Cool season crops are cold-hardy vegetables that thrive at lower temperatures. They are typically planted in very early spring for an early summer harvest. This includes things like lettuce, spinach, peas, carrots, radishes, beets, cabbage, and onions.
Such seeds are normally planted as soon as the soil can be worked. It is usually still cold and they can even get snow and frost. The seeds of cool-season crops don’t seem to have a problem dealing with these harsh conditions. I’ve been gardening for 50 years and have given classes on vegetable gardening and made many videos on the topic. I have always told people to plant their cool-season seed as soon as they can. That is the advice everyone gives. But ….. we may have been doing it wrong!
Key Takeaways
- All seed germinates better warm.
- Planting cool-season seed in cold soil does not make sense.
The Old Science is New
I was looking through some seed starting information and came across a list of vegetables and the best temperatures for germinating seed. I noticed almost all vegetables germinated best at about 85°C. I checked out some cool-season crops like peas and lettuce. The temperature they preferred was about the same as warm-season crops like tomatoes.
That made no sense at all based on common gardening advice. Why are we planting pea seeds in cold weather when they prefer a warm temperature for germination?
My first step was to check this data to see if it was right. Several US Extension offices report the same data. In fact, it was identical data, so they are clearly repeating it. I tried to track down the original study, with some success. I never found the original scientific paper online; however, the tables I did find were created by a JF Harrington who has published numerous papers on starting seeds. I even found his thesis, called Drying, Storage, and Packaging Seed to Maintain Germination and Vigor. I feel confident in accepting the germination/temperature data as published. The following table shows the time required (in days) for germination at various temperatures. Testing was done in soil and based on seeing a shoot above the soil line. The fastest germination in almost all cases was at 86°F (30°C).
Why Do We Plant Seeds in Cold Soil?
The data shows us that planting seeds in cold soil is a bad idea. They take far too long to germinate – so why do it? Our advice to plant in cold soil is based on the plant’s need to grow in cool weather. Many of these cool-season crops need to finish growth by early to mid-summer, because they can’t take the heat. But they also germinate better in warm conditions.
Rather than plant cold and grow cool we should be germinating warm and growing cool. But how do we do that?
One solution is to germinate indoors and plant out seedlings, but that is a whole lot more work, requires more space and equipment. Another option is to germinate warm and plant seeds that are just starting to germinate. This is easy to do with my baggy method.
Start seeds with the baggy method, in a warm room, and plant out the seeds once they germinate. It is a bit more work, and it is a bit harder planting wet seeds, but if you are only planting a small number, it is quite easy. The benefit of this new method is that you get seedlings growing much more quickly, as you’ll see in my test below.
Testing: Germinate Warm, Grow Cool
I decided to run an experiment to test two things.
- Is the data in the above table correct? Does cool-season seed really germinate so much slower in cold conditions?
- Will warm germination speed up early plant growth?
I took 40 pea and 40 beet seeds and split them into two groups. Group C (cold) was germinated cold, and group W (warm) was germinated warm. Both groups were put into baggies for germination, following my standard method (see above video). Group W was kept at room temperature, and group C was put in the fridge to mimic cold soil (7°C/45°F). They were started in early May in zone 5. This year we had a very cold, cloudy May, so everything was behind in the garden.
Seed Germination
Group W peas germinated in about 2 days. You can easily see the radicle develop under the seed coat. The beets took 3-4 days. They were then planted in a container outside. Group C peas took almost 3 weeks to germinate, and even after three weeks, the Group C beets were still not germinating.
Growing Seedlings
By May 31, Group C would not be showing above the soil level. Group W sprouted a couple of weeks earlier and are now good-sized seedlings.
Is The Data in The Table Correct?
My fridge was 7°C (45°F). Based on this temperature and the data in the table, peas should germinate somewhere between 13 and 36 days (this is not a linear range). Beets should germinate somewhere between 17 and 42 days. Keep in mind that since the data in the table depended on a shoot showing above the soil line, its “days to germination” are a bit longer than the real germination time you get by looking at seeds in a baggy. My germination times were in the ballpark of these numbers. There was definitely a big difference in germination time between the fridge and room temperature. Based on my results, I have no reason to doubt the data in the table, which I am sure are more accurate than my numbers.
A Better Way to Grow Cool Season Crops
This is not a very rigorous experiment, and should be redone with more types of seed, but both the germination data we have and this little demonstration convince me that we should be growing our cool-season crops differently. Germinate warm using the baggy method. Once germination occurs, plant outside. This new method will get plants growing a couple of weeks earlier, giving them a longer growth period early in the season when temperatures are still cool. Does this produce a larger crop? We don’t know that from the data presented, but since gardeners jump through hoops to get an early start for these crops, I would be surprised if it didn’t improve yield. This is especially true with leafy greens and peas that just can’t take the heat. Things like beets and carrots are not as heat-sensitive.
