Photos by Lainie Kertesz, Johnny's Territory Sales Representative
Getting Your Crops Through the Summer Heat
"Is it hot enough for you?"
You've probably 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 and stay there. This 2-part series on heat-tolerance basics can help you put a plan in place now for hotter days ahead.
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.
Part 1. Production, Harvest, & Post-Harvest Practices to Offset Heat
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.
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.
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 a 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™ Bender 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.
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 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 ideal, though heavy rain can damage tender seedlings and increase soil compaction.
Another effective way to reduce heat is to transplant onto heat-reflective or light-reflective mulch.
White-on-Black Mulch is a two-sided product you can apply white-side-up to keep the soil cool, and black-side-down to suppress weeds. Metallic Silver Mulch repels insects, cools the soil, and reflects light.
Other mulching options include straw or other organic matter to help retain moisture and lower soil temperature slightly.
Irrigation as Cooling
Keeping freshly-planted beds evenly moist works to decrease soil temperature in two ways:
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.