Let azaleas dry out a bit in fall before winter sets in, but continue to water rhododendrons. Adding a mulch of pine needles, oak leaves, or fir bark around the plants will help retain moisture.
Regular feedings with a complete fertilizer or one formulated for azaleas and rhododendrons will also help keep plants in good health. Fertilize in early spring and then again as the blooms start to fade. Add compost over the top of the soil in fall to add nutrients.
Healthy rhododendrons and azaleas are fairly pest-free, and those problems that do develop can usually be easily mitigated. You’re most likely to have problems with leaf miners, root weevils, and thrips. Look for low-toxic insecticides to control these before they get out of hand. For aphids, lace bugs, mealybugs, and scale, use insecticidal soaps or horticultural oil.
Diseases can vary, depending on where you live. Root rot is one of the most common problems; solve this by providing fast-draining soil. If mildew is a problem, make sure there is plenty of air circulation around the plants. If you’re in an area where conditions make your plants particularly prone to mildew, plant resistant varieties.
Petal blight is an occasional problem. Botrytis petal blight, which starts with water-soaked spots that become dry and fragile, can be controlled by an appropriate fungicide. Azalea petal blight, which rhododendrons can also get, starts with water-soaked spots and can result in slimy petals. To keep the blight from spreading and restarting, remove all infected flowers.
Neither azaleas nor rhododendrons need much in the way of pinching or pruning. Pinch the tips of young azaleas and rhododendrons to encourage bushiness. Evergreen azaleas especially should be pinched just after they finish blooming to keep them compact.
You’ll want to prune to remove any damaged branches, or to open up the plant to allow for more air circulation. Start at the bottom and inside, and then work your way out and up. Remove any weak or crossing branches and any that are diseased or dead. Remove about one-third of the healthy foliage if plants are overgrown.
Evergreen azaleas can be pruned at any time, but you’ll usually only need to do it to remove an occasional wayward branch. Deciduous azaleas should be pruned when they are dormant, but after any danger from frost.
You can remove branches at any time, but do more extensive pruning of rhododendrons in late winter or spring, as long as the weather is above freezing. Bear in mind that doing so right before the plant blooms will lessen the number of flowers. Cut back to a side branch, a cluster of buds, or a leaf.
Azaleas and rhododendrons can be harmed by cold weather, especially when accompanied by strong sun and wind. Protect tender plants with a windbreak. In areas where the ground freezes, mulch the root area once that has happened. Remove the existing mulch and add a new layer once the danger of frost has passed.