The Rhododendron is a genus characterized by shrubs and small to (rarely) large trees, the smallest species growing to 10–100 cm tall, and the largest, R. giganteum, reported to over 30 m tall. The leaves are spirally arranged; leaf size can range from 1–2 cm to over 50 cm, exceptionally 100 cm in R. sinogrande. They may be either evergreen or deciduous. In some species the underside of the leaves is covered with scales (lepidote) or hairs (indumentum). Some of the best known species are noted for their many clusters of large flowers. There are alpine species with small flowers and small leaves, and tropical species such as section Vireya that often grow as epiphytes.
Basic Conditions Required by Rhododendrons
Rhododendron growing in natural, open field conditions have a root mass of very fine, hair-like roots that grow in a thick mass very close to the surface of the surrounding soil. They tend to spread out in a circle to a radius of 2 to 3 feet from the center stalk of the plant. To survive and flourish, Rhododendrons continuously require air, moisture and nutrients at the roots. Healthy development of rhododendrons requires that the following conditions be met.
- Soil should be loose and crumbly with a very high percentage of humus. A minimum of 50 percent extra- coarse, chunky, peat moss is recommended, plus other organic material. When amending existing soil, especially hard clay or very fine sandy soil, use equal parts of original soil, compost and coarse chunky peat moss.
- Oxygen in the root system is a vital factor in maintaining healthy rhododendrons.
- Soil must be consistently moist but never soggy. Adequate humus should hold enough moisture between rains, but watering may be necessary in drought conditions.
- Sharp drainage is critical. Rhododendrons are mountain plants, growing where there is always down grade for excess water to drain away.
- If existing garden soil is very heavy and/or poorly drained, keep the entire root system 6 to 8 inches above grade by creating a mound or berm, or a raised planting bed using a retaining curb such as logs, timbers or rocks.
- Some sun, or at least bright light is necessary to encourage bud formation and compact growth.
- Plants should be sheltered from the prevailing winter winds which desiccate the foliage.
- Ideally, shade should be provided during the hottest part of the day in summer.
- Shade in late February and through March, when the ground is frozen, may be important for some varieties as the strengthening sun may scorch the leaves.
Rhododendrons should be planted with either a root ball, if the plants are field grown, or with their container mix, if container grown. In favorable climates, rhododendrons can be planted almost any time of the year with reasonable success. In colder areas, early spring planting is recommended with early fall planting being second choice. In hot areas, fall planting is preferred.
When planting, first consider the nature of the soil. This may mean working in organic matter and acidifying the soil if it's too alkaline (pH higher than 6) by adding agricultural sulfur or ferrous sulfate. The amount of sulfur to add depends on local soil conditions, therefore it is advisable to consult with your local Extension Agent for proper rates for your soil. Do not use aluminum sulfate to acidify the soil because aluminum is harmful to the plants.
Your plants will come burlaped or in plastic bags or containers. The burlap may be left on the root ball unless it is plastic or otherwise non-biodegradable. Open up the biodegradable burlap and lay it well back from the trunk, taking care to remove any plastic or other types of cord or string. Plastic bags or containers must be removed. With container grown plants, it is especially important to cut any encircling roots and loosen the outer roots so that they will be in good contact with the soil.
Rhododendrons and azaleas are easily damaged or killed by planting too deeply. The top of the root ball should be at the surface of the ground in ideal planting sites or an inch or two above the surface of the ground or even in raised beds in less than ideal sites. Never plant rhododendrons or azaleas deeper than they were grown in the nursery as is sometimes recommended for other types of plants.
In very light, sandy, acid soil which is high in organic matter and ideal for rhododendrons, they may be planted in a hole a little larger than the root ball. Where the native soil is less porous than the material in the root ball, the soil should be improved by adding organic matter or perlite to make it more porous. Where the soil is clay and holds water in the bottom of a dug hole or is alkaline, it is advisable to plant on top of the ground in a mound made of a mixture of soil, coarse peat moss, bark, sand or perlite. The mound of soil may taper off at the edges or be confined by planks or logs in the form of a planter. Such raised beds require special watering attention during the summer.
In hot climates, root rot organisms flourish in wet soils and can kill rhododendrons. Under these conditions, raised planting beds that incorporate 50% or more fine pine bark can be helpful in suppressing Phytophthora root rot. Extreme cases may require the use of fungicides such as Subdue or Aliette.
Before planting, dry root balls should be thoroughly soaked in a tub of water. Under normal circumstances, it is not necessary to break apart a soil root ball, however, some loosening of the outer roots should be done to get the fine roots out of the existing root ball and into the new soil. This is particularly true if the plant was previously grown in heavy soil. With container grown plants, it is important to loosen and even cut some of the outer roots, especially if the plant is root bound. Cutting the roots encourages new roots to grow out into the soil. Hosing off the outer part of the soil or planting mix can be helpful in loosening roots for growth into their new planting hole.
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