Part 1: Cultural Best Practices to Prevent Heat Stress in Vegetable Crops
Hawai'i photos courtesy of a retired seed sales representative
Beat the Heat
- Part 1: Growing practices that protect vegetables from heat stress
- Part 2: Helping cool-season crops through summer heat
"Is it hot enough for you?"
"Hot enough for you?" You've probably heard it plenty during the dog days of summer. Like most growers, you likely worry less about yourself (you can get out of the sun!) than about your crops. Most vegetables, even so-called heat lovers, struggle when temperatures climb into the 90s or above and stay there. This two-part series on coping with heat can help you plan now for hot days to come.
Part 1 covers basic growing strategies for helping crops through summer heat, an increasing challenge for many growers. In Part 2, we look at what cool-weather crops need as temperatures rise, compared with naturally heat-loving crops, though each group has its own variations, and suggest cool-season varieties that hold up best in heat.
Part 1 • Production & Harvest Practices to Offset Heat
For growing lettuce, brassicas and other cool-season crops in warm weather, see: Part 2: Heat-tolerant cool-season crops
For handling summer harvests after picking, see: Cooling methods and storage conditions chart
Whatever you grow and wherever you grow it, certain growing, harvest and post-harvest practices give the best results in high heat. These basics help limit the damage heat does to yield, flavor, appearance and marketability.
Germination
Every crop has a known minimum, maximum and ideal temperature range for germination, as shown in the germination guides on product pages. You've probably used the minimum temperature to time spring sowings as the soil warms. Watching the maximum temperature as well helps make sure the soil is cool enough for summer sowings.
To cool the soil, shade, water and mulch beds for several days before planting. When starting seeds under cover in a greenhouse or other structure, use the coolest spot available, such as a shaded northeast corner, for summer sowings.
Shading
In hot regions worldwide, growers raise vegetables and cut flowers under shade cloth to cut heat and light intensity, which improves quality and yields. Use it for a crop's whole life or just at an especially sensitive stage as a heat shield.
Shade cloth is a durable woven or knitted fabric that comes in densities from about 12% to 90%, the percentage of light it blocks; 47% shade cloth, for instance, blocks 47% of the light. In strong heat, most vegetables do best under 30–50% shade. Cloth denser than 50% is mostly used for shade plants or as a windbreak.
To work well, shade cloth has to sit high enough above the crop for air to move and keep heat from building up underneath. That's easy in a greenhouse, hoophouse or purpose-built shade house like those at nurseries. Low tunnels built with hoop benders are a low-cost option: drape shade cloth over the hoops and leave the sides open for airflow. Low tunnels go up and come down quickly as conditions and crops change.
With good ventilation, shade can also bring temperatures closer to ideal for starting seeds in summer and, as noted, shelter crops from drying winds.
Windbreaks
Whatever the temperature, wind dries plants out and can push tender crops beyond the point where they can recover from wilting.
Planting on the sheltered side of a natural, planted or built windbreak works much like shade, reducing water loss without cutting the light crops receive.
Transplanting
When you can, transplant during the cool hours, early morning or late in the day. During heat waves, skip midday planting entirely.
Plants usually handle afternoon transplanting fine if they're well watered in. Keep water coming throughout the process to cool young plants and slow their water loss.
On hot days, cloudy weather or a light rain is a good time to transplant, but heavy rain can damage tender seedlings and compact the soil.
Mulch
Planting into heat- or light-reflective mulch or ground cover is another good way to keep things cooler.
White-on-black mulch is a two-sided film laid white side up to keep soil cool and black side down to block weeds. Reflective woven ground cover reflects heat while keeping weeds down underneath.
Straw and other organic mulches also hold moisture and cool the soil a little.
Irrigation as Cooling
Keeping newly planted beds evenly moist cools the soil in two ways:
- Cool water lowers soil temperature directly.
- Extra moisture during hot spells (but not in humid or foggy weather) increases evapotranspiration, which pulls heat up and out of the soil and creates a cooler microclimate.
Ways to keep moisture even while saving water:
- Drip tape put down before or at sowing or planting.
- A soil moisture sensor, so you can adjust watering promptly.
- See watering and irrigation supplies.
Harvest Strategies
- Pick at the right time. Harvest happens when it has to, but as with transplanting, cooler parts of the day are best, especially during heat waves.
- Cool produce right away. Most crops picked in hot weather should ideally be rinsed in cool or cold water right after harvest to remove field heat. Alternatively, get produce into a cooler quickly, pack it in an ice chest, or top-ice it. What matters is removing field heat fast.
- Keep harvested produce out of the sun. In hot weather, get picked crops into the shade as fast as possible; the longer they sit in sun or wind, the more they wilt and dry out, and soon they won't be sellable.
- For more, see our cooling methods and storage conditions chart.
Connecting With Your Weather-Decision Support System
Most of these tips may seem like common sense, but thinking ahead helps you make a better plan and put it into actionbefore hot weather arrives.
More broadly, connect with others in your region, such as your extension office, neighboring growers and your state climatologist. Sharing local data on temperature, rainfall, heat warnings, cloud cover, evapotranspiration, soil moisture and pest forecasts helps pinpoint what farmers need and improves weather and crop forecasting tools.
