As spring is almost upon us in the northern hemisphere, most home gardeners in dry Mediterranean climates will be looking at their lawns and wondering if they should be turning on the sprinklers. The question is, is it necessary or desirable to irrigate the lawn at the first hint of warm weather?
Most lawns grown in hot, dry summer climates are perennial grass species that develop deep and extensive root systems by means of underground stems known as rhizomes. Common examples are the Bermuda grasses, Zoysia varieties, Kikuyu grass and Paspalum. They are able to take up water at depths well beyond the topsoil layer. Furthermore, their underground perennial organs make it possible for them to withstand drought to a considerable degree, often recovering splendidly from a brief period of neglect.
The preferred irrigation regime for these grass types is one based on deep, but relatively well-spaced watering, as this induces the roots to grow down into the subsoil. Other than encouraging the lawn to be more drought tolerant, there are numerous benefits to such a regime, such as increased hardiness to pests and disease, and preventing salts to accumulate in the soil.
In early spring therefore, providing a significant rain has occurred within 3 weeks or so, and assuming that the soil depth is over say 75cm, it is best not to water at first, but to wait for a period of time to elapse before doing so. The question is of course: how long should one wait?
For established lawns, it is possible to delay watering until the grass displays the first signs of stress, typified by a change of color and a loss of leaf turgidity. In fact, it is possible to save more water by delaying irrigating for a few further days, and deducting those extra days from the calculation that determines the amount of water that is to be applied. It is safer though for home gardeners not to do this and to open up the sprinklers, when the grass starts to yellow.
It must be made clear, that perennial grass types like St Augustine, (Stenotaphrum) that grow by means of stolons as opposed to rhizomes, develop shallower root systems and so are less suited for this type of schedule. Furthermore, conclusions should not be drawn from this and applied to other groups of garden plants. Most flowers, perennial or annual, struggle to recover after the wilting point has been reached, and so water is applied in order to pre-empt stress. With regard to woody plants as well, both trees and shrubs, the initial signs of water stress, often indicate a level of internal damage, from which the plant may never satisfactorily recover.