Marin is in the midst of a drought. Our county's Mediterranean hot, dry and often windy summers can cause significant damage to trees and shrubs unless steps are taken to minimize the effect of these conditions.
The cost to trees this year is especially high as our typical summer weather follows an especially cold, low rainfall winter that brought several spells of debilitating frost. Frost disrupts the movement of fluid within trees. Ice crystals rupture cell walls making water unavailable to plant tissues. Drought reduces the amount of water available in the soil that trees can draw up to replace daytime water loss. In both conditions, damage to trees occurs when more water is lost through transpiration than the plant can extract overnight from water-depleted soil.
Symptoms of drought- or frost-stress can be sudden or can take a year or two to appear. Symptoms of drought include wilting, off-color sparse canopies, undersized leaves, leaf scorch, small, poorly formed buds, yellowing, and premature fall coloration. Prolonged drought causes branch, root death and dieback ultimately resulting in the death of the tree. Frost damaged trees exhibit limp, dry and dark brown or black leaves. Both drought and frost stress weaken trees making them more susceptible to attack by fungi and opportunistic wood-boring insects.
Some species of trees and shrubs are more susceptible to drought, while others can withstand dry conditions. Trees with large, flat, green leaves, like sycamores and maples, tend to be water-needy. Trees with small, thick, waxy, hairy or light-colored leaves like blue oak (Quercus douglasii) and European olive (Olea europaea) or that have extensive root systems, are usually more resistant to drought. However, any aging, drought-stressed tree can be injured and go into irreversible decline if its root zone is limited, compacted or enclosed in paving. Deciduous trees may develop scorch, brown outside edges or browning between veins. Evergreen needles may turn yellow, red or purple.
Apply a 3- to 4-inch layer of mulch out to the branch tips (drip line) to help retain moisture in the soil, protect against temperature extremes and control weeds that compete for water and nutrients.
Water requirements vary with the age and species of tree, soil type, irrigation system, weather and the amount of evaporation of plant water from the tree's leaves. Deep water trees early in the day two to three times per month to a depth of 12 inches below the soil surface using a drip system or soaker hoses. Water slowly, dispersing the water flow so that it gets down to the tree's roots. Allow for thorough drainage and partial drying before the next irrigation cycle.
Monitor the moisture content of the soil frequently by using a soil probe to a depth of 12 inches. Most deep-rooted trees and shrubs can handle dry soils to 6 to 10" down, surviving on water in soil at 10 to 12 inches or below. Adding organic matter to soil improves the soil's nutrient capacity and increases the ability of water to flow through the soil and to hold water. The critical watering time in Marin is during the longest exposure to sunlight or June through August.
Prune broken, dead and diseased branches that further weaken a drought-stressed tree. Do not fertilize a tree that is under drought stress. Salt in the fertilizer may burn roots and stimulate top growth that the tree cannot maintain in drought.
Monitor drought-stressed trees and shrubs for insects and disease. Look for cracks in trunks and branches, small holes in trunks where wood-boring insects emerge, sawdust or sap flow, and branch dieback.
The water required to establish a new tree greatly exceeds that needed to maintain established trees. Take preventive measures now while you can. By recognizing the symptoms, properly irrigating our landscapes and following recommendations to minimize the injurious effects of drought and frost, gardeners can prolong the beauty and benefits of mature trees in the landscape.