When It’s Too Darned Hot: Tasty Soil, Tender Mulch, and Just Enough Water

Have you felt yourself wilting, lately?

A day in the nineties or triple digits, makes me glad I’m a mobile creature. I can retreat to the shade or the relative cool of an interior. But what happens to plants when the temperatures soar?

Plants, like people, differ in their responses to heat.

As you can imagine, plants which evolved in steamy tropical rain forests and searing deserts survive the heat without much apparent fuss. It’s the plants from temperate zones which stress and cause the gardener to stress even more.

Like afternoon naps for the gardener, one of plants’ heat survival strategies is dormancy. A great example is the natural response of bluegrass varieties. When the heat turns up, bluegrass turns brown and goes to sleep. The water we pour on it through the hottest months isn’t so much to keep it alive, but to keep it cool. As long as irrigation water is evaporating and cooling the lawn, it stays relatively green. According to University of Colorado Extension, a healthy bluegrass lawn can get by on twice weekly irrigation and can survive in dormancy for weeks or even months without irrigation. Once cooler temperatures return, begin regular watering, and the green will follow.

Many flowering annuals take a break in the heat. While they aren’t totally dormant, they save their energy for the opening and closing of stomata, a kind of pore on the undersides of leaves. Perennials that normally bloom in summer’s heat, such as lilies, may wrap up their season in a hurry. When a plant wants to rest from the heat, it’s important to refrain from fertilizing. This is especially true of roses.

The Golden Jubilee Agastache pictured above is in a serious heat wilt. It is also more sensitive to soil moisture than many of the plants around it. I call it a signal plant. When it droops, I watch it as the evening cools. If it perks up, soil moisture is not yet an issue. If it doesn’t, you’ll see a sprinkler out in the morning.

During a heatwave, the natural impulse of many a gardener is to turn on the tap. While it’s important to keep garden soils from drying completely, we often water more frequently than necessary. In sandy, porous soils, three times a week might suffice. In clay-based soils, twice a week could do it. The trick is to irrigate for longer periods during extreme heat, making sure moisture penetrates to a depth of 6-8 inches.

To muddy the waters a bit: All watering rules are relative. How quickly do the temperatures rise in the morning? How far did they go down overnight? What is the relative humidity? Has it been windy? Any of these factors can change your watering regimen.

Knowing when to water is told by the soil. Put a blade in to a depth of 3-4 inches and make an opening. Feel the soil at the bottom of the opening. If it is cool and damp, all is more or less well. If it is dry, it’s time to water. Relying on plants to tell you when to water can be misleading. While it’s true that plants droop when they need a drink, they also droop in heat. They can also droop from drowning.

The same principles apply to the vegetable garden. According to University of Colorado Extension, a properly prepared veg garden should be able to tolerate intervals of 2-7 days between watering.

These watering intervals might seem counterintuitive at best, preposterous at worse, but the key is to follow other practices as well.

The foundation to any lawn or garden survival through these conditions is living soil. Soils that are properly amended save on water and support stronger, more resistant plants.

In addition, the use of mulch is crucial, even in the veg garden. The goal should be no exposed soil surface. Even half an inch of humus, leaves, straw, or fine gravel will shade the soil, helping to keep it cool and reduce evaporation.

Timing and delivery methods of watering are also important. Most experts agree: water in the morning and deliver the water at the soil’s surface, under the mulch, if possible. This helps mitigate fungal and disease issues as well as conserve water.

Keeping up with watering is sometimes the only thing a hose-dragger like me can accomplish during the hottest weeks. The comfort is knowing that as the day’s grow shorter, longer nights and cooler overnight temperatures give both gardens and gardeners a much needed respite.

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