corms
cover crops
fruit trees
grains
root crops
shrubs
some alliums
tuber crops
There are things you can’t effectively winter sow. These are things like:
garlic
spring flowering bulbs
seeds that require double dormancy or take multiple years germinate
Understanding the windows for winter sowing
First window: when the weather in your area becomes consistently cold (group 1 seeds)
One distinct characteristic about many native plants (and many perennials as well) is that after producing seed, the seeds remain dormant—meaning, they won’t germinate. In order to break dormancy, certain conditions have to be met. When used at the right time, the winter sowing method provides the perfect conditions for seeds that require it, to be exposed to a period of cold moist stratification, and as a result, effectively break dormancy.
When you think about the first window for winter sowing, think dead of winter. This first window typically starts around winter solstice and—for some seeds—continues through to the entire length of the second window. The number of days required for cold stratification is what determines how large your window to sow seeds from this group is. What you’re aiming for with this window is to winter sow seeds from group 1 either just before and/or throughout the coldest parts of your winter, allowing them enough time to complete their stratification requirements. This will vary from seed to seed.
Are you wondering if your winter is cold enough? As long as you choose seeds native to your region and start them in this window—even if your days swing well above freezing, they will still achieve cold stratification.
Because we’re talking about the idea of windows for sowing, these time frames for winter sowing—while fairly lengthy—can be missed. In other words, at some point, the window can and will close. For example, if you’re wanting to start seeds for a plant that requires 90 days of cold stratification, and you don’t get to them until March—apart from areas with very late last frost dates—you most likely will have missed that window for the year, for that specific seed.
To learn more about winter sowing seeds that require periods of cold stratification, visit my guide on winter sowing native plants.
Group 2 seeds in the first window
Whenever I get to it, toward the middle to end of the first window, I like to begin starting group 2 seeds. You can absolutely wait to start these seeds until the start of the second window—and you can also start them earlier—but I personally like to get these seeds out somewhere mid to end of the first window for a couple reasons. Firstly, because seeds from this group are hardy enough to take freezing or below-freezing temperatures without special care. In fact, seeds from this group benefit from colder temperatures at the start. Many plants from this group of seeds, if well established, I will transplant before the last frost date. Secondly, they’ll have a longer chance to establish and grow before transplant—this means earlier blooms and harvests.
The reason I personally wait a bit, and don’t start seeds from this group with my natives at winter solstice, is because I am winter sowing in a climate with milder winters. It’s not uncommon for my ‘dead of winter’ to experience days in the 50s and 60s, and nights that drop into the twenties or even teens. By waiting, I avoid unnecessary chances of killing temperatures that could effect early germination—which is entirely possible based on my experience winter sowing in the southeast. Hypothetically, it would also be really easy to miss germination during this time, and then if the seedlings end up being killed, come spring, you might be wondering why those jugs were a dud.
When I was winter sowing in a climate with very cold winters, it really didn’t matter if I put these seeds out with my natives; they weren’t going to do anything until after those initial thaw cycles begin closer toward the second window. If you’re in an area with a climate like mine, or in an area that seems like springs are getting warmer, and winters are becoming milder, but you’re still experiencing really harsh cold nights, you might consider waiting like I do.
Second window: the final weeks leading up to your last frost date (seeds for group 2 and 3)
The second window for winter sowing is both for group 2 and group 3 seeds. Again, there may still be time to winter sow seeds from group 1 in this window, but it’s important to make sure there are enough days left in order to fulfill the necessary length of cold stratification. If temperatures are still consistently fluctuating back to freezing or (ideally) below freezing, and you’re still a significant ways out from your last frost date (at least as many days as are required for cold stratification) then it’s typically not too late.
The easiest way to find the start of this window (that is, the earliest I would recommend starting seeds from group 3), is to count back 6 to 8 weeks from your last frost date. Whatever date (or week) you get at that point is a good time to start (if you haven’t already) seeds from group 2.
