Tips for Beginners: Rhododendron Culture, Planting and Transplanting
Federal Way, Washington
Reprinted from the Rhododendron Species Foundation newsletter, October 1995.
There are three criteria to be aware of when deciding where to plant a rhododendron: visualization, light conditions and drainage.
An important aspect of site selection is to visualize what the plant will look like in five to twenty years. Find out how large the rhododendron will be at maturity; this will help in deciding where to place the plant. All too often one sees a large plant growing in front of a window or encroaching into a walkway. What was once a small plant in a one-gallon container may become a huge shrub growing in the wrong place. Plants do grow!
The light a plant receives is important to its health and growth habit. A site with morning sun and afternoon shade or filtered sunlight throughout the day is optimum for most rhododendrons. A standard guide common among rhododendron growers is that rhododendrons with smaller leaves will grow well in locations with more sunlight and those with larger leaves do better with more shade. These guidelines are for general use only. The level of sunlight under which rhododendrons will grow best depends upon the area of the world in which they are being grown and the length of time a particular species can endure full sun without injury. One cannot assume that if a plant grows in full sun in the wild it will tolerate that condition in your garden. There may be a significant difference in altitude and climate between the two growing areas. For example, while the summer air temperature in alpine regions may be very warm, even hot, the temperature of the soil is typically much cooler because of moisture from melting snow. In addition, there are often more days with fog and cloud cover. At lower elevations, where the majority of gardeners live, the sun can cause the soil temperatures to rise, injuring the roots in the process.
The Rhododendron Species Botanical Garden alpine area is situated on the east side of a ridge that runs the length of the garden, and most of the alpine rhododendrons are protected from the hot afternoon sun during the summer. Nevertheless, there is an additional problem with heat because of the infrastructure rock. This area was landscaped with 200 tons of granite from the nearby Cascade Mountain Range and these boulders absorb and reflect a considerable amount of heat before the sun moves west. To help compensate for this the area is watered often during periods of high temperatures and no rainfall, typically every other day.
Most species rhododendrons do not grow well in dense shade. The new growth tends to be very long and lanky, which can be unattractive. Also, the lack of light often reduces the potential for flower bud formation.
Rhododendrons grow best in well-drained soil. The roots require sufficient oxygen for growth and will suffocate and die if grown in areas that are constantly saturated with water. Plants with inadequate drainage will also be more susceptible to fungi which cause a disease commonly called root rot. The principal fungus that causes this disease in rhododendrons is typically Phytophthora cinnamomi, often referred to as Phytophthora.
To test the drainage of the site, dig a hole 15 to 20 inches (38 to 50cm) deep and fill with water. Watch to see how long it takes for the water to drain away. If the water remains for more than ten minutes the drainage is probably poor. Layers of clay are a common cause of poor drainage. Sometimes you can dig and break through to better draining soil or cut into the clay to create a drainage channel so the water will flow away from the root zone of the plant. If the foregoing is not possible, other solutions might be to build a raised bed or to create a berm.
Early blooming rhododendrons may be subject to spring frosts. Planting under a tree canopy may provide enough protection from mild cold snaps and shade the flowers from the morning sun. Beware of planting near trees with shallow roots because the competition is often detrimental. Sites near buildings walls, fences, or pavement can often be a source of intense reflected heat which can be damaging. Also, areas under the eaves of buildings usually receive little or no rainfall, and plants in these areas must be monitored year-round for moisture. Excessive wind is possibly another factor to consider in your area. You may need to protect your rhododendrons with screening such as a fence, wall, or other plants.
The pH of the soil should be slightly acid, or in the range of 5.0 to 6.0. Contact your local agricultural extension service for information on soil testing.
Organic matter should make up to 10 percent of the soil. If necessary, you can increase the organic content by using decayed leaves, compost, pine needles, decomposed sawdust, coarse peat moss, composted or processed manure, shredded wood, or bark that is salt-free. Add organic matter and gypsum to heavy clay to soil to improve drainage and reduce compaction. Add extra organic matter to sandy soil for moisture retention and nutrient-holding capacity. If fresh cedar, fir, or hemlock sawdust is used as an organic addition to the soil, one should add a nitrogen supplement such as sulfate of ammonia (21-0-0). This supplement is necessary because bacteria are breaking down the sawdust and in the process they are utilizing most of the available nitrogen in the soil.
