If they are listed as drought-tolerant, shouldn’t this be a guarantee against mortality in the dry period?
Trees listed as drought-tolerant are those that have genetically adapted to sites in their native habitat that regularly experience prolonged dry spells. However, all newly-planted trees can use some help from us.
During the first couple growing seasons the tree is expending a lot of energy trying to get its roots established in the soil. We can make this easier by providing water and covering the soil with wood-chip mulch. Proper planting and then keeping the soil moist to a depth that includes all the roots will speed root establishment and hasten the day when the tree really can withstand droughts on its own.
We need to quickly add that it is equally important to not over-water. Moist is different than soggy, and you can judge this by feel or use of a moisture meter. A damp soil that dries for a short period will allow adequate oxygen to permeate the soil. This aeration is as important as water. It has been said that more trees in urban settings drown rather than die of drought. This can be prevented by checking the soil during the summer and being careful that lawn irrigators are not unintentionally providing too much water on young trees.
Some Drought-Tolerant Species
For a more complete list, please visit the Drought Tolerant Tree List