Your soil has now been well and properly prepared, with good moisture-holding capacity. So how much should you water?
There is no simple rule for when and how much to water — but no watering at all can spell disaster for newly planted shrubs, trees, and perennials.
The best way to know is to literally stick your finger in the ground next to a plant, below the mulch, into the top inch of good soil. Is it damp? Wet? Dry?
If it's dry, water well. That means about a gallon of water around the base of each small plant, and two to three gallons around the base of shrubs and small trees. If you do this every other week whenever there's been no significant rainfall, your plants should be in great shape. (A quarter-inch of rain counts as a good rain.)
Supplemental watering is critical for the first two to three months after planting in spring, and through the warm summer months. By fall, most of these plants will have healthy, vigorous root systems able to find their own moisture — except in extreme dry spells with no significant rain for three weeks or longer.
A few tips
I like to use a watering wand — a two- to three-foot aluminum pipe with a gentle, low-flow spray head, ideally with a shut-off valve at the handle. The length lets you tuck the head under shrubs and direct water to the roots, where it's supposed to go. Watering the foliage instead serves no purpose, wastes water, and can encourage diseases like leaf spot and powdery mildew.
Morning is the best time to water. But if a plant is clearly stressed from dryness, water it right away rather than waiting for the ideal time of day.