Brown leaves on trees and shrubs can be indicators of leaf scorch. Leaf scorch occurs during hot weather when the rate of water uptake by roots is exceeded by the rate of water loss from leaves. Plants respond to rapid water loss by willingly letting some leaves die back so that water intake equals the amount of water loss. Not all trees have the same level of leaf dieback—it varies from tree to tree, by circumstance and by species.
Watering if the soil is dry makes sense but watering a whole lot more does not. This is because constantly soggy soil doesn’t force plant roots to take in water faster. Worse, constantly soggy soil causes root dieback, compromising water uptake even more.
There are things humans do that can make leaf scorch worse—excavating soil/adding soil around roots, using lawn herbicides containing dicamba around trees and shrubs, and adding rock mulch around the base of plants, to name a few. In these cases, leaf scorch will be an ongoing problem.
While there is nothing that can be done to induce browned leaves to regain their health, there are some things that will gain ground on good tree health. Water when there is an absence of rainfall by irrigating with 1 inch of water per week, applied all in one application. Remove rock mulch and landscaping fabric, replacing it with 2-3 inches of wood chips or shredded bark. Refrain from fertilizing trees and shrubs because fertilizers contain nutrients as salts that can make scorch worse.
Leaf scorch is hardest on small and newly-planted trees. In these instances, trees need every leaf they have to help with establishment. For older and well-established trees, occasional leaf scorch does not cause any long term stress.