Treat drought-, heat-stressed plants carefully

Richard Bogren, Owings, Allen D.  |  8/28/2015 9:37:02 PM

Azaleas frequently showed summertime stress and were more prone to problems this year due to high rainfall this spring. (Photo by Allen Owings, LSU AgCenter)

Centipede grass lawns were damaged by heat and drought from early July through August. (Photo by Allen Owings, LSU AgCenter)

Indian hawthorn that was pruned too often shows more heat and drought damage than plants less regularly maintained. (Photo by Allen Owings, LSU AgCenter)

News Release Distributed 08/28/15

By Allen Owings
LSU AgCenter horticulturist

HAMMOND, La. – What a rough July and August in Louisiana for hot temperatures and droughty conditions. The LSU AgCenter Hammond Research Station in Hammond had 43 straight days of temperatures 95 degrees or above. During that time we measured only 0.60 inches of rainfall. Similar high temperature durations and low rainfall totals were recorded across the state.

As a result, most landscape plants have suffered heat-stress damage. This damage may or may not be noticeable. Heat damage occurs gradually over a longer period of time than cold damage occurs. Heat stress over an extended period – anywhere from a month or so to a couple of growing seasons – can cause significant damage. Some of the stress indicators include withering flowers or flower buds, drooping leaves, loss of green foliage color (bluish-gray color), diminished root growth and increased attractiveness to insects.

Ornamental plants were more prone to heat and drought stress this summer due to the cooler, cloudy days accompanied by above-average rainfall in the spring.

Environmental factors other than heat may affect our plants, but we can minimize these problems and maximize success. Humidity, water availability, oxygen exchange, light quality and quantity, day length, wind movement, soil conditions and available nutrients all play a role in success with your garden plants not only through summer but also at other times of year. This will be very important as we go into the fall season with landscape plants that may not be looking good.

Water availability – or limiting water stress – goes a long way in eliminating plant stress. Maintaining optimal soil moisture levels is critical. And it’s hard to do without proper irrigation management. Most landscape plants need about 1 inch of irrigation or rainfall weekly during early spring through late fall. It is best for irrigation or rainfall to come once or twice weekly rather than every day.

The most common symptoms of a plant needing water are wilting or pale gray-green foliage. Unfortunately, these symptoms are also common in plants that become highly stressed as a result of too much water. This can be quite confusing and frustrating to gardeners when trying to determine if they are watering too much or not enough.

When irrigating, water should be applied slowly and uniformly and preferably around the base of the plants. Watering at the base rather than from overhead helps eliminate water accumulation on the foliage, which in turn reduces opportunities for leaf diseases to occur. Watering at the base also maximizes water use efficiency while reducing runoff.

Handheld hoses, drip irrigation systems, soaker hoses and micro-sprinklers are excellent techniques for watering landscape plantings and usually consume about 30 percent less water than overhead sprinkler systems. Most garden centers have do-it-yourself irrigation kits that readily attach to water faucets.

Mulching landscape plants also conserves soil moisture and lessens soil temperature fluctuations in the root zone.

It is always best to water plants in late evening, at night or in early morning. This maximizes the amount of water made available to the plant compared with watering at midday when 30 to 40 percent of the irrigation water can be lost to evaporation.

We have had many inquiries about what to do with plants damaged by drought and heat. It is best to take a “wait-and-see” approach.

Many trees will be losing their foliage prematurely this fall due to drought stress. Shrubs, such as gardenias, azaleas, Indian hawthorn and loropetalums, are showing discolored foliage and wilting. Sasanquas and camellias that were not irrigated during our driest six weeks this summer are behind on setting flower buds for the upcoming fall and winter bloom season.

Plants that are over-pruned or otherwise over-maintained in the landscape are probably showing more damage because they have had less foliage available to maximize plant photosynthesis this summer. And many lawns across the state have considerable loss of turf cover due to the drought and heat.

As we move into fall, do not prune plants in a way that will stimulate new growth but only selectively thin out dead branches. Be sure plants are well irrigated without being over-watered this fall. September, October and November are typically are some of the drier months of the year, and most gardeners do not irrigate enough in the fall.

On damaged lawns, raise the mowing height and avoid nitrogen fertilizer. Be aware that damaged lawns will be more prone to weed infestation this fall and next spring.

You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals at both sites.

Rick Bogren

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