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Cultural Practices to Beat the Heat
Hirabara Farm, Kamuela Kona (Big Island), Hawai'i
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.

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.

Shade Cloth & Reflective Mulch
At-Your-Service Aquaponics
Reflective Mulch & Shade Cloth Keeping Things Cool
Lihue, Kauai, Hawai'i
Photos by Lainie Kertesz, Johnny's Territory Sales Representative
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.

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 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.

Windbreak - Hawai'i
Crops in the Protective Zone of a Windbreak
Chris Robb's Farm, Kamuela Kona (Big Island), Hawai'i

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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.

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.

Irrigated Fields - Hawai'i
Transplanting Broccoli
Kekela Farm, Kamuela

Chard on Reflective Mulch
'Bright Lights' Chard
on Reflective Mulch
Hirabara Farm, Kamuela

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Transplanting

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.

Mulch

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:

  • 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.
  • View our Watering & Irrigation Supplies »

Harvest Strategies

    Irrigated Fields - Hawai'i
    Irrigated Greens in Field Production
    Kekela Farm, Kamuela
  • 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, crops 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, or placing it in an ice chest. The key is to remove any excess field heat from the crop.
  • Post Harvest Sun Is the Enemy. 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.

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.

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Johnny's
Germination Guides
Germination Guides for Optimal Temperature
Use our Germination Guides to schedule your hot-weather plantings.

For information about growing Lettuce and other cool-weather crops in warmer weather, see
Part 2. Heat-Tolerant Cool-Weather Crops »
View All Our
Heat-Tolerant Varieties
 »




Shaded Flowers at Market
Keep Flowers
from Wilting
Cut flowers are prone to wilting in summer, especially when on display at a farmer's market or farmstand on a hot day. And nobody wants to buy a bunch of wilted flowers.

To prevent premature wilting, follow good harvest and post-harvest procedures. Here are the basics »

•  Thoroughly clean your buckets and clippers. Bacteria that reside in dirty buckets proliferate quickly in summer heat, and will clog up the stems and shorten the flowers' vase life.
•  Pick flowers early in the morning or late in the evening. Don't pick at midday, when the sun is beating down on the flowers.
•  If it's really hot, fill buckets with cool water a little deeper than normal. The pressure of the deeper solution helps push water up the stem.
•  Work quickly, minimizing the time between cutting stems and placing them in water. 
•  Keep the buckets of harvested flowers in the shade until you can transport them to the packing shed.
•  If you are having problems with certain varieties wilting, try using a hydration solution in your picking bucket. A hydration solution is a commercial product that helps the cut stems take up water after they are cut. We offer FloraLife floral preservative/ hydrator. Follow the label instructions carefully. (NOTE: Certified-organic growers may not be able to use floral preservative products, even though they are used post-harvest. Check with your certifier if in doubt.)
•  Cool the flowers as soon as possible, either in an air-conditioned room or a 50°F/10°C cooler.
•  Don't mingle flowers with ethylene-producing fruits and vegetables. And be aware that some summer flowers will suffer chilling injury if placed in a cooler set below 40°F/4.4°C.
•  Before displaying flowers, be sure the water in the bucket is deep enough to keep the flowers hydrated all day.
•  Using light-colored buckets and placing the buckets up off hot, paved surfaces will help to keep the water cool. Adding frozen gel packs or containers of ice to your buckets will also help to keep the water cool.
•  Above all, keep the flowers out of direct sunlight at market. Keep in mind that sleeves act like little greenhouses in the sunlight, making flowers uncomfortably hot, so you may want to put flowers into sleeves after your customers purchase them.
Flower Grower's Library »


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