Don't rely on mother nature to water you new Landscape! Rain fall can be very deceiving. You think the 45 minute thunderstorm just watered your plants beautifully, but half of it ran off into the lawn because it rained so hard, and only the top couple of inches of mulch is wet. Unless it rains a light soaking rain for a least 4-5 hours, Don't even consider natural precipitation as watering.
Here are some watering guidelines which Landshapes recommends.
First and foremost water must be applied gradually to allow water to soak in deeply and not run off, this is very important.
Perennials, grasses, and medium size shrubs 2'-4' in height: Apply 2-3 gallons of water per foot height of plant. Water daily for the first two weeks, every other day for the next 2 months and twice a week for 1 full year.
Trees 2"-3" in caliper: Apply 2-3 gallons of water per inch caliper of tree trunk. Water daily for 1 month, every other day for 3 months, and twice weekly for 1 full year.
Trees 4" and greater in caliper: Apply 2-3 gallons of water per inch caliper of tree trunk. Water daily for 6 weeks, every other day for 5 months, and twice weekly for 1 full year.
Notes: Note that supplemental watering during dry periods may be need of up to several years.
Take your soil type into account. If you have heavy, dense, clay, soils less water will need. Sandy soil may require more water.
Unless you get a minimum of 2"-3" of rain per week this watering protocol should be followed.
Tree and plant establishment times.
Herbaceous perennials, grasses, and ferns 1-3 Months.
Medium shrubs 6 months to 1 year
Large shrubs and caliper size trees 3-6 months per foot height or per caliper inch of trunk.