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What to know about brightening your garden with rhododendrons

Before your decide which rhododendron is right for your garden, you need to understand the plant's needs.
Before your decide which rhododendron is right for your garden, you need to understand the plant's needs. Centre Daily Times, file

My neighbors, Dale and Bev Lippincott, have some rhododendrons on their property that are beautiful and are also inside a fenced enclosure to keep our four legged friends from pruning them and ruining them. Since it is snowing outside as I write this column, I thought that an article on rhododendrons might brighten your day.

The genus rhododendron has more than 900 species because it hybridizes freely, has nearly as many cultivars or cultivated varieties. Over the years, horticulturalists have developed improvements in foliage, flower color and quality, hardiness and growth.

Rhododendrons are native to many parts of the world but are found primarily in China, Japan and the eastern United States where the soil and atmospheric conditions are ideal. I remember seeing beautiful plantings of rhododendrons when I was stationed in Japan at the end of the 1960s. Many species of this broad-leaved evergreen shrub are popular landscape plants valued for their color, shape, low maintenance and spectacular flower display in April to May.

Flower colors vary from deep red to clear white, including various shades of lavender and purple. Sizes vary from 2 to 3 feet high to as large as 15 feet. They frequently have a spread equal to their height so it is very important to match your selection to the size of your planting site. But before your decide which rhododendron is right for your garden, you need to understand their needs.

For best growth and development, you will need a site that is protected from strong winter sun and wind, where humidity is high and where the soil is well-drained, acidic and has higher than normal amounts of organic matter.

Plant rhododendrons in a northeastern or eastern exposure or in other sites if you can protect them with other plants, buildings or screens. You need to protect foliage from winter sun and wind or you will have problems with drying leaves.

I would try to adjust the soil pH to as close to 5.5 as possible. Soil can be acidified with applications of sulfur or ferrous sulfate. Rhododendrons also respond to applications of iron chelate if the soil pH is on the high side. The best ways to know your soil pH and to know what you need to add for optimum growth and good leaf color is to take a soil test.

Adding organic matter to planting soil helps to maintain a uniform moisture level throughout the year. Organic matter absorbs and holds moisture and supplies additional pore space so excess water can drain away from shallow roots.

An organic mulch on the surface also helps to conserve moisture and supplies a limited amount of fertilizer as it breaks down. However this should be supplemented with a pound of 0-20-20 fertilizer per each 100 square feet of bed area every year or so. Take a soil test every three years to help you keep the nutrients in balance.

Now that we have discussed growing conditions, let’s look at several cultivars and varieties.

The first is Catawba Rhododendron (Rhododendron catawbiense). It's a native rhododendron that has been freely hyberdized and is probably one of the most available large varieties. It has a mature height of 8 to 10 feet and a spread of equal size. Leaves are medium-sized, about 3 to 6 inches long and 1 to 2 inches wide, dark green above and light green below. The true catawbiense species are generally not available. However, many cultivars are. They produce flowers in a variety of colors most of which open in mid-May. There are slight size and flower variations but plant labels usually list specific plant characteristics and you should read them carefully.

The second one is Carolina Rhododendron (Rhododendron carolinianum) is a slightly smaller and often less aggressive plant. This grows to about 6 feet after 10 years, a quality well suited to smaller sites. The leaves are small, about 2 to 3 inches long and 1 to 1.5 inches wide. The have a light color change in the winter from a dark green in this area to a purplish tinge in cooler climates. Flowers are white to pale rose or a lilac rose which bloom in mid-May. As with other rhododendrons, the Carolina has a few improved cultivars.

A third one is “PJM” Rhododendron, named after the breeder Peter James Mezzitt who introduced the plant in 1943 and is a popular one. It has many of the same leaf and shape characteristics as its parent, however, flowers are a consistent bright lavender pink in late April to early May. Plants produce little or no seed after flowering so you can expect heavy flowering each season without having to remove the seed pods. As an extra bonus, its flower buds are very hardy.

A fourth one is Cloudland Rhododendron (R. impeditum) that is good for even smaller garden sites and has purple or bluish-purple flowers in later Arpil to May.

Others that are good for small areas are “Purple Gem” and “Ramapo.” They are dwarf to compact and rounded with light purple and violet to pink flowers respectively.

The last three have small leaves no more than an inch long. For some reason rhododendrons with smaller leaves will take more sun than larger leaf species like the Catawba or Carolina.

All rhododendrons are easy to maintain. They need little or no pruning to keep them in bounds if you have made a choice according to your space constraints. Plants have a naturally informal and spreading habit and can be encouraged to spread even more by a little finger pruning.

Before they go into active growth in the spring, remove the terminal leaf bud. These are the small pointed buds, smaller in diameter than a twig, and they snap off easily with your thumb and forefinger. Removing these on a few selected branches each year will encourage several side shoots instead of a single twig to develop. I hope that this has brightened your day.

Bill Lamont is a professor emeritus in the department of plant science at Penn State and can be reached by e-mail at wlamont@psu.edu.