Why is my kale so slow and sickly? I thought it was supposed to be so cold-hardy! And why did my spinach and mustard just make a stalk of flowers last year with hardly any leaves?
The answers to these questions have to do with the natural life cycle of plants in the wild. Once you know what the plant does naturally, you can give it the conditions that will make the best crop, and you can plant at the proper time.
Kale is ultra-hardy, surviving freezes down to 0 degrees--when it is an adult plant. In nature, kale seeds fall on the ground in early summer. They sprout in warm summer soil, and go into winter as adult plants. They need to be very cold-hardy to live through the winter and make seeds the next spring. But they don't need to be cold-hardy as babies--they sprout after the soil has warmed up. The optimum temperature for kale seed to germinate is actually the same as for squash--86 degrees.
Lettuce has the opposite life cycle. Wild lettuce seeds fall on the soil in the autumn and overwinter as bare seeds in the soil. When the soil warms just a bit in spring, those seeds sprout. The lettuce plant grows in wet cool soil, which provides lots of water for fast growth and juicy leaves. As summer heat comes on, the plant shoots up to make a seed stalk, producing ripe seed before fall. The lettuce plant doesn't live through the winter; mature lettuce plants are less cold-hardy than spring baby lettuce. Lettuce seed has an optimum germination temperature of 68 degrees--almost 20 degrees colder than kale.
You can take advantage of seeds' natural patterns by planting a main crop of lettuce while the soil is cool. Lettuce seed is programmed to go dormant in hot weather, and is hard to sprout. In fall, you can fool lettuce by putting the seed in the fridge, to make it think it is spring. In the same way, you can sprout kale indoors to make it think it is summer.
An additional factor can be day length. Lettuce bolts--stretches into a seed stalk--when the weather gets hot. But many plants bolt when the days start to get long. Most mustards and Asian greens are this way, and so is spinach. You need to plant them very early, before the long days of May and June make them want to bolt. Once the days are getting shorter--in late summer to fall--you can plant them again.
Cool-soil germinators (45-70 degrees): Lettuce, Peas, Mizuna, Tatsoi, Bekana, Mustard, Bok Choy, Miners' Lettuce, Minutina, Mache, Arugula, Cilantro, Calendula, Poppies, Turnips, Radishes, Spinach, Calendula, Sweet Peas, Milkweeds
To see our guide to growing lettuce in hot weather, click here.
In-between(70-80 degrees): Broccoli, Onions, Carrots, Beets, Chard, Nasturtiums, Chickory, Radicchio
Warm-soil germinators(75-90 degrees): Beans, Corn, Kale, Cabbage, Collards, Chinese Cabbage, Peppers, Eggplant, Tomatoes, Squash, Brussels Sprouts, Cauliflower, Melons, Watermelons, Marigolds, Zinnias, Basil, Sunflowers
This work by Bountiful Gardens is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.