It’s been almost five decades since I had a college level plant physiology class and studied the process of how plants move water through their vascular systems and how they adapt to drought conditions so they can survive the dry summers. After some research, I am happy to say that the science hasn’t changed much, although I did encounter some new and strange words like xylem embolism and cavitation, which I don’t remember from a half century ago.
Typically, plants absorb water almost exclusively through their roots. The water is translocated up through the xylem tissue to the uppermost tips of the branches and ultimately released through openings on the underneath sides of the leaves called stomata. The primary function of the stomata is to breathe in carbon dioxide and expel water, and only in some rare situations does water actually enter through the stomata. There is also evidence that plants take in water through their leaf cuticles, which explains why foliar feeding of plants is an effective way to fertilize them. I am furthermore inclined to believe that plants use the morning dew as a source of water to help carry them through the dry season.
To make a long story short, plants that are considered to be “drought tolerant” have evolved over the years so that they are very effective at absorbing water and ultimately hanging on to it. They tend to have thicker, smaller leaves, with dull leaf surfaces. They can fold or roll up their leaves to conserve moisture and have formed symbiotic relationships with soil micro-organisms that enhance their ability to absorb water exponentially. As gardeners, when we see a plant labeled “drought tolerant” it should tell us that ONCE ESTABLISHED, that plant will tolerate more abuse and neglect, but it is not a license to completely ignore it for the whole summer.
We tend to think of native plants as being drought tolerant and in their natural environment where they grew from seed and matured in the native soils. This is true, although they can still die from lack of moisture. In the home landscape where the soils have little resemblance to our forest soils and the plants were raised in a nursery in a gallon container, I have to say that native plants are no more drought tolerant than imported ones. While native plants have other positive attributes, don’t be deluded into thinking that you can plant and forget them. It won’t have a happy ending.
We can minimize the trauma of drought by doing some supplemental watering in July, August and September using drip or soaker hose applicators once or twice a month. Applying a one to two inch layer of mulch goes a long way to conserving moisture, as does simply forming a watering basin around individual plants to focus the water. In drought conditions, keeping the fertilizer to a minimum and sticking to organic slow release forms can help as well. Arranging our plants into wet and dry zones makes good sense both horticulturally and aesthetically speaking. In desperate situations, there is even a product called Wilt Proof that can be sprayed onto plants to reduce water loss.
The term “drought tolerant” was never meant to be a free pass to ignore our landscapes. If we manage our soil, water, and fertilizer and pick the right plant for the right place, there should be absolutely no reason not to have a beautiful and healthy landscape.