Glossy green leaves and large attractive flowers make the rhododendron (Rhododendron spp.) a show-stopper in the garden. Rhododendrons, which grow in U.S. Department of Agriculture plant hardiness zones 5 through 9, come in both deciduous and evergreen varieties. They don't tolerate drought stress and they thrive in well-drained soil that doesn't dry too quickly. Frequent, deep watering when necessary ensures the shallow roots receive the moisture needed for healthy growth, while keeping the soil from becoming too wet, which causes root rot.
Lay a soaker hose around the base of the rhododendron shrub, about 10 inches away from the main trunk. Turn on the water and irrigate the shrub for about 30 minutes, or long enough to provide about 1 inch of moisture.
Insert the blade of a trowel 6 inches into the soil 10 inches away from the trunk. Verify that the moisture penetrates the entire depth of the hole. Water longer if the bottom of the hole is still dry.
Water the rhododendron about once weekly during the summer and early fall. The shrubs may not require any irrigation in spring or early summer in areas with sufficient rainfall to keep the top 6 to 8 inches of soil moist. Water up to twice weekly during hot, dry weather or if the rhododendron foliage begins to yellow or wilt. Checking soil moisture once or twice a week using the trowel method allows you to judge soil moisture and adjust the watering schedule as needed.
Spread a 2-inch layer of mulch over the soil, covering the rhododendron's root zone. Replenish the mulch each spring so it remains 2 inches deep. Mulch slows evaporation and helps retain the soil moisture.
Reduce watering in mid-fall to once every two weeks as the shrub begins to go dormant. Water evergreen varieties deeply after the first hard freeze so the plant doesn't begin winter in dry soil. Rhododendrons rarely require further winter watering after this final deep watering.