Living on the Edge

If you've ever taken a walk on the Delaware & Lehigh Trail in the Lehigh Valley and Poconos, you've probably seen the slate cliffs covered with wild Rhododendrons - Rhododendron Carolinianum and Rhododendron Catawbiense. These evergreens, which are found in growing wild in the mountainous areas of the Mid-Atlantic states including eastern Pennsylvania, do exceptionally well in Mid-Atlantic gardens.

Rhododendron Carolinianum plant is a perfect choice for areas where you need broadleaf foliage and a low-growing form. It can be used for a low border shrub or for massing where its display of spring flowers will add interest. It can also be used as a fill-in planting in front of other larger broadleaf shrubs. Periodic pruning will keep the plant 3 to 5 feet tall and equally wide. It tends to be round and open, with upright spreading branches. The foliage is not too dense, 2 to 3 inches long, and about 1 inch wide. Summer color is dark green, and winter color varies from green to purple-green. The flowers are one of this plant's main features. They are produced in clusters up to 3 inches across starting in mid-May. Flowers are pure white, pale pink, or rose depending on the variety.

The foliage and flowers of Rhododendron Catawbiense make it ideal for shrub borders or as accent specimens. A proper blend of compatible flower colors can create an impressive landscape design. Rhododendron Catawbiense, which are larger than Rhododendron Carolinianum, can be maintained at heights of 6 to 8 feet and spreads of 5 to 8 feet. They are dense, upright, and spreading plants with thick layers of foliage covering the branches. The branches often extend to the ground to produce a rounded form. Individual leaves are broad and elliptic, 2 to 4 inches long, and about 11/2 inches wide. Leaves are somewhat coarse and leathery with dark green upper surfaces and lighter coloring below. Individual flowers range from 2 to 21/2 inches across and are produced in clusters up to 6 inches in diameter by late May. Flower color on the species is lilac-purple. The flower clusters should be removed as soon as they fade. This will prevent seed development and increase the number of flower buds set for the following season.

Both varieties require the same cultural needs: moist, well-drained acidic soil; light shade in the winter; protection from drying winds all year; periodic applications of soluble iron fertilizer; and plenty of water in the fall before the soil freezes. The Rhododendron Catawbiense is suitable for hardiness zone 5. Rhododendron Carolinianum will do well in hardiness zone 6, without any particular treatment and hardiness zone 5 with winter protection. The Lehigh Valley and southern Pocono areas fall in zones 6 and 7.

Rhododendrons are healthiest in light shade, especially under oaks and pines with the lower branches trimmed. Do not choose a location near maples, elms, ashes or other trees with shallow competitive root systems. Avoid planting them in a south, southwest or west exposure, especially if heat and light are reflected on the plants from a nearby building. In dense shade, the plants tend to grow spindly and do not bloom profusely. Protection from winter's cold drying winds is also desirable. Plants that have been raised in soil in the field will establish themselves more quickly than plants which have been grown in containers in a light-weight mix.

Fall or early spring are the best planting times for rhododendrons, though spring planting is preferred in really cold areas. Plantings made in late spring and summer will require more frequent watering than plantings made in fall or early spring. It is not advisable to plant material in active growth.

Rhododendrons do not tolerate as much fertilizer as many other plants. Over-fertilizing can burn the roots and can encourage severe root disease. If the leaves are light green or yellow, but not yellow with green veins, the plant should respond to a light application of fertilizer. About one-fourth to one-half of the amount recommended for other kinds of plants should be adequate. A split application, one-half in March or early Apri and one-half in mid-May is most desirable. Scatter the fertilizer lightly around the outer edge of the root ball. Do not apply fertilizer in late summer or fall as the plant may be stimulated into growth and be killed during the winter. Special rhododendron fertilizers are available. If in doubt, do not fertilize. Many more rhododendrons die from too much fertilizer than from none at all.