Q&A: Heat and Drought Tolerance

Q. Do plants "feel" the heat index temperature?

A. Strictly speaking, no, plants can't "feel" the heat index temperature. The term heat index that meteorologists use in their nightly weather reports refers to what the combination of heat and humidity feel like on the human body. Conversely, plants don't feel the combination of the wind and cold, known as the wind chill factor either.

Of course, plants do respond to extreme temperatures, and in the case of extreme heat, wilt is the most obvious sign. But wilt isn't always a sign of a lack of water. In fact, too little or too much water can cause plants to wilt, as can a number of different pest and disease problems. For many gardeners, the most frustrating situation occurs when the soil appears to be plenty moist, but the plant, especially if it is relatively young, wilts.

The cause of such a curious wilting situation is actually fairly straightforward. Due to the considerable surface area of the leaves, the plant transpires, or loses moisture, faster than the plant's roots can take it up. And in the case of container plants, too small a container can aggravate the situation. In time, landscape plants will develop a root system large enough to maintain the moisture level within the plant. As for container plants, a larger pot or a shadier location should help to solve the problem.

Q. What is an urban heat island?

A. Urban heat island is another meteorological term that refers to places with lots of paved surfaces and relatively few plants, such as shopping malls, industrial parks and many downtown areas. And in places like that, the air temperature can be as much as 8, 10 or 12 degrees higher than in rural areas. But smaller versions of the same incidence can occur on patios, which is why some container plants need to be watered everyday during the summer months. It's also why many so-called drought-tolerant plants wilt.

Q. What does drought-tolerant really mean?

A. The term drought-tolerant is used to describe plants that will grow well with little, if any, supplemental water. However, what is often lacking in that description is one key phrase, namely, "once established." I'm not aware of a single landscape plant that you can simply stick in the ground and expect to grow without water, especially during that first critical year.

It's during that first critical year of growth that plants become established, meaning they develop a healthy root system and become acclimated to the soil, the weather, basically the whole environment. Some plants take longer than others to become established. So make sure you water new plantings routinely, as often as everyday or as little as once a week, depending on the plant requirements, until it seems capable of supporting itself with only the occasional watering or from rainfall alone.

Q. What are some of the most heat-tolerant perennials?

Figure A

A. Thankfully there are lots of them, and the following plants are a few of my favorites that are readily available, establish themselves quickly and grow magnificently throughout much of the country. Topping the list are the ornamental grasses (figure A). Once established, ornamental grasses are among the most drought-tolerant and carefree plants on the planet. Once more, they're beautiful in almost any setting throughout much of the year.

Figure B

Daylilies are another sure thing. Because they bloom for months, you get a lot of bang for your buck (figure B).

Figure C

Despite its dainty appearance, coreopsis is tough as nails and can really take the heat (figure C).

Figure D

The same can be said for coneflower (Echinacea) (figure D).

Figure E

A black-eyed Susan (Rudbeckia) can tolerate heat too (figure E).

Figure F

In fact, you could create a terrific drought-tolerant garden with just those five plants. Of course you could also add iris (figure F).

Figure G

Yarrow (figure G) and a number of evergreens, such as junipers, spruces and pines are great additions as well.

Figure H

Finally, there are the obvious choices, cacti (figure H), which make outstanding additions to the landscape or look great in containers, too.

Figure I

Succulents also look great in containers (figure I).

Q. My nursery wouldn't sell me a Japanese maple in July. Why not?

A. In many areas of the country, in particular the southern and south-central states, most planting comes to an abrupt halt during the hot summer months because it is extremely difficult to establish plants under the conditions of extreme heat. In other words, the nursery did you a favor because chances are the Japanese maple wouldn't have survived anyway. Wait until the fall or even the following spring before planting your Japanese maple, and the odds of it surviving will increase dramatically.

Q. During the heat of summer, isn't it better to water landscape plants every day?

A. Watering daily during the summer is absolutely not a good idea, because even in the middle of summer, you can overwater plants. Even if you water lightly, the water won't penetrate very far which means roots will hover near the surface of the soil. It's important to water once or twice a week in a manner that it percolates deep within the subsoil. By deep-soaking, you encourage roots to likewise venture deeper into the subsoil in search of water. And as a result, those plants are far better equipped to handle drought conditions.

Q. How can plants make the environment cooler?

A. Plants make the environment cooler, thanks to a process called evapotranspiration. Take an oak tree, for instance. As it secretes, or transpires, moisture through its leaves, this water draws heat as it evaporates, thus cooling the air in the process. Incredibly, a large oak tree can easily release up to 40 gallons of water in a single day. When combined with the shade it casts, trees can make a huge difference in energy costs when planted on the south and west sides of a house.