This window is also the time to start anything that is frost sensitive (group 3 seeds). The key here is to try not to start them too soon. Annual plants have two main jobs each season: to flower and set seed—annual plants are fast growing. With fast growth being in their nature, starting them too soon can lead to problems. In the climate I’m winter sowing, where we have springs with very warm days, I try to tame my spring exuberance as best I can and hold off until about 6 weeks before my last frost date. The reality is, even at six weeks, that can sometimes be too early to start some tender annuals if you’re in a climate with a mild winter. Some things that come to mind are things like zinnias and sunflowers. These are absolutely wonderful candidates for winter sowing, and you’ll get earlier blooms if you choose to winter sow them, but they also tend to grow very fast if days are warm, and crowded jugs can become an issue.
I’ve winter sown in two very different climates. I’ve found that for things that take longer to develop into an ideal size for transplant, but area also ultra-sensitive to cold (things like peppers and tomatoes), I like to start those right around 6 weeks—maybe even a little before that if it’s a warm spring. For pretty much all other tender annuals, I’ve found that starting them 4-6 weeks before my last frost date is the sweet spot. This timing allows plenty of time for significant growth while not running into problems with plants bursting out of jugs which can sometimes lead to leggy plants. This timing also leaves the smallest window for having to watch for nights that drop below freezing when tender plants will need to be covered.
A note on zones and winter sowing
While knowing your zone is a great way to determine which things you might want to grow in your own garden based on which plants are perennial in your area, zones are essentially irrelevant to winter sowing. Your climate and last frost date however, are relevant. If you want to learn more about why zones aren’t relevant to winter sowing, I’ve wrote about that here.
Winter sowing in your garden: understanding the nuances of winter across different climates
Winter sowing in a climate with wet winters
Things to keep in mind in climates with wet winters, especially with temperatures that fluctuate above and below freezing, starting group 3 seeds at the recommended time is especially important in order to avoid seed and root rot.
Winter sowing in a climate with cold or harsh winters
When following the recommended windows for winter sowing, winter sowing in a climate with cold or harsh winters is very straight forward. For places with later last frost dates, or historical frosts beyond the average last frost date (some parts of Alaska experience this), winter sowing is a great way to extend your growing season and the mini-greenhouse can serve as an ideal place to protect from late frosts, if you choose to wait to transplant.
Winter sowing in a climate with mild winters
Climates with mild winters tend to have a warmer start and are susceptible to erratic temperature surges and plummets—this can last for several months. As you become familiar with the windows for sowing, aim to remember that these unexpected extreme fluctuations in temperatures can sometimes cause early germination for seeds that produce a tender plant. For this reason, in areas with mild winters, I recommend following the windows very closely for seeds in group 3.
Winter sowing in a climate with warm winters
If you’re winter sowing in a climate with warm winters, early last frost dates, and very few—or even no frosts whatsoever—modifications might be necessary at times. Some winter sowing modifications you might want to use in a climate with warm winters are:
Start winter sowing sooner than winter solstice
Monitor moisture levels more frequently
Apply less drainage holes to the base in order to retain more moisture
On especially hot days, break up the seal on the tape a bit in order to encourage air flow
Keep jugs in the shade on especially hot days
Keep jugs with seeds requiring cold stratification out of direct sunlight during the stratification phase
Winter sowing in a dry climate or on a year with a dry spring
Across any climate, as temperatures start to rise, you might find yourself winter sowing in place that is experiencing a dry spring. When first assembling your mini-greenhouse, it’s always important to make sure the soil is thoroughly saturated. If you find yourself winter sowing during a spring that is especially dry and temperatures have been on a warm trend, it becomes very important to monitor moisture levels more frequently.
Why you should winter sow within windows
So after all this, you still might be asking, why not winter sow my seeds all at once?
The beauty of winter sowing is that as you leverage both nature’s way of growing and the greenhouse effect, you have a lot of flexibility as for when to begin winter sowing. For the most part, winter sowing is fairly forgiving. However, because we’re dealing with nature, while you can winter sow things in the “wrong” window and it turn out to be total success, it’s also true that if you winter sow things outside their ideal window it may turn out to be a flop—an avoidable one at that.
The reality is that when you take the approach of winter sowing things outside their ideal window, you will get varied and inconsistent results over the years. I’m at the point where intuitively, I know which seeds to start in their respective optimal window—I get to them when I have the time and energy, but do so within their most favorable windows so that I have the best chance at success each year.
I hope this guide has been helpful and I’d love to hear what questions you have in the comments below!
How does winter sowing work? Learn here!
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