Henry Thoreau wrote, "The splendid rhodora now sets the swamp afire with its masses of rich color." It’s no wonder rhododendrons have always been one of our most popular shrubs. Versatile, vibrant, with incredible flowerheads and often evergreen during the winter, it is a plant for all seasons. One can use these beautiful plants for energy-saving planting as well as the plainer ones.
The rhododendron family is large, with over 2000 listed varieties and hybrids that include rhododendrons and azaleas. They grow in every temperate country in the world, from Lapland to Australia, with twenty-seven species in North America alone. The ones we mostly grow in our gardens have come mainly from the Orient — some were probably cultivated in Chinese gardens centuries ago.
In many parts of the world, rhododendrons are hard to grow and so, in that perverse human way, they are more prized. They can be made to grow in most of the United States, except for hot regions that have no cool winter. The East Coast and the Pacific Northwest are blessed with just the right soil and climate, which makes them easy to grow.
In other regions they often need protection, so special microenvironments have to be created for their survival. Some winters might be too cold for most varieties. Wind protection is usually necessary. The summers may be too hot, so open shade may have to be established. It is important that specific varieties be chosen for each region. Special soil may have to be created for the rhododendron as well. In areas with hard alkaline ground water, acid rainwater may have to be used.
If it gets too complicated, it might not be worth the effort to try to get them to grow outdoors, because they are easy to grow as potted plants, in special soil — especially azaleas. In places with naturally acid soil, adequate rainfall and humidity and a temperate climate where they will happily thrive, however, rhododendrons are a most glorious shrub.
Rhododendrons must have acid soil, 4.5 to 6.5 pH. It must also be friable and soft, so the very fine roots, which grow quite near to the surface, can push through it. A heavy clay soil can be improved with large quantities of acid peat moss, bog soil, rotted oak leaves, pine needles, or acid woodland soil. Or a new bed about 2 feet deep may be excavated and proper soil brought in.
To keep it acid, don’t add lime, gypsum, wood ashes, bone meal or calcium carbonate to the soil. Also beware of new concrete as rain can splash the calcium off of stucco or cement and into the soil, which will make it alkaline.
Most rhododendrons prefer a dappled shade, but they will grow in sun, as long as they have enough water. If the shade is too heavy, they will not flower well. Some varieties can stand more sun than others, but for all rhododendrons water is essential. The shallow roots easily dry out, which is why a mulch is always recommended.
It is essential to water new plants with 5 gallons of water once a week for the first year in the spring, summer, and early fall. Rainwater does not sufficiently moisten the soil. The second and third year they are still fragile and so must continue to be watched carefully and given supplemental water, because it takes at least that long for the roots to become established. You can tell when they are dry, for the leaves droop like limp hands.
Once established, rhododendrons need little care, except for the occasional soakings between June and September in dry years. Interestingly a good deep soaking in August makes them more winter hardy, but too much water after mid-September promotes winter damage. The plants need to get good water in late summer, to make as much carbohydrate as possible and to have time to harden off before winter.
Although rhododendrons need a moist root run, they will rot and die if they sit in standing water. For this reason, it is a good idea to plant the crown an inch or so above the ground level and slope the soil into a saucer to hold the 5 gallons of water for the first year.
The best mulch is 1 to 2 inches of oak leaves, for it keeps the soil acid yet it doesn’t mat down to form a mushy mess. (Maple leaves are not good for mulching purposes.) Pine needles, peat moss and pine bark mulch also work well. The mulch should never be more than a few inches deep and should not touch the bark but it should begin a little bit away from the stems. If it is too deep it encourages rot; if it is piled around the bark, it can interfere with the transport of sap and nutrients.
The best practice is to leave the mulch until it rots, just adding a little more on top as needed, instead of raking everything up each year and putting on new mulch. For some reason most landscapers feel compelled to remove all the fallen leaves and old mulch and then charge for both the unnecessary cleanup and the new mulch. Or they pile on a few more inches each year until it becomes too deep, unhealthy and smothers the soil.
You can save money by letting nature proceed at its slow pace and adding fresh mulch only for cosmetic reasons or when it decomposes and is too thin.
Fertilizing should be done in spring and be finished before the end of June. After that, soft new growth is encouraged, which is susceptible to winterkill. In warm climates, however, late-winter fertilizer can be applied.
A balanced fertilizer is best. Too much nitrogen will promote vegetative growth and fewer flowers. Adequate superphosphate and potash will promote flower bud formation and improve winter hardiness and vigor. One trick the commercial nurseries use to enhance flowerings is to apply a light top dressing of superphosphate in early August.
