Some varieties of rhododendrons are naturally leggy. Some varieties are naturally tall. Plants that are grown in shade are almost always leggy. To have a healthy well shaped rhododendron, there are several things you can do.

First, prune off lower branches of shade trees so that you have "high shade" above your rhododendrons. This is ideal for a healthy rhododendron bed.

Second, once the rhododendrons reach the height you want, then each spring after the flower buds start to swell and the smaller leaf buds are obvious, break off leaf buds on the top of the plant to prevent it from growing taller. You must be careful not to damage any flower buds.

Third, if the plant is taller than you like, you can prune it after it blooms. It is best to prune within two weeks of when they stop blooming. This is to prevent removing next years flower buds. As with most spring bloomers, they form next years buds this summer.

Fourth, in the future grow plants that are the height that you want. Unfortunately, most rhododendrons never stop getting taller, but their height is quoted for plants that are 10 years old and by that time most varieties have slowed down their growth considerably. But if you choose plants that are the right size to begin with, they are relatively maintenance free.

Fifth, if they have additional problems treat them. When rhododendrons are not planted in ideal locations they may develop chlorosis. Chlorosis is yellowing of a leaf between dark green veins. It is caused by malnutrition that can be caused by a wide variety of conditions. They include alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency, magnesium deficiency, nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer. In any case, a combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. HollyTone contains these elements and some 4-6-4 fertilizer. It is best applied in the spring prior to blooming to make sure the plant is healthy when forming next years flower buds. If you missed applying it in the early spring, it can be applied up until mid summer. Rhododendrons do best when left alone in the right conditions. Don't use HollyTone or any fertilizer unless the plant shows signs of malnutrition.

Sixth, of course the annual maintenance of a rhododendron bed includes maintaining the mulch on the bed. Rhododendrons do best when they have about a 2" to 3" layer of mulch to hold in moisture, prevent weeds, and keep the roots cool. Since most mulches are organic, they need to be topped off periodically, usually about every year or two. Do not make the mulch over 3" thick. Keep the mulch about 2" to 3" back from the trunk/stem of the plants to avoid bark split and rodent damage. Do not use peat moss as a mulch. It is a soil amendment to be used when preparing the soil in a bed and can cause severe problems when used as a mulch including dehydrating the soil and preventing moisture from reaching the soil. It also tends to blow around. It is best to mulch with a 2-inch layer of an airy organic material such as wood chips, ground bark, pine needles, pine bark or rotted oak leaves. A year-round mulch will also provide natural nutrients and will help keep the soil cool and moist.

Seventh, protect from deer. I use deer netting in the winter and for plants that are susceptible to damage in the summer, such as azaleas, I have a couple beds protected by deer fencing.

Eighth, move plants that are planted in the wrong location. Some rhododendrons like more sun and some like more shade. If you didn't note this before planting, the plants may be struggling and should be moved to a better location. Rhododendrons are easy to move. They have shallow roots. When transplanting a large plant several steps should be followed. First, it is best to stimulate a tight root ball by root pruning the plants to be moved about a year before moving. This is accomplished by cutting a circle around the plant stem with a shovel to cut off roots that extend beyond this point. This radius is usually slightly smaller than half way to the drip line. Second, it is best to move when the plant is dormant and not stressed. This would be in the spring and fall when the plant is still dormant but the soil is not frozen. Moving in the fall before the ground freezes is preferable if you don't have a problem with frost heaving. Sometimes winter freezing and thawing cycles can actually lift a transplanted plant out of the ground where the roots are then desiccated and the plant dies. For this reason, it is safer to transplant in the spring after the ground thaws in climates where frost heaving is a problem. Third, take precautions to preserve the integrity of the root ball. Tie the ball together and support is so it doesn't fall apart. The very safest approach is to dig a trench up to 12 inches deep, around the dripline of the plant. Then undercut the plant to form a cone, removing the soil an inch or so at a time, moving all around the plant, until you begin to see that you are removing roots. If possible, then get a square of burlap under the plant. Tilt the plant to one side, put one edge of the burlap close to the center of the plant, wadded up so that only half of it is on the open side of the plant, then rock the plant the other way and pull the burlap through. Tie the corners of the burlap to each other across the plant. Tie the burlap tightly to keep the soil around the plant roots undisturbed. Then lift the plant by the burlap and the bottom, not by its stems. Finally, pruning the top helps match the demands of the top to the capability of the roots after they are stressed by the move. People have been known to cut the top off wild rhododendrons before moving and the plants have come back with superior shape. This is drastic and not recommended for a plant you don't want to risk loosing. Rhododendrons and azaleas have dormant buds beneath the bark which sprout to form new growth after severe pruning, hence severe pruning which removes 1/3 to 1/2 of leaf area is quite common when transplanting. Make sure you watch the plant after it was moved like you would a new plant. Its roots are compromised and it will need a reliable source of moisture. If the weather has a dry spell, make sure you water any newly planted rhododendrons, large or small.

This brings up the last point, don't forget to water your plants during droughts and other unusually dry periods, summer or winter. Only water when necessary and in hot weather always err on the dry side, but don't hesitate to water plants that look wilted in the morning. In hot weather it is normal for rhododendrons to look slightly wilted in the heat of the day, but if they look wilted in the morning, then they are too dry.

Here is a link that might be useful: How to care for rhododendrons