Part 1: Cultural Best Practices to Prevent Heat Stress in Vegetable Crops

"Is it hot enough for you?"

You've likely heard this greeting more than once during the dog days of summer. If you're like most growers, your main concern is less for yourself (you can get out of the heat!) than for your crops. Most vegetables—even the ones we think of as "heat-loving"—don't do well when temperatures soar into the 90s or higher and stay put. This 2-part series on heat-tolerance basics can help you put a plan in place now for hotter days ahead.

Here in Part 1, we cover basic production strategies for getting your crops through summer's heat—a mounting challenge for many growers. In Part 2, we address the needs of cool-weather crops as temperatures climb, as distinct from inherently heat-loving crops—though there are differences among both. We also provide recommendations for cool-weather varieties that perform best in the heat.

Johnny's Germination Guides

Use our Germination Guides to schedule your hot-weather plantings. The guides appear on each of our vegetable product pages.

For information about growing lettuce, brassicas, and other cool-weather crops in warm weather, see: Part 2. Heat-Tolerant Cool-Weather Crops

For details on post-harvest handling of summer-harvested crops, see: Chart of Cooling Methods & Storage Conditions

No matter which crops you grow or where you grow them, there are production, harvest, and post-harvest handling practices that allow for an optimal outcome when temperatures soar. These are basic practices that help to diminish the detrimental consequences of too much heat on crop yield, flavor, appearance, and marketability.

Germination

For every crop there is a well-established minimum, maximum, and optimum range for germination, as depicted by the Germination Guides we include on product pages. You've probably referred to the minimum temperatures to schedule plantings in spring, when the soil is just warming up. By keeping an eye on the maximum germination temperatures, you can be sure your soil is cool enough for hot-weather plantings as well.

Soil temperature-reducing measures include shading, irrigating, and mulching the beds for several days before you plant. If you're seeding under cover, in a greenhouse or other structure, use the coolest available location. Here at Johnny's, we use the northeast corner of the greenhouse to get our summer plantings up and growing.

Aquaponics set-up in Lihue, Kauai, Hawai'i

Shade cloth laid over hoops, with the sides rolled up, helps keeps things cool.

Shading

In hot climates around the world, vegetables and cut flowers are grown under shade cloth to reduce heat and light intensity, resulting in better quality and higher yields. It can be used throughout a crop's lifecycle or at a particularly vulnerable stage to provide a heat-protective barrier.

Shade cloth is made of a weather-resistant, woven or knitted fabric in densities ranging between 12% and 90%. The density listed designates the percentage of light blocked by the cloth; for example, a 47% shade cloth blocks 47% of the light. In high heat, most vegetables should be grown under 30% to 50% shade. Shade cloth with density of greater than 50% is generally used for shade-loving plants or as a windbreak.

The key to success with shade cloth is to hang it high enough over the plants, to provide enough ventilation to prevent heat from building up beneath it. This is easily accomplished in a greenhouse, hoophouse, or other specially-built shade structure, such as those seen at nurseries. Low tunnels made with Johnny's Quick Hoops™ Benders offer an inexpensive alternative. Shade cloth can be laid over the Quick Hoops, with the sides left uncovered for maximum ventilation. Low tunnels are easily assembled and disassembled, to accommodate changing conditions and crop needs.

Together with proper ventilation, shading can also help reduce temperatures to more optimal levels for seed-starting in the summer and, as mentioned, provide protection from drying winds.

Chris Robb's Organic Farm • Kamuela Kona (Big Island), Hawai'i

Crops growing in the protective zone of a natural windbreak.

Windbreaks

Regardless of temperature, wind has a drying effect, and can push tender crops past their permanent wilting point.

Planting in the lee of a natural, planted, or other physical windbreak serves a similar function to shade, decreasing evapotranspiration, without decreasing the light the crops receive.

Transplanting

Kekela Farms • Kamuela Kona, Hawai'i

Transplanting broccoli into an irrigated field.

Transplanting in the cool part of the day, where possible, is best, either early in the morning or at the end of the day. In very hot spells, avoid midday planting altogether.

With adequate watering-in, plants usually fare okay when transplanted in the afternoon. Growers should provide plenty of water throughout the transplanting process, however, to help keep the young plants cool and reduce their rate of transpiration.

On hot days, transplanting during overcast conditions or during light rain events is also preferable, but doing so during heavy rain can damage tender seedlings and increase soil compaction.

Mulch

Hirabara Farms • Kamuela Kona, Hawai'i

'Bright Lights' Swiss chard on reflective mulch.

Another effective way to reduce heat is to transplant onto heat-reflective or light-reflective mulch or ground cover.

White-on-Black Mulch is a two-sided film you can apply white-side-up to keep the soil cool and black-side-down to suppress weeds. Reflective White/Black Ground Cover is a woven fabric that reflects heat while suppressing weed growth beneath.

Other mulching options include straw or other types of organic matter to help retain moisture and lower soil temperature slightly.

Irrigation as Cooling

Use a moisture meter to test soil water content.

Keeping freshly-planted beds evenly moist works to decrease soil temperature in two ways:

• The cool water itself reduces soil temperature on contact.
• Increasing plant bed moisture during warm spells (though not in muggy or foggy conditions) helps increase plant evapotranspiration, which in turn creates a microclimate by drawing heat up and out of the ground.

Measures that help ensure even soil moisture as well as water conservation include:

• Drip irrigation tape, laid in advance or concurrently with sowing or planting.
• Use of a moisture sensor to assess soil moisture, allowing for adjustments to be made in a timely fashion.
• View our Watering & Irrigation Supplies.

Harvest Strategies

Kekela Farms • Kamuela Kona, Hawai'i

Irrigated greens in field production.

• Time your harvest. Harvesting has to be done when it has to be done. Just as with transplanting, however, it is best done during the cooler times of the day, especially during hot spells.
• Immediately cool your harvest. Ideally, most crops harvested in warm weather should be immediately washed upon harvest—in either cool or cold water—to remove field heat. Other options include harvesting and promptly bringing the produce to a cooler, placing it in an ice chest, or top-icing. The key is to remove any excess field heat from the crop.
• Post-harvest sun is the enemy. In warm weather, move harvested crops out of direct sunlight as soon as possible—the longer they sit in the sun or wind, the more they will wilt, dry out, and before long, become unmarketable.
• For more details, refer to our Chart of Cooling Methods & Storage Conditions.

Connecting With Your Weather-Decision Support System

While most of the recommendations we've laid out above will seem intuitive, a reminder of what lies ahead can help you lay a better plan and put that plan in place—before the heat hits.

On a higher level we advocate for connecting with other stakeholders in your region, such as your local cooperative extension agency, growers in your area, and your state climatologist. By working together, sharing site-specific temperature, precipitation, heat stress warnings, cloud-cover/sunshine, evapotranspiration, soil moisture, and pest forecasting, you can help to better identify farmer needs and further the development of weather data and crop forecasting tools.

• Part 2 • Getting Your Cool-Weather Crops through Summer's Heat • Article
• Cooling & Storing Your Summer-Harvested Vegetables • Charticle
• Heat-Tolerant Varieties • Product List
• Lettuce & Greens for Southern Growers • Webinar Resources
• Summer-Plant for Fall-Harvest Varieties • Product List