For water conservation and plant health, the smartest xeric strategy is to water the roots of the plant and avoid watering the plant’s leaves.
Let’s look at the water savings first: Water evaporates when exposed to air, and occurs at the water surface area. The smaller a drop of water, the higher the percentage of the drop’s surface area. Add the effect of wind on tiny drops of water from sprinklers and you might as well just pour that water down the drain. And if you irrigate a plant from above or with sprinklers and spray emitters, much of the water lands on the leaves, where it can evaporate. In fact, water constantly evaporates from a plant’s leaves as it is, in a process called transpiration. It’s a plant’s natural way of cooling off on hot summer days.
Feed a plant’s roots
It’s much better for a plant to take new water in through the plant’s roots, where the water picks up soil nutrients and works its way up the trunk and stems back to the leaves to do its cooling and feeding work. There’s another reason not to spray water on plant leaves, especially late in the day or during cloudy, humid weather: wet leaves can harm many plants.
Fungal diseases such as powdery mildew on roses or apple scab on apple trees and crabapples is partly a result of water on the leaves. Sometimes, there is nothing a gardener can do. We have an old apple tree with some scab that likely came from spores in old leaves left on the ground and a week or more of cool, cloudy and humid weather in late spring.
Change how you irrigate
Often, simply changing irrigation practices can improve a plant’s health. When we first moved to a home in Albuquerque many years ago, the previous owner had installed sprayheads in all of the flowerbeds. We eventually had to replace all of them with bubblers and rework the plantings. Bubblers or drip irrigation might have required a little more planning, better leveling of the soil and more parts or emitters. But in the end, the homeowner would have saved money on his water bill and I bet on plants! Farmers know this is the way to go, and many are learning new ways to improve irrigation techniques to reduce water use.
We recently visited a nursery in search of tomato cages and noted that the tomato plants they still had in stock looked awful, even though they had a few large fruits on the plants. I thought at first it was too shaded in the greenhouse, but then Tim noticed the cause: an overhead spray watering system. The leaves were spotted and nearly goldish-brown in color. I don’t know how they are healthy enough to continue feeding the plants. Granted, these plants have been in the greenhouse way past the typical time, and there are many more than you would have to deal with in your garden. Still, it seems to me they would sell more tomato plants if they watered differently.
And in case I haven’t convinced you, here’s yet another reason to water with drip emitters or by hand near the roots of a plant instead of broadcast or spray irrigation: weeds. When you spray water, you water everything around, including weed seeds. Watering only around your vegetables’ or ornamentals’ roots confines weed growth, making it easier to pick small weeds out by hand.
As I said before, you can’t control rain, which obviously comes from overhead. But keeping plants exposed to proper sun, trimmed and cleared to give them sun and airflow and cleaning out debris from around the bottom of trees and plants can help reduce risk of fungal diseases. Choose mulches carefully, depending on local recommendations for a given plant.