Rhododendrons and azaleas have no tap roots.  They have fibrous roots.  When you tip one out of the pot, you typically see a dense mass of hair-like roots.  These fine roots anchor the plant and provide its feeding system so there have to be a lot of them.

Rhododendrons and azaleas are very shallow rooted (about 1-2 feet deep), and the root zone may dry out during hot weather even though deeper rooted plants show no signs of drought stress.  Therefore, rhododendrons should be well watered during the hot months, especially the first year after planting when the roots have not yet gotten out of the original root ball and into the surrounding soil.

After transplanting the roots can take longer to grow out into the surrounding soil than most other kinds of plants.  They act like temperamental children that don't want to leave the house.  Because of this, the newly planted rhododendron will get its water out of the original root ball and if the root ball is allowed to dry out it can be difficult to get it wet again.  One way to rewet a dry root ball is to place a dripping hose at the base of the trunk of the plant and let it run for several hours.  Unless you have a very wet climate, doing this in addition to the regular watering the plant gets during the first growing season will help get the plant off to a good start.

Fibrous roots can't readily grow into crusty, hard soil.  The texture of the soil surrounding the rootball should allow easy penetration of the fine roots.  They can't compete with the big kids in the neighborhood, so don't plant where there are other competing roots.

Rhododendrons and azaleas ideally grow in damp, never soggy soil.  Think of taking a wet sponge and squeezing it.  The remaining damp sponge is the nearly perfect air and water environment for rhododendron roots.  Too much water promotes root rot that can cause plant death.  Too little water, the plant will suffer and can be damaged.  Low available water in the root zone can cause leaves to sun burn.  Sun burn appears as splotches of brown, tan or off-white areas on the leaves.

Some foliage droop is normal in dry weather, especially on warm afternoons, but when leaves still show signs of drooping in early morning, the plant is showing a need for water and should be irrigated.  When air temperatures go above 95°F (or even lower for alpine types), rhododendrons and azaleas appreciate a misting to prevent desiccation of their foliage.  In cold climates, watering or misting of foliage during warm days in the spring or on windy days when the roots are still frozen will help to keep rhododendrons in good condition.

A year-round mulch of some type of organic matter is desirable to conserve moisture and eliminate the need for cultivation.  Because of their shallow roots, little or no cultivation should be done around rhododendrons.  Weeds should be carefully pulled, or in extreme cases shaved off with a sharp hoe.  A fairly deep mulch of leaves, pine needles, chips, bark or other organic material will practically eliminate weed growth.  (Peat moss should not be used as a mulch because it sheds water when it dries out.)  The coarser the mulch the better, as water and air are admitted while the mulch still retards evaporation by providing shade and reducing wind velocity over the roots.  A mulch also helps to maintain a moderate soil temperature in the root zone.

For further information on watering rhododendrons and azaleas consult the following Journal ARS articles:

  Tips for Beginners: How to Transplant a Container-Grown Plant by Steven Feryok
  Watering: A First Priority by H. Edward Reiley
  Rhododendrons With A Drinking Problem by Dr. Bill Letcher
  Armchair Watering by Jean Minch
  Mulching: Tips for Beginners by Terry Richmond
  Mulch Ado About Nothing or To Mulch Or Not Too Mulch Is That The Question? by Robert A. Murray