During February, tree and shrub irrigation requirements often get overlooked. Many gardeners tend to think these types of plants do not need water during winter or early spring, because of lack of warm temperatures, or because they are dormant. However, the root systems still need moisture to sustain themselves. The question then becomes, how often should the plant be watered. Irrigation needs are driven by several factors, rainfall, temperature, and plant metabolism.
During winter, plants are dormant, or their metabolism is slowed significantly; therefore, irrigation can be less frequent. Established evergreen shrubs and trees continue to lose moisture through their foliage and may need irrigation monthly if there is inadequate rainfall.
However, as temperatures increase, as in late spring, summer, and fall, established plants will inevitably use more water. Therefore, irrigation needs to be more frequent (once per week with 1 inch of irrigation water) if there is inadequate rainfall. Newly planted shrubs and trees may need to be watered more frequently. Always mulch plants to conserve soil moisture. For more information, see HGIC1604, Mulch.
Care should be taken during any time of the year to thoroughly water the roots deeply and give time between irrigations for soil drying to prevent the chance of root rot diseases. These deep, infrequent irrigations also promote stronger, deeper root systems.