Part 2 • Getting Cool-Loving Crops through Summer's Heat
Beat the Heat
- Part 1: Growing practices that protect vegetables from heat stress
- Part 2: Helping cool-season crops through summer heat
Heat tolerance varies, by both crop & variety within crop
Plenty of us slow down in midsummer, and so do many vegetables. Crops also differ, as people do, in how sensitive they are to heat and humidity and at which stage heat hurts most. For some it's germination; for others it's flower bud formation, fruit set or another phase. Breeders working on heat-tolerant varieties for hotter summers need to understand these stages, and growers can use the same knowledge to plan their crop schedules.
Before sowing, check the germination guide for each crop to see whether the soil is cool enough. Surprisingly, many cool-season vegetables sprout at very high temperatures. Brassicas, for example, usually germinate fine in a very warm greenhouse without shade cloth. Cabbage and cauliflower can germinate at up to 100°F (37.8°C), carrots and onions at up to 95°F (35°C), and turnips at up to 105°F (40.6°C). Most varieties won't thrive if it stays that hot, though, because later growth stages are more heat-sensitive.
Varieties within a crop also differ in how well they handle heat. This article covers the basics of carrying cool-season crops through summer, focusing on varieties known to do better than others in warm conditions.
Recommendations for Cool-Weather Crops
Heat tolerance trialsA heat-tolerant designation is meant to flag varieties that clearly outperform others in hot conditions.
Seed trials in a range of locations, including very hot regions, help identify cool-season varieties that tolerate heat. Each crop is judged on its own heat-tolerance criteria, with flavor counting as much as performance and yield. Trials are timed to coincide with the stresses of summer: strong sun, heat, humidity and long days.
Broccoli
Before transplanting, lay white-on-black mulchwhite side up to help broccoli get through the shift from leafy growth to flowering (it also suppresses weeds, holds moisture and helps deter pests and diseases).
- Broccoli's most heat-sensitive window lasts only about 10 days, while the growing point switches from leafy growth to forming flower buds. This change can't be seen from outside; it happens about 10 days before a tiny crown shows in the center of the plant, roughly 3–4 weeks after transplanting. Several days above about 95°F (35°C) during this window can cause uneven, misshapen heads. Growers can use this to decide which broccoli varieties to plant in which slots for the best quality and yield.
- Heat-tolerant choices include 'Imperial', a standout, and 'Green Magic', another strong performer. For moderate heat, 'Gypsy'.
- also does well. In extreme heat or humidity, as in the Southeast, Deep South and Florida, though, none do well unless transplanted after the peak of summer heat.
- Browse our Heat-tolerant broccoli.
- For help timing broccoli plantings for warm conditions, see our broccoli harvest planner.
- For more on timing, temperature, fertility and watering for broccoli and other brassicas, see our article on growing brassicas well.
Brussels Sprouts, Cabbage & Chinese Cabbage
- Brussels sprouts, cabbage and Chinese cabbage all handle moderate summer heat reasonably well.
- Where heat or humidity is extreme, as in the Southeast, Deep South and Florida, these brassicas only do well if transplanted after the hottest part of summer.
Cauliflower
This variety comes from Asia, where green-stemmed, loose-curd cauliflower is a common crop in hot, humid regions.
- Once past germination, cauliflower hates heat.
- Where summers are moderately hot, white varieties that cope reasonably well include 'Snow Crown' and 'Amazing'. For colored cauliflower, try 'Cheddar', 'Graffiti', and 'Vitaverde'.
- In extreme heat or humidity, as in the Southeast, Deep South and Florida, none of these do well unless transplanted after peak summer heat.
- One type of cauliflower comes from Asia, where it's widely grown in hot, humid areas. 'Song TJS-65' is a sweet, early example. From above it looks like ordinary white cauliflower, but its curds are looser, a bit smaller and easy to prep, so it's sometimes called "loose-curd cauliflower." Flip it over and the stems are pale green rather than white, which is why others call it "green-stemmed" cauliflower. Curds may show a faint orange tint, which is normal and doesn't affect eating quality.
- For help timing cauliflower for warm conditions, see our cauliflower harvest planner.
- For more on timing, temperature, fertility and watering for cauliflower and other heading brassicas, see our article on growing brassicas well.
Greens
Many greens have some heat tolerance and do quite well as long as you follow sound growing practices and the recommended post-harvest cooling and storage methods.
A few more specific suggestions:
A heat-loving stand-in for spinach.
- Mustard greens that do well include the distinctively mild 'Mizuna' and the well-known 'Red Giant' and 'Red Splendor', all from Japan. 'Green Wave' is the slowest to bolt.
