Your newly planted shrubs need to be carefully hand watered after being planted! Root stimulator at a rate of 3 tablespoons per gallon of water should be applied every three weeks for the first three months.
Watering is not as simple as waiting until a plant wilts or using some predetermined schedule. Watering is influenced by a number of factors. Soils differ in how quickly they dry out after a rain or watering. A clay soil will not need watering as frequently as a sandy soil since clay soils drain slowly and sandy soils drain quickly. The addition of organic matter will increase drainage in clay soil and moisture retention in sandy soil. Subsoil also affects water drainage. Mulches help keep the soil cool and reduce water loss through evaporation thus extending the time between watering.
Different plants have different water needs — some are quite drought tolerant. Plants with large leaves (hydrangea) or with shallow root systems (azalea) are usually the first to suffer during drought periods. Shrubs under large trees are especially susceptible because of the large volumes of water taken up by tree roots. Plants are more prone to suffer from drought during their first two growing seasons after transplanting. Late spring or summer transplants are the most susceptible, because their roots have a shorter time to become established prior to summer stress. When there is an extended period without rain during the summer, new plants should be watered at least twice a week.
It is important to water thoroughly and to allow the soil to dry between watering. Frequent, light watering wastes water, does little to satisfy the water requirements of most plants, and leads to development of a shallow root system thus increasing susceptibility to drought. By allowing the soil surface to dry out somewhat between watering, major root development will be at greater depths where soil moisture is higher.
Your automated sprinkler system will most likely not provide sufficient water for your container grown shrubs larger than 1 gallon. Wetting the soil to 6 inches deep requires 1 to 2 inches of surface water which will vary with soil type, compaction and slope. Conversely, standing water can cause drowning due to oxygen depletion and poor drainage can cause root decay and root rot diseases. Carefully monitor your water program for happy, healthy, beautiful shrubs for years to come!