Lack of moisture usually affects all the leaves on one or more branches. Leaves affected first and most severely are those exposed to afternoon sun and prevailing winds. When leaves drop as the plant tries to conserve water, reduced foliar cover per unit of ground surface area helps the plant conserve water further. During 1975-1977, California suffered a severe drought, with annual rainfall below 10 inches. Most trees that were lost had shallow root systems or were growing on shallow or poor soils.
Plants show increased susceptibility to winter injury.
Sun scald and frost splitting of tree trunks Winter burn of conifer foliage Dieback of overwintering broad-leaved plants. Plants can be injured or totally killed by low temperatures at any time of year, but especially in spring and autumn, in the coldest part of winter, and when low temperatures follow warm winter periods. If rain is lacking for three weeks or more, and if you put in plants during the last 10 years, they need supplemental water. If the plant has been transplanted in the past 13-26 weeks, it will need water every five to 10 days. If the soil is shallow or structurally poor, however, you need to water once every one to two weeks.