Why? New plants do not yet have sufficient root systems to sustain themselves. The idea is to water regularly so that the roots will grow deeply into the soil, eventually providing enough root structure to be self-sustaining in all but extreme drought conditions.
When? Water deeply 3 times weekly for the first several weeks. Then, water 1 or 2 times per week until late October. Water in early morning preferably, or evening. Do not water in the middle of the day when the sun is beating on the leaves – this can scorch the foliage. Plants should dry off before nightfall to avoid mildew and other complications.
How? It is best to water at the base of the plant rather than using a sprinkler over top. Perennials should have about ½ – 1 gallon of water. Small shrubs about 1 – 2 gallons, large shrubs 3 – 4 gallons and trees about 5 – 10 gallons. Keep in mind that a brief thunderstorm does very little for newly planted plants. It might wet the mulch layer, but will not get into the root zone.