At first glance, it seems obvious that warm-weather crops prefer to be planted and to grow in warm weather; likewise for cold-weather crops.
This is one of the most misunderstood concepts in gardening. Understanding the life cycle of plants in nature (without the intervention of gardeners) helps us know a couple of things: when to plant and when to grow. These are not always the same. For example, kale is an extremely cold-hardy plant and will survive the freezing weather of Yakima winters, down to zero degrees, without the protection of a greenhouse or cold frame. (It helps to plant in a sheltered area out of the wind.)
The most cold-hardy plants are ones we could think of as human teenagers and young adults, not small children or senior citizens. So you want kale to be mostly grown plants but not at the end of their life cycle. In nature, kale survives the winter and blooms in spring. The seeds fall on the ground in summer when the weather is very warm. That is when the seeds germinate and baby plants take off in the warm weather, gradually maturing and becoming more cold-hardy as the fall weather arrives. So kale is a warm-weather germinator and a cold-weather crop.
The big surprise for most people is that kale and squash both like to germinate at 85 degrees; but unlike kale, squash also needs to grow at warm temperatures. In nature, squash germinates and grows in warm weather. It takes all winter for the squash fruits to decompose enough for the seeds to sprout in late spring, growing all summer and starting the whole cycle again, which makes squash a true warm-weather crop.
Lettuce has the opposite life cycle. In nature, lettuce plants bolt and set flowers in late summer. The seed heads don’t drop the seeds on the soil until autumn. They overwinter in the cold winter soil. In the spring, as the soil warms from cold to cool, those seeds sprout. Lettuce likes to germinate and grow in cool (not cold) weather. Lettuce seed will germinate in temperatures from 45 degrees to 75 degrees with an optimal germination temperature of 68 degrees. This is almost 20 degrees colder than kale likes for germination.
You can take advantage of lettuce seeds’ natural life cycle patterns by planting a main crop of lettuce in the early spring while the soil is cool. For a second crop, in late summer, you can fool lettuce into thinking spring has arrived by putting the seeds in the refrigerator for a couple of weeks. Then plant them outside, when the nights are cooling and the days are getting shorter; the seeds will sprout and continue to grow as the weather cools down in the fall.
Day length can also affect germination and when a plant will “bolt” (begin the process of producing blooms). Many plants bolt when the days start to get long. Most mustards, Asian greens and spinach bolt when the days start to lengthen, even if the weather is still cool. You need to plant them very early, before the long days of June arrive. Once the days begin getting shorter in the fall, you can plant again.
For a fall crop of any plants that prefer to grow in cool weather, make sure the plants have reached the stage of young adult plants before really cold or freezing weather arrives.
The following list shows the temperature range where most common vegetable garden plants germinate. A very general rule (that has many exceptions) is that root crops and brassicas germinate in warm weather and grow in cool weather. Plants that have fruits or pods usually germinate in warm weather and grow in warm weather.
A good book on seed saving will explain the natural growing cycle of plants and will help you understand when to plant and when to grow.
Don’t make the common mistake of planting everything on the last frost date — somewhere between the 10th and 20th of May. Some things could have been planted a month earlier and others will need another two or three weeks before the soil is warm enough to grow well.
- Cool soil germination (45-70 degrees): Arugula, bok choy, cilantro, lettuce, mache, milkweeds, mustard, peas and sweet peas, poppies, radishes and spinach.
- Moderate soil germination (70-80 degrees): Beets, broccoli, cauliflower, carrots, chard, chicory, nasturtiums and radicchio.
- Warm soil germination (75-85 degrees): Basil, beans, beets, corn, eggplant, kale, marigolds, melons, okra, peppers, sunflowers, squash, tomatoes, turnips, watermelons, zinnias and zucchini.
We hope this information will help you”work smarter, not harder” in your vegetable garden.