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Heat affects landscape plants differently

When you walk outside this time of the year, the heat and humidity are almost unbearable. At times you can hardly breathe. Imagine you are a plant in your landscape. You can’t just go inside and cool off. Instead, you have to stand there and take the heat day after day, night after night.

It is at this time of the year that we can see clearly which plants are well-adapted to our area and which won’t make it. Other than tropical plants, which we all know have problems with freezing temperatures, it is often the late-summer heat rather than the cold of winter that takes its toll here. If daytime highs in the 90s weren’t bad enough (and they certainly are a problem), it is the nighttime temperatures in the mid- to upper 70s that also give plants a particularly hard time.

Although plants don’t actually sleep, cooler night temperatures allow their metabolisms to slow down so they can kind of catch their breath, so to speak. When night temperatures stay very warm, plants’ metabolic rate tends to stay high, burning energy and using up food the plant has created. Tropical plants are well adapted to this situation, and for them it’s not a problem. But plants from cooler climates rely on the cooler nights. When nights stay warm, these plants become weak because they use up their food too quickly. The food I’m referring to is the food created by the plant for itself through photosynthesis, not fertilizer.

Add to this situation high humidity and frequent rain showers, and you have the ideal conditions for weakened, stressed-out plants to be attacked by a variety of insects and diseases – particularly crown and root rots that are so often fatal. These intense environmental and pest pressures mean that only those plants that are well-adapted to our summer conditions stand a good chance of surviving and thriving here.

Many attractive and useful plants that are considered reliable and even easy to grow in other parts of the country will not thrive here. When we choose hardy trees, shrubs, ground covers, lawn grasses and perennials for our landscapes, we must primarily keep in mind the temperatures they will be subjected to during our summers.

It doesn’t impress me in the least to read that a plant is hardy down to minus 10 degrees. It doesn’t get that cold here. Show me something that tolerates five months of hot, humid days and sultry nights. If they can’t take the summer heat, they are questionable for use here.

August is a good time to walk around the landscape with a critical eye to how things are going. Even tried and true plants that are reliable here may not look their best this time of the year, so don’t be too critical. In particular, look at any new or unusual plants you’re trying out.

Unfortunately, even well-adapted plants may do poorly this time of year if they are newly planted shrubs and trees going through their first summer in the ground. Watering properly is critical. Overwatering can be fatal. It is better to slightly under-water, waiting for the plants to express slight drought stress before irrigating, than to water too much and too often.

Keeping the soil too moist, along with the heat, encourages attack by fungal root rot organisms. If this happens, common symptoms include plants that appear wilted even though the soil is moist or sections of a plant that wilt and die. These root diseases are destructive and often fatal, and we don’t have effective controls. Be careful not to water too generously. Plants can recover from slight drought stress, but they can be damaged or killed if kept too moist.

Plant selection, then, is extremely important to a successful garden or landscape. However, in the information age of magazines, books, television and the Internet, you will be exposed both to plants that will do well here and to those that won’t. How do you know which is which?

Finding good plants primarily by trial and error is both frustrating and expensive. Make sure you check with local sources, such as gardening books for our state, staff at local nurseries and friends knowledgeable about gardening here. Your parish LSU AgCenter Extension office is an extremely valuable source for locally appropriate gardening information and diagnoses of plant problems. Check out the LSU AgCenter website.

When selecting books, look for titles that have the word South, Southern or Louisiana in them. Even South in the title is no guarantee when you realize what a large geographic area the South is. “The Southern Living Garden Book” (Oxmoor House) takes this into account, and that makes it very useful. Each plant is rated on how suitable it is for the Upper South, Middle South, Lower South, Coastal South or Tropical South. Other books to help you select well-adapted plants for our area include “Southern Plants” by Odenwald and Turner (Claitor’s Publishing), “Louisiana Gardener’s Guide” by Dan Gill and Joe White (Cool Springs Press) and “Month-by-Month Gardening in Louisiana” by Dan Gill (Cool Springs Press).

As I mentioned earlier, one group of plants in our gardens that loves this hot, steamy time of year is the tropicals. It would be hard to imagine gardening without gingers, palms, elephant ears, cannas, bird-of-paradise, angel’s trumpet and so many others. However, we do not live in the tropics, and serious freezes can occur here, particularly in north Louisiana. You should always keep in mind the limitations of using tropicals in locations where hardy plants would be more appropriate. If a permanent or long-term planting is needed, such as a shade tree, hedge or ground cover, for instance, stick with reliably hardy plants.

During summer we retreat into our air-conditioned homes and mostly enjoy our gardens by looking out the windows. But plants in the landscape can’t do this. Fortunately, there are lots of well-adapted plants that will take the heat and provide us with beautiful landscapes throughout the summer.


Dan Gill is a horticulturist with the LSU AgCenter. He can be reached at DGill@agcenter.lsu.edu

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