At the chosen site prepare the area by digging and loosening the soil. The size of this area is determined by the size of the root ball, whether it is growing in a container or in the ground. If the upper surface is a foot or less in diameter, the area to prepare should be approximately three times as wide as that dimension and several inches deeper than the height of the root ball. For example, the size of the area to be prepared for a rhododendron growing in a container that has a diameter of 5 inches (12.5cm) would be 15 inches (38cm). For a plant with a root ball larger than 12 inches (30cm) measure the diameter and add 20 inches (50cm). Therefore, if the plant has a root ball with a diameter of 36 inches (90cm), the diameter of the area will be 56 inches (140cm). At this stage you may wish to add organic matter. Thoroughly mix any additions into the soil at the chosen planting site; this will aid in forming a transition zone that helps water flow through the root zone and away from it. If the soil is dry, add water and let it drain.
Most of the rhododendrons at the RSBG are growing in sawdust [in 1995, when this article was first published; since then soil renovation has been done in much of the garden]. In 1974, 37,000 cubic yards of sawdust were used to create the planting beds. After twenty years this sawdust is in various stages of decomposition. There are a few areas of the garden where one can dig down a foot or two and still find the sawdust relatively unchanged, but most of the surface sawdust has decayed. The decomposed sawdust has the consistency of mud, which is not conducive to drainage. In this situation there is little or no oxygen available to roots because of the lack of pore space. In areas where this problem is particularly severe, the decomposed sawdust will be removed and replaced with a better medium.
For planting a rhododendron that is growing in a container, first remove it and check to see if the root ball is dry. If it is, soak in water until thoroughly moist. Plants that are root- or pot-bound should be scored, that is, scratching the outer surface of the root with your fingers or a sharp instrument. Follow these steps for planting:
1. Dig a hole and place the plant with the top of the root ball slightly higher than the surrounding ground surface.
2. Look at the rhododendron and determine if it has a "best" side, in other words, a side with a more attractive shape, such as the foliage being fuller. Turn the plant so that side faces the direction from which it will be seen the most. For smaller plants this step may not be as important as with more mature specimens.
3. When you are satisfied with the placement, bring the soil up to the edge of the root ball and firm in gently with your hands.
4. Water in well.
5. Mulch the root zone with medium to coarse bark up to two inches (5cm) deep, leaving clear an area of about an inch or two around the trunk to prevent rot. You may want to stake the plant if it is over 3 feet (1m) tall or has a less than substantial root system.
Pay special attention to root-bound plants after they are planted to be sure the root system stays moist because they have a tendency to dry out until they are established. In mild climates, container grown rhododendrons can be planted any time of the year if the ground is not frozen. If they are planted during a warm, dry period, monitor them carefully for water.
The roots of rhododendrons are shallow, form a compact and fibrous mass underneath the plant, and extend beyond the dripline. The dripline is the edge of the canopy of leaves of a plant. Rhododendrons have no taproot which makes them relatively easy to move. Before moving, the root zone should be moderately moist. For plants growing in the ground, begin digging a minimum of 2 inches (5cm) beyond the dripline. Of course, the larger the root ball you can dig, the better for the plant. Push the shovel straight down and work all the way around in this manner. If the root ball is larger than 3 feet (1m) across, dig a small trench a minimum of 2 inches (5cm) beyond the dripline. Next, push the shovel under the root ball, gently lifting it up as you go around again until it is completely free. If the root ball is very large, use a long metal shaft to loosen the area underneath the trunk of the plant where the shovel cannot reach. For moving large plants have one person lift one side of the root ball and the other work a tarp underneath as far as possible. Then switch sides, lifting the root ball and pulling the other half of the tarp through to the other side.
After digging out the rhododendron, follow the steps in the paragraph above under "Planting." For a plant being moved on a tarp complete step two before you remove the tarp.
The optimum time for planting and transplanting is autumn for many areas, particularly those with mild climates. Spring is the primary time for areas with cold winters.
Rick Peterson is Co-executive Director and Garden Manager of the Rhododendron Species Botanical Garden in Federal Way, Washington.