So let’s get this season started as soon as possible with cool-season crops. These are the plants adapted to grow best with cooler daytime temperatures, 65⁰ F-75⁰ F, compared to warm-season crops like tomato and chile pepper. They are planted in Maryland from early March through mid-May and again from July through September. Thinking about these crops in January gives us time to plan and prepare for success!
Some cool-season crops are hardy, very frost-tolerant, like pea, spinach, and onion. Others are semi-hardy, more easily injured by cold temperatures, like beet, carrot, and lettuce (although this depends a lot on cultivar, stage of development, and growing conditions). Cool-season crops germinate at lower soil temperatures (40⁰F-45⁰F) than warm-season crops like cucumber and squash (55⁰F-60⁰F).
Some cool-season crops need both cool and warm temperatures. Onion and garlic need cool temperatures for rapid leaf growth, and warm temperatures for bulb enlargement. Leafy greens grow well in spring and fall while broccoli and cauliflower tend to produce better yields and higher quality heads in fall. That’s because leaf and root growth is favored by warm temperatures and head development is best with cooler weather.
We’re mostly interested in eating the leaves, stems, and storage roots of cool-season crops. Cool temperatures cause these crops to produce more sugars which makes them more cold-tolerant and better tasting. Rising temperatures can reduce crop quality, causing bitter or off-flavors, and force plants to send up flower stalks and produce seeds (bolting). Increasing daylength may also induce bolting in lettuce, spinach, radish, potato, and carrot.
A few cool-season crops are perennials (rhubarb, horseradish, and asparagus) but most are grown as annuals, even though many are biennials.
Most commonly-grown cool-season annual crops by plant family:
- Bean family (Fabaceae)- garden pea, fava bean
- Cabbage family (Brassicaceae)- radish, broccoli, cabbage, Brussels sprouts, cauliflower, kale, mustard, turnip, collard, kohlrabi, Asian greens, Chinese cabbage, rutabaga, arugula
- Garlic family (Amaryllidaceae)- garlic, leek, onion, shallot,
- Beet family (Amaranthaceae)- spinach, beet, Swiss chard
- Carrot family (Apiaceae)- carrot, parsnip, celery, celeriac
- Lettuce family (Asteraceae)- lettuce, endive, escarole, radicchio
- Tomato family (Solanaceae)- potato
See HGIC’s Vegetable Crop profiles for specific planting and care information.
Tips for success:
Seeds or transplants– most gardeners plant cabbage, broccoli, cauliflower, and Brussels sprout transplants but sow seeds directly in the garden for all other cool-season crops. This year, try growing or buying transplants instead. Even peas can be started indoors under fluorescent or LED lights.
Growing lettuce, kale, pea, arugula, etc. for just 2-3 weeks indoors will give you a head start on the growing season and eliminate the need for thinning excess plants (tedious!). Transplants are also less likely than seeds to be washed away in a storm. Even carrot and beet can be transplanted as long as the tap root doesn’t hit the bottom of the container while growing indoors.
Soil preparation and nutrients– it’s possible to plant and grow warm-season crops, like tomato and squash, in cloddy, clayey soil. The same is not true for most cool-season crops. Whether you plant seeds or transplants the topsoil should be loose, well-aerated, and high in organic matter. Heavy, cloddy soil slows seed germination and restricts root growth.
Preparing a bed in spring requires minimal tillage- cutting winter annual weeds at the soil with a weeding tool, fluffing the soil with a garden fork, and raking the soil smooth. If a cover crop is in place, cut it to the ground with a string trimmer, cover the area with a tarp or weed barrier for 2-3 weeks, remove it and plant through the cover crop residues.
The majority of cool-season crops have a medium to high requirement for nutrients. Fertilize seedlings and transplants in spring with a soluble fertilizer to get them off to fast, strong start. Nutrient release from soil organic matter is low at this time and root systems are too small to adequately mine the soil for nutrients may be inadequate. Fertilize fall crops once they are established.
If the soil is not suitable, plant in containers filled with a mixture of compost and soilless growing media (lightweight “potting soil”).
Protection– climate change is giving us a longer fall season allowing big harvests of cool-season crops into December. But climate change is also making spring weather even more unpredictable than “normal.” The average last frost date is earlier but seedlings and transplants are subject to wide temperature swings, excessive rainfall, and late freezes (as we saw in 2020).
Floating row covers and cold frames provide a buffer against unfavorable and rapidly changing weather conditions. They allow us to extend the gardening season so that we can plant earlier, harvest latter, exclude insect pests, and increase yields. Protective covers, whether, glass, clear plastic, or polyester fabrics, increase the air temperature around plants and reduce damage from wind and driving rainfall.
Timing– planting calendars are helpful but planting decisions should also be based on the 7-day forecast. Transplants give you some added flexibility as they can be set out earlier or held back depending on conditions. The key is to have all the pieces in place- seeds or transplants, prepared soil, protection, and fertilizer- when conditions are right for planting. Some years you may find yourself planting with a headlamp or flashlight!
Future articles will focus on planting and caring for specific cool-season and warm-season crops, and how to extend the gardening season and adapt to climate change.
By Jon Traunfeld, Extension Specialist. Read more by Jon.