What should be done in the garden in August? “As little as possible.”

The period from late June to mid-September is the most stressful time of the year for gardeners and their plants. Indeed, our hot summer season essentially defines what trees, shrubs, ground covers and perennials we can grow successfully in our landscapes.

When you walk outside this time of 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’s enough to make you wonder how anything survives.

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, often the late summer heat rather than the cold of winter takes its toll here. If daytime highs in the mid- to upper 90s weren’t bad enough — and they certainly are a problem — it’s 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, and they can kind of catch their breath, so to speak. When night temperatures stay very warm, plants’ metabolism rates tend to stay high, burning energy and using up food they have created. Tropical plants are well-adapted to this situation, and for them the weather isn’t a problem. But plants from cooler climates rely on cooler nights. And if nights stay warm, these plants become weak because they use up their food too fast. The food I’m referring to is the food the plant creates 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 often fatal. This intense environmental and pest pressure means that only those plants that are well-adapted to our summer conditions stand a good chance of surviving and thriving in Louisiana.

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

This month also is a good time to walk around your lawn and garden 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. But in particular, look at any new or unusual plants you’re trying out. Which has collapsed? Which looks scorched and unhappy?

Plant selection, then, is very important to a successful garden or landscape. But in the information age of magazines, books, television and the Internet, you will be exposed both to plants that will do well here and those that won’t. How do you know which is which? As much as gardeners like to try new things, finding good plants primarily by trial and error is both frustrating and expensive. Make sure you check with local sources such as gardening books and magazines for our state, staff at local nurseries and friends knowledgeable about gardening here. Your parish LSU AgCenter office is an especially valuable source for local information and gardening pamphlets. Also, check out our website at www.lsuagcenter.com.

When selecting books, look for those that have the word South, Southern or Louisiana in their title. Even South in the title is no guarantee when you realize what a large geographic area the South is. “The Southern Living Garden Book” from 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; “Gardening in the Humid South” by Ed O’Rourke and Leon Standifer, LSU Press; “The New Orleans Garden” by Charlotte Seidenberg, University Press of Mississippi; “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.

When it’s this hot, sometimes the best thing to do is relax inside where it is air conditioned and read a good gardening book, look out the windows and enjoy the view, and dream about cooler weather.

Dan Gill is a LSU AgCenter Horticulturist.