THE COASTAL GARDENER

08.04.06

 

Endless summer heat wilts plants, gardener

By Ron Vanderhoff

It seems like a long time ago, but you may recall that March and April were unusually cool. Average temperatures were four degrees below normal. March's average daily temperatures were the coldest of the year; colder than December, January or February.

How soon things change. In a reversal of spring, summer has been unusually hot. June temperatures were well above normal. July temperatures were among the hottest on record. As I write this, the mercury at John Wayne Airport hit 98 degrees, a record. The onshore breezes that usually moderate the temperatures of our coastal gardens have been especially weak this summer. Our gardens are showing the stress of a hot summer.

As a gardener, you feel the urge to do something, anything, to respond to the strain many of your plants may be showing this summer. But knowing how to respond requires knowledge and an understanding of what is really going on, at least from the perspective of the plants in your garden.

After a long, especially hot day at work this week, I got home at about 6:30 p.m. to see many of the plants in my garden in various stages of decline. The leaves of my alstroemeria, fuchsias, hydrangea and impatiens were limp. So were several collectable plants, such as Ligularia "Britt-Marie Crawford", Mallotus japonicus, Parochetus communis, Ruellia graecizans and Brugmansia "Super Nova."

These plants were all suffering from heat stress. Even with moist soil the day was just too hot for them. These plants evolved from climates with cooler summer temperatures and higher humidity than my garden is offering. Water, although a critical requirement for these plants, wasn't the issue today. More water wouldn't have made much of a difference.

More than 99.9% of all the water used by a plant is absorbed through its roots and ultimately exits through its leaves, as vapor. Less than 0.1% of the water absorbed is used to create plant tissue. The movement of water into a plant's roots and out of the plant through its leaves is called evapotranspiration.

Water that has moved through a plant becomes atmospheric moisture, eventually accumulating as rainfall. It falls to the soil again, is absorbed by the roots of other plants, moves up the plants and out through their leaves and return to the atmosphere. It is a never-ending process and is the foundation of life on Earth.

If water moves through a garden plant's leaves at a rate faster than the plant can absorb water through its roots, the plant will wilt. During a very hot day, a day with especially low humidity, or a dry, windy day, many plants will wilt even though there is water available to the roots. That's because evapotranspiration is very rapid on such days.

As I continued tallying the day's damage to my garden, I mentally sorted out another group of unhappy plants. My elephant ears (Colocasia), plumeria, bougainvillea and hibiscus, at a glance, were just as distressed as those mentioned earlier. But the cause was not heat. These tropical plants all evolved with temperatures routinely in the 90s. Oddly, even though they were stressed, they actually enjoyed today's temperatures. These heat-loving plants were just dry and needed a drink. I was at fault, not the weather. As long as they didn't wilt too far, a good soaking and they would be fine.

Keep your garden well irrigated during summer's long hot days. But be aware that even with adequate irrigation, there may be plants in your garden that will stress and it's not water that will save these. For plants that are accustomed to a cooler summer, anything you can do to bring the temperature down a bit will help. Thick organic mulch over the roots is a good place to begin. We should all be mulching regularly anyway, for many reasons. A couple of midday sprinklings with a garden hose will help immensely. Likewise, some quick blasts, for only a minute or two, from a sprinkler system will bring the temperatures down as much as ten degrees and uses very little water.

So, as a gardener there may be something that you can do to provide some summer relief to those sad plants during our hot summer. Knowing whether they are too hot or too thirsty will give you some insight on how to respond.