The deeper the water, the better for the roots

By Angela O'Callaghan

Wednesday, April 1, 2009 | 2:49 p.m.

Although horticulture specialists may feel like broken records when they say this, it remains an important rule for irrigating plants: When you water anything, water it so that the whole root zone receives irrigation. An essential part of the Water Smart program is Water Deep.

Roots cannot grow through dry soil. They will follow the path of water.

When water is applied shallowly and frequently, it remains in the top few inches of soil and then evaporates. In order to obtain the water they need, plant roots will grow in that same top few inches. This is the usual way to irrigate turf. For trees and shrubs, shallow watering can lead to a number of problems, particularly as we experience the windy season followed by the high heat of summer. With plants like vegetables and flowering annuals, this is less of an issue since their root systems generally do not grow very deeply. In either case, however, deeper is better.

Since the top few inches of soil are those most exposed to the dry air, roots in that area will be most likely to become dry quickly. In extreme conditions, they will desiccate and become useless to the plants that rely on them for survival. There are so many reasons why deep watering can benefit plants.

When trees and other woody plants are only shallowly irrigated, they are less able to withstand the pressure exerted by the fierce winds of the Mojave. These plants can actually be toppled when their root systems are located primarily in the top of the soil.

If roots can only mine a shallow area, they will lack more than water. Fertile soil is considerably more important to plant survival than fertilizers. The soil is the primary way that plants obtain essential nutrients. As roots mine the soil for these elements, the greater an area they can explore, the more access they will have to this nutrition.

We seldom think of the other life that exists in the soil, and what a major role these microorganisms play in the life of plants. We all know about legumes, those plants that form associations with certain bacteria for the purpose of converting inert nitrogen into a form that plants can be taken up by plants to use for building proteins and other compounds. It turns out that many other bacteria and bacteria-like organisms will also fix nitrogen, making it available for plants growing nearby. Certain fungi are also essential for plant survival, since they link with roots to create a massive root-like system enabling plants to grow larger and stronger.

When only a small shallow area is moist, however, there is only a small area can house these significant creatures. Plants can survive as long as conditions are mild, but they will not thrive, and they will certainly suffer under extremes of temperature, dryness and wind.

In other words, when you water, water deep.

Angela O’Callaghan is the area specialist in social horticulture for the University of Nevada Cooperative Extension. She can be reached c/o the Home News, 2360 Corporate Circle, Third Floor, Henderson, NV 89074, or [email protected].