Hot weather effects on the landscape

The longer temperatures remain in the 100s with low humidity and drying winds, greater injury is possible to plants. This is especially true for un-adapted, non-native ones and those further stressed by poor maintenance.

Hot, dry winds cause much moisture loss. Some evaporation from foliage is expected in the desert, but if moisture is lost faster than the plant’s ability to replace it, leaves will begin to dry and wilt. Drought-hardy plants have a root structure able to absorb moisture from the soil quicker than evaporation takes place.

Proper watering year round which creates deep root systems is key to summer hardiness. A general recommendation is to water long enough each time to allow moisture to penetrate completely through the root area, but no more often than necessary to prevent wilt.

Older leaves will show hot air injury by drying and browning of tips and edges. Next, new tip growth will wilt, eventually dying back. Tender foliage can turn black with rapid moisture loss.

Evaporation cools the leaves, but they must have enough water from the roots to provide this effect. Otherwise, newer growth wilts and older leaves burn.

Soil heat buildup harms plant growth, especially for those in containers or with shallow roots. Persistent afternoon wilting followed by foliage burn is evidence of heat exposure and hot soils. Deep roots can reach more moderate soil temperatures and moisture, so to help create this environment, mulch the soil surface around plants. Mulching helps stabilize soil temperatures, in containers or in the ground.

There are some gardeners who believe the sun’s radiation has become more intense over years. Arizona has mostly bright, cloudless summer days that can overwhelm sensitive plants. Also added to this problem is reflected heat from walls. Too much intense sunlight can cause a yellow-white “burn” on the upper surface of older leaves and eventual stunted plant growth.

It is important to always install a new plant in the same direction it was formerly facing. Both the skin and foliage have become accustomed to sun exposure and are less likely to burn on that side. Reputable growers mark their plants or containers on the south side, so they can be planted in the same orientation. Even indoor plants will burn if exposed to direct sunlight.

Another summer plant problem is the buildup of naturally occurring soil salts. These dissolve as water is added to the soil, and some get into the plant with the water taken up by its roots. However, most salt remains in the soil accumulating on ridges and around plants, showing as white deposits.

By late summer, soil salt injuries become apparent as persistent wilting, yellowing, then browning at tips or between veins of older leaves. This can be combated by deep, penetrating irrigations that wash the salt deposits downward out of the root area. Because of its increased salt content, drainage water from evaporative coolers and swimming pools should not be used for irrigating plants.

Application of fertilizer during hot weather should be done with caution, if at all. Fertilizing in autumn helps plants recover from summertime exhaustion. Spring fertilization encourages vigorous growth to prepare for summer stress.

The best way to get hot weather plant vigor with minimum care is to use plants that are adapted to the desert climate. The reality is that some plants simply don’t do well in our area. This becomes most obvious during winter cold spells or extreme summer heat. Plants suffering the torments of a harsh summer are not a pleasing sight. Native or desert-adapted plants are by far the most practical choices for our arid climate.

Mary Kidnocker is a Master Gardener who writes frequently about the subject from a local perspective. Her columns are featured each Sunday.

2010 Garden Tour

Got a favorite Green Valley garden? The Green Valley Gardeners are starting the process of selecting special gardens for the April 10 Garden Tour 2010 and would like your help. Large or small, trimmed or natural, anywhere in the area. If you have a suggestion, please call Co-Chairman Leslie Campbell, 396-4466.