Cool-season crops are vegetables that perform best and produce the highest quality harvest in mild weather. This group includes leafy greens such as lettuce and spinach, root vegetables like carrots and beets, and brassicas, which are members of the cabbage family. These crops are best grown in the spring and fall, avoiding the summer heat that causes them to fail, bolt prematurely, or become bitter.
Defining the Ideal Air Temperature Range for Growth
The most vigorous vegetative growth for most cool-season crops occurs when the average air temperature falls between 60°F and 75°F (15°C and 24°C). Temperatures within this range support a steady rate of photosynthesis while minimizing the stress that triggers the plant’s reproductive cycle. An optimal average daily temperature for sustained growth is often cited around 70°F (21°C).
Maintaining a slightly cooler nighttime temperature is an advantage for these plants. Lower night temperatures reduce the rate of cellular respiration, the process where the plant consumes its stored energy reserves. By slowing respiration, the plant conserves sugars produced during the day for leaf, stem, and root growth, resulting in a sweeter, more tender harvest. Conversely, temperatures consistently above 80°F (27°C) often cause leafy vegetables to prematurely bolt and accumulate bitter-tasting compounds.
Critical Soil Temperature Requirements for Planting
Successful planting begins with the soil, as seeds and young roots require a specific thermal environment for germination and establishment. Cool-season vegetable seeds are adapted to sprout in cold conditions, with many varieties germinating at a minimum soil temperature of 40°F (4°C). The ideal temperature range for rapid and consistent germination is higher, typically between 50°F and 60°F (10°C and 15.5°C).
A soil thermometer is a useful tool for confirming that the planting area is ready, as air temperature can be misleadingly warm. Planting seeds when the soil is too cold and saturated can be detrimental, leading to imbibitional chilling injury. This occurs when seeds rapidly absorb frigid water, damaging internal tissues and making them susceptible to fungal diseases. Cold soils also restrict the plant’s ability to access certain nutrients, like phosphorus and zinc, hindering strong early root development.
Coping with Temperature Extremes and Stress
When air temperatures exceed the optimal range, cool-season crops experience heat stress, which manifests in several ways. Prolonged exposure to temperatures above 85°F (30°C) signals the plant to cease vegetative growth and begin the reproductive phase, leading to bolting in crops like lettuce and bitterness in greens. For brassicas like broccoli and cauliflower, excessive heat can result in poor quality, such as small, discolored, or unevenly formed heads.
To mitigate heat stress, focus on cooling the plant and the root zone. Applying a thick layer of organic mulch, such as straw or pine needles, helps reduce soil temperature and conserves moisture. Temporary shading with a 30% to 50% shade cloth can lower the ambient temperature and prevent leaf scorching during the hottest part of the day. For cold stress, cool-season crops tolerate a light frost down to 32°F (0°C), but a hard freeze below 28°F (-2°C) requires protection. Row covers, cloches, or cold frames are effective methods for trapping ground heat, insulating the plants, and extending the growing season.
Classifying Cool Season Crops by Cold Hardiness
Cool-season crops are categorized based on their tolerance to freezing temperatures, which guides planting times and the need for protection. The “Hardy” classification includes vegetables that can withstand a heavy frost, surviving temperatures below 28°F (-2°C) for short periods. This group contains most brassicas, along with spinach and onions:
- Kale
- Collards
- Brussels sprouts
- Cabbage
- Spinach
- Onions
The “Semi-Hardy” crops tolerate a light frost, typically surviving air temperatures between 28°F and 32°F (-2°C and 0°C). These plants require protection if a hard freeze is anticipated. Examples of semi-hardy vegetables include:
- Carrots
- Beets
- Parsnips
- Swiss chard
- Leaf lettuce
For root crops in this group, the tops may die back, but the underground portion is protected by the soil and remains viable.