New planting of trees and shrubs can be especially susceptible to drying out if not watered correctly. Mulching new plants can help conserve water a great deal.
Whenever you put in a new tree, shrub or perennial its a good idea to plant it about 1-2" higher than the surrounding grade. This will allow you to add mulch to the planting area. Mulch acts to keep down weeds and conserve water. Weeds can take light, water and nutrients from a new plant and this can add a bit of stress to a plant that needs to acclimate to its new home.
Try using 2" of organic mulch around the base of your new plantings. Be sure that the mulch does not come up any higher than the buttress roots of the tree or the height that the shrubs and perennials were growing in their containers or burlap.