The residual by-products of some fertilizers make the soil alkaline, so check with your nursery before buying. There are special fertilizers formulated for acid-loving plants, the by-products of which are acid; however, overuse can cause problems. Alternating chemical and organic fertilizers is probably best, or you can use mostly organic fertilizers, which will make bigger bushes with fewer flowers.
Once the plants are well established in good soil, are not overwatered and get enough sun and if the mulch underneath is left to compost naturally, they will bloom perfectly well without any fertilizer.
Pruning a Rhododendron
Pruning is not essential. Once established, the shrubs are best left alone except for removal of broken or dead branches. In case of winterkill, always wait well into June to prune out dead wood, because there are almost always live buds which will sprout, although very slowly, from the branches. If the brown winter-killed leaves are offensive, trim the brown edges off with scissors or cut off the individual leaves. But leave the terminal end of the twig, which will usually leaf out again.
Unpruned rhododendrons flower best and are graceful large shrubs that have a natural, woodsy look. Some grow upright, some are spreading, some grow fast and some slowly. If they get too large or grow over windows or into paths, they can be pruned and clipped back. Pruning gives them an Oriental look and fewer flowers.
Many varieties can get quite large and really outgrow their space, yet one hates to remove such beautiful plants. There are several ways to deal with this problem. One is to remove whole branches at ground level. Most members of this family will sprout new shoots from the sides of big, old branches from trunks that are exposed to light. It may take a couple of years, but usually the sides become green again.
Another way is to prune out all the lower side branches, leaving just exposed tall trunks. This creates a beautiful flowering tree, much like the treasured tree rhododendrons from the Himalayas in the magnificent greenhouses at Kew Gardens outside London. As an added bonus, if one prunes old shrubs this way a shady new bed is created underneath that can be filled with ferns, flowers, small shrubs or ground cover.
The final way to deal with hopelessly overgrown rhododendrons is to cut back everything to about 6 inches, removing the whole shrub. New shoots will eventually appear about 75 percent of the time. Many nurseries cut large wild rhododendron and mountain laurel back to the ground this way and allow new shoots to grow from the large vigorous root system. After a few years, they dig them up and sell them. You can identify these "cut backs" by the large trunk stubs in the root ball. Cut backs will grow into nice bushes but take several years to flower.
Threats and Dangers
Hardiness and winterkill can be problems with rhododendrons. Not all varieties are winter hardy, but many can stand quite a bit of cold. Most good catalogues and specialized rhododendron lists will include the lowest winter temperature each particular plant will stand; you can then check it against the lowest winter temperature in your area. Because rhododendrons have been so highly bred, each has a different cold tolerance.
If you can’t find this information, check with your local nurseries, the Agricultural Extension Service in your area, or the American Rhododendron Society. An empirical way to find out which varieties are hardy in your area is to check which ones bloom well in your neighborhood.
Even varieties that are hardy and will survive in your area can still suffer from winterkill, which is the drying and browning of leaves and buds. Evergreen varieties are most affected. Usually just the outer edges of the leaves turn brown, but sometimes whole leaves or irregular blotches may become discolored.
The important thing is to not assume the twigs are dead and make the mistake of pruning off stems or tips too soon. New shoots usually appear, although they may be a month later than the new shoots elsewhere on the plant. It is best to wait until late June on Northern areas before pruning for winterkill.
The reason for winterkill is lack of water. Evergreen leaves transpire even in winter. When the weather warms up, or when winter sun reflects off a wall and warms the leaves, the breathing holes (stomata) open up and the sap begins to run. But if the ground is frozen, the roots can’t take up any replacement water. This lack of water in the leaf cells causes them to turn brown.
Another weather event that causes browning is repeated warm and cold temperature fluctuations, especially in early spring. It can happen on several days, or it can be just the difference between the warmth of the late afternoon sun (around 65 degrees) and a sudden drop at sunset (often below freezing). These changes confuse and stress the leaf tissue, causing cell death, which appears as brown leaf edges.
Winter wind is very damaging because it also dries and desiccates the leaves and twigs. Although hardy rhododendrons don’t need protection from low temperatures per se, they may not survive if there is winter wind blowing over them.
There are so many varieties of rhododendrons that it boggles the mind. There are deciduous, semi-evergreen and evergreen varieties. The deciduous ones are generally called azaleas, while the evergreen ones are usually called rhododendrons. Their periods of bloom start with the first warmth in spring (PJM Rhododendrons) and run through the early part of summer (Swamp Azalea), a span of about 3 months. Each plant blooms for one one or two weeks, but with a carefully chosen selection a garden can have a succession of blooming azaleas and rhododendrons for a long period.
In each area of the country certain varieties will do better than others. It is always necessary to check your local conditions before ordering from catalogues to make sure the varieties you choose will thrive for you.