Keeping Crops Cool During Hot Weather: 13 Ways to Beat the Heat

Tips on keeping crops cool during hot weather. Hot summer sun can be hard on garden plants and their roots. If watering and mulching are not doing enough to keep your plants cool enough to continue producing, consider shading the plants or at least their roots. Late summer also is the time to plant your fall-producing crops, such as spinach, lettuce, broccoli, carrots, beets and radishes.
By Barbara Pleasant
August/September 2008
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Learn about keeping crops cool during hot weather. Some plants stop flowering and setting fruit when temperatures soar. To keep them cool, use old screens, lattice and sheets to shade vulnerable mature plants from the effects of mid-summer sun. Cover new seedlings with opaque covers to protect them from the sun until their roots are established.
Illustration by Elayne Sears
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Keeping Crops Cool During Hot Weather

If you want to keep your garden productive well into fall, then late summer is a busy  season that must be embraced. Hot temperatures are as rough on plants as on the gardeners who grow them, but here’s a roundup of techniques you can use to help you and your crops cope. And the sweat you invest when you seed beets or transplant broccoli will be richly rewarded in a few short weeks. The late summer planting list is a long one with plenty of choices — chard, spinach, lettuce, kale, broccoli, kohlrabi, carrots, beets, radishes and turnips. Now is the time to give cool-season vegetables a second run in your garden.

Sizzlin’ Summer: Preventing Heat Stress in Plants

At this point you may be thinking it’s all you can do to nurse tomatoes and other pet crops through a mean summer. We know your pain! But even if you keep the soil evenly moist by using lots of soakers or drip irrigation hoses covered with a thick mulch, many vegetables will abort their blossoms rather than set fruit in extreme heat. Temperatures in the 90s cause many beans to hold back flowers, and tomatoes, peppers and eggplants start having trouble completing the pollination process when temperatures rise above only 86 degrees Fahrenheit.

Commercial growers often use sprinkling to cool plants down. A late afternoon sprinkling may be an excellent idea, but strategically placed shading devices are often more practical, more water efficient and much less time-consuming. For example, you can easily cool down the sunny sides of tomatoes by installing a short run of snow fencing or pre-assembled sections of picket fence along the south or west side of the row.

Shade covers made from lightweight cloth (such as old sheets) also will help keep struggling plants cool, though they must be held several inches above the plants to keep them from retaining heat. When using a cloth-type shade cover over plants, tie or staple the corners to wood stakes. You can probably buy shade cloth at your garden center, or a 6-by-12-foot piece that blocks 50 percent of light is available for $17.50 from Lee Valley Tools. Scavenge before you buy. Old window screens make good shade covers, too, as do narrow panels of wood lattice. If plants are so tall that installing a shade cover over them is impractical, simply situate a sun screen alongside them to shield their bases from afternoon sun.

Controlled shade also may help you grow lettuce and other leafy greens despite 90-degree heat. In a 2001 study conducted on two Kansas City-area organic farms and at the Kansas State Research and Extension Center in Olathe, high tunnels covered with 40 percent shade cloth doubled the survival of transplanted lettuce seedlings and kept the plants from developing bitter flavors. You can learn more about high tunnel culture, hosted by Kansas State University.

Whether you use shade screens or not, be sure to remove weeds to reduce competition for limited moisture. Ripening fruits demand huge loads of water and nutrients, so pick thoroughly and often to make it easier for the plants to keep up with the business of staying alive.

When to Plant for Fall Harvesting

Many of the best crops of fall — including salad greens and various cabbage cousins — will refuse to germinate in soil warmer than 85 degrees, so the seeds are best started indoors (on a 90-degree day, surface soil temperatures may actually be 110 degrees or more). The procedure for growing seedlings is the same as in spring (see Seed Starting Basics), but care for transplanted seedlings requires some extra hardware. Insect pressure in late summer is severe, so use homemade or purchased lightweight row covers to exclude critters. (See The No-Spray Way to Protect Plants) It helps to shade each seedling with a light-colored flowerpot, pail or other cover for two to three days after transplanting. Protected from scorching sun, the seedlings can concentrate on growing reliable roots.

To determine your optimum dates for sowing fall crops, add three weeks to the “days to maturity” ratings given on the seed pack to compensate for days that are getting shorter. Then count back from your first frost to find the date you should sow each crop. (To find your average first fall frost date, search for “frost dates” in the box above.) Cool-season plants will continue to grow a little after nights turn chilly, but they will make most of their new growth early on, while the weather is still warm.

Cabbage and broccoli seedlings need to be planted at least eight weeks before the first fall frost, and as long as their roots are kept moist, they seem to benefit from a spell of hot weather as late summer days become shorter. When set out so late that they miss the last warm spells, cabbage cousins tend to stop growing too soon. Whether you’re working with seedlings you grew yourself or bought at a store, get them in the ground as soon as you can.

Improving Seed Germination

You may be able to direct-seed fast-growing kohlrabi and kale only six weeks before your first frost arrives. Leafy greens, such as arugula, spinach, lettuce and Asian mustards, are easy and fast to grow from seeds sown right in the garden. Root crops, such as carrots, beets, radishes and turnips, grow best from seeds tucked into moist garden soil. As the ground cools in September, lettuce can be put back onto the planting list too.

When choosing sites for small, late summer plantings, make use of partial shade from neighboring plants that will be sent to the compost pile soon. After the new crop is up and growing, you can remove the “screen” plants to open the site to stronger sun.

To help seeds germinate fast, soak them in water overnight, which will coax them out of dormancy. If you can’t plant your plumped-up seeds right after a drenching rain, thoroughly water the bed or row the day before putting them in the ground.

To further insulate seeds from the drying effects of strong sun, it helps to plant the seeds slightly deeper than you would if planting them in spring. One trick that works every time is to plant seeds in a slightly recessed furrow, water well, and then cover the seeded furrow with a board for three to four days. When sowing in wide beds, shade the seeds with an old blanket or a piece of wood lattice held aloft over the bed with bricks for several days.

The repeated watering needed to get seeds up and growing sometimes causes a crust to form over germinating seeds, slowing or blocking their timely emergence. A light sprinkling of grass clippings will help prevent crusting, or you can cover seeds with potting soil or screened compost to keep the surface crust-free.


How to Keep Cool During Heat Waves

When summer sizzles, it’s best to limit your outdoor work to the cool of the morning or late in the evening, but sometimes working in the sun is the only way to get things done. Wear a wide-brimmed hat to shade your face, and cool down your neck and wrists with loosely tied wet bandanas. A tie-on neck cooler will stay wet longer if you sew it up as a tube, fill it with a tablespoon of the same water-absorbing polymer crystals used to improve the moisture retention of potting soils (also used in diapers) and then sew up the ends. Or, thoroughly dampen clean dish or hand towels, shape them into rounded collars on cookie sheets, and pop them in the freezer. A frozen collar draped around your neck will cool you down for more than an hour.

Even with these tricks, remember that it’s unwise to do hard outdoor work when both the temperature and the humidity are high. When the two numbers added together equal more than 160, stay indoors during the middle of the day.


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