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Rhododendrons and azaleas grow best in a sheltered site that is not windswept, and one that receives dappled sun in the summer with little or no early morning sun in the Great tips for managing your garden, from initial planning straight through to planting and maintenance. |
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Growing Rhododendrons and Azaleas
Submit your comments, tips, or suggestions you'd like to share with other users regarding this article. Reference: Ohio State University Extension The elegance and beauty of azaleas and rhododendrons in bloom at the nursery have caused many an unsuspecting gardener to purchase these plants on impulse without knowledge of their cultural requirements. These plants are often planted in heavy Ohio clay soil, and the gardener is puzzled as the plants languish, dying branch by branch until they are only a shadow of their former beauty. Rhododendrons and azaleas are referred to as "ericaceous" plants in the genus Rhododendron, of the family Ericaceae. It is possible to grow azaleas and rhododendrons successfully in Ohio, providing that their strict cultural requirements are met – well–drained, light, acid soil; ample moisture during the growing season; and protection from winter winds and early morning winter sun. In general, rhododendrons and azaleas have the same cultural requirements, but azaleas are somewhat less stringent and thus "easier to grow." Deciduous azaleas are easier to grow than the evergreen types. Site Selection
The best sites for these plants are on the north side of a building, the east side, or last, the west side. The south side of a building should be avoided unless it is protected from winter sun. If not planted near a structure, the preferred sites are those protected by windbreaks, fences, or evergreens. Rhododendrons should not be planted under a building's eaves where little or no rainfall wets the soil. It is also advisable to plant rhododendrons and azaleas in small groups rather than as individual specimens. Soil Preparation Perhaps the most important requirement for the successful culture of rhododendrons and azaleas is soil preparation. The more time and effort spent preparing the soil prior to planting, the less time will be required caring for the plants in the year's to come. It is best to prepare beds for rhododendrons and azaleas, rather than plant them in isolated holes with amended soil. However, this subject will be addressed later, as most rhododendrons and azaleas will be planted in this manner regardless of the recommendations! The soil for these plants must be moist, well–drained, well–aerated, and have a pH of 4.5 to 5.5. Fifty percent of the planting medium should be organic material. Varying combinations of sphagnum peat moss, pine bark mulch, compost, and aged, chopped leaves should be worked into the soil to a depth of about 12". Pine bark is particularly good because substances in the pine bark are thought to inhibit fungi that cause root rot. The large amount of amendment added will raise the bed considerably, which will allow for good drainage and aeration of the soil. Inorganic materials that may also be added to soil include perlite, vermiculite, and Profile™.
Individually prepared planting holes are not recommended, as they tend to form a "bathtub" effect and accumulate water that does not drain through the surrounding soil. If the gardener insists on planting azaleas and rhododendrons in this manner, and if the surrounding soil drains poorly, dig the hole 36" wide and 15" deep. Remove half of the native soil and incorporate an equal amount of organic matter and a handful of sulfur with the remaining soil. In the bottom of the hole, add a 3" layer of pea gravel or stones (not limestone) to facilitate drainage. Fill the hole with the soil mixture and water thoroughly to settle the soil. As an alternative, raised beds built on top of the native soil to a depth of 12"¡18" and held in place with timbers or stones are good for rhododendron culture. Raised beds require special attention to watering during the summer as they dry out faster than beds prepared at the original grade. Although not a requirement, a cup of alfalfa meal added to the planting hole at the time of planting may be beneficial. Alfalfa contains a potent growth hormone that stimulates luxuriant growth in rhododendrons and azaleas. Alfalfa meal can be purchased at most feed stores. Planting
Dig a hole in the prepared bed and place the plant in the hole so that 1" to 1–1/2" of the root ball is above the soil line. This is important since more rhododendrons are killed by planting too deeply than for any other reason. Gently firm the soil around the roots in the hole. Do not use your foot to tamp the soil. Place a 2" layer of pine bark mini–nuggets or hardwood mulch around the portion of the root ball that is above the soil line, being careful not to pile the mulch around the stem of the plant. Water the plant well. Note: Do not use peat moss as a mulch for any plants. When it dries, it sheds water like a duck's feathers and is difficult to rehydrate. To avoid compacting the soil, do not walk in the beds after planting and mulching rhododendrons and azaleas. Watering It is imperative that rhododendrons, azaleas, and other evergreens have sufficient water before the soil freezes in the winter. Because they are evergreen, during the winter they lose water through the leaves that cannot be replaced when the soil is frozen. If sufficient rain has not occurred in late November, by Thanksgiving, water the plants well. Fertilizing
Phosphorous is important for bud formation in young rhododendrons. If a soil test indicates the need for phosphorous, it is best incorporated into the soil at planting time as superphosphate or triple superphosphate. Phosphorous does not easily move down or through the soil from surface applications. Iron chlorosis (yellowing of the leaves) occurs in ericaceous plants when the soil is not maintained at an acid pH. Chelated iron may be applied as a "quick fix." Products such as Sequestrene™ or Greenol™ supply iron in the chelated form. This is only a quick fix, and annual applications of agricultural sulfur to acidify the soil to a pH level below 6.0 is less expensive and longer lasting. Maintenance Deadheading is necessary for the large leaf rhododendrons and deciduous azaleas, to remove the withered flower trusses after they have finished blooming. This improves the appearance of the shrub and will direct energy to the production of more flower buds for the next blooming season. Plant Selection Rhododendrons are divided into two groups – large leaf and small leaf, while azaleas are divided into evergreen and deciduous. The best large leaf rhododendron for ease of culture and excellent growing habit in Ohio is 'Roseum Elegans' or related cultivars, which include 'Roseum Pink,' 'English Roseum,' or 'Roseum Superbum.' Others that do well in Ohio are:
Note: 'Nova Zembla' is a red hybrid sold extensively in much of Ohio, but it is not a good selection for the beginner. It is very susceptible to stem dieback, and impressive specimens of this rhododendron are rarely seen. 'Caroline' is a light lavender hybrid that is not sold locally but is worth the effort to find. It is the most resistant rhododendron to Phytophthora root rot and dieback diseases.
Insects and Diseases Stem dieback affects the twigs and branches. Two fungi, Phytophthora and Botryosphaeria, are common causes of this problem. A canker develops and girdles the stem. Leaves and stems above the canker wilt and die while the rest of the plant looks normal. Diseased stems should be pruned well below the canker, and the pruning shears must be disinfected with rubbing alcohol (70%) before each cut is made. As mentioned earlier, water–stressed plants are more susceptible to this disease. Labeled fungicides may be helpful. Powdery mildew is a problem on some deciduous azaleas. It can be controlled with fungicides, but it is better to select mildew–resistant cultivars such as 'Crimson tide,' 'Yellow Pom–Pom,' 'Northern Hi–Lights,' and the species R. viscosum. If ordering deciduous azaleas from a catalog, look for those specifically mentioning mildew resistance. Root weevils, particularly the black vine weevil, are the major insect pests in Ohio. Adult weevils feed at night and cause small, irregular or semicircular notches on the leaf edges that give the leaf a jagged appearance. Damage to the leaves is not a serious threat to the plants, but the larvae feeding on the roots and stems can cause reduced plant vigor and/or death. Orthene™ is a pesticide registered for weevil control.
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| Data Source: Ohio State University Extension. Articles and resource may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide and it is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. |