- For a komatsuna with good heat tolerance, try 'Carlton'.
- 'Shungiku', an edible chrysanthemum species with finely toothed, very aromatic leaves, is another heat-tolerant Asian green.
- Pac chois also like cool weather but do fairly well in warmth. Those that handle heat better than most include 'Win-Win Choi', 'Joi Choi', 'Mei Qing Choi', and 'Black Summer'. Compare maturity, color, size and spacing in our pac choi chart.
- Specialty greens handle summer heat well, except in the very hottest regions. 'Red Leaf Vegetable Amaranth', a much-loved Caribbean green, is tough and stands up to heat and some drought. For a spinach substitute in heat, try 'Red Malabar Spinach', which truly loves hot weather.
- Browse heat-tolerant greens.
Spinach
- In warm weather, spinach needs irrigation to germinate well, since watering keeps the soil cool. The advantages of irrigation covered in Part 1 apply especially here.
- Spinach prefers cool weather for germination and growth, but slower-growing varieties suit late-spring and summer sowings. Good summer choices include 'Seaside' and 'Lizard' . 'Space' also fits this slot. 'Flamingo Improved' works if you want arrow-shaped leaves; it's one of the slower-bolting Asian types, though still quick to bolt.
- For spinach varieties to plant in sequence, see our spinach harvest planner.
- Harvest spinach promptly, before it bolts, or stems turn stringy and flavor suffers.
Lettuce
- Use all the basic practices from Part 1, such as irrigation, shade cloth and sowing in cooler spots, for lettuce too.
- Most lettuces turn bitter in summer, but Summer Crisps, also called French crisp or Batavia, stay juicy and sweet. 'Fusion' is a top summer performer. 'Muir', a Batavian type, also handles heat very well. Other summer crisps include 'Cherokee' and 'Magenta'.
- Among other full-size head lettuces, these hold up better than most in heat: 'Tropicana', 'Panisse', 'Salvius', 'Adriana', and 'Skyphos'.
- For varieties to plant in sequence, see our head lettuce harvest planner.
- For bolt, tipburn and heat tolerance details, see our full-size lettuce chart.
- See all heat-tolerant lettuce.
Carrots
- Carrots can be grown all year, but summer heat makes them more bitter and less sweet. 'Romance' is a strong choice for midsummer heat.
- In hot, dry weather you can also grow short, quick-maturing types, such as the round Parisian market carrot 'Atlas'.
- Growing carrots in the shadier, cooler part of the farm is key to getting them through heat.
- Watering carrots well from sowing to harvest keeps soil cooler and improves flavor; well-watered roots are milder and less bitter.
- For more, see our four-part carrot growing guide.
Radishes
A Japanese-style daikon for mid- to late-summer harvest.
- Besides flavor, heat tolerance in radishes is judged by how long they stay crisp and how quickly they turn pithy in warm weather.
- For Round red radishesare also judged on how well they keep a neat, uniform round shape. 'Rover' and 'Sora' are good heat-tolerant conventional and organic picks, respectively. Most small round radishes struggle in extreme heat.
- For Specialty Types and Daikons, these can do well farther north: 'Shunkyo Semi-Long', 'Nero Tondo', 'KN-Bravo', 'Alpine', and 'Summer Cross No. 3'. See our specialty and daikon radish planner for more.
- All radishes taste hotter and harsher when grown in heat than in cool weather, specialty and daikon types most of all.
- Browse our Heat-tolerant radishes.
Choosing varieties for heat tolerance—One of a set of variables in the summer-success equation
Even the most heat-tolerant cool-season varieties have limits. In the Deep South, none of the crops covered here are grown in midsummer, except maybe summer carrots, and lettuce always does better in cool weather. Still, people want fresh salads and slaws all summer, so it pays to have varieties that hold up through tough summer conditions. The varieties suggested here are still affected by intense heat, but they generally do better than others.
Succession planting is also key to summer success, including sowing varieties to plant in summer for fall harvest. Succession planting is usually thought of as season extension, but here it's about working with the seasonal temperature cycle: taking advantage of some crops' high germination temperatures and timing harvest for cooler weather. Brassicas, carrots and turnips are great candidates for summer sowing. For more, see the articles and calculator in our succession planting overview.
Wherever you farm, having a plan in place before hot, humid weather peaks will help limit heat damage. Choose varieties known for heat tolerance, time plantings well, follow good hot-weather growing practices, and cool and store summer crops properly, and keep track of what works best in which conditions, and you'll get the most from your cool-season crops in warm and hot weather.
