Q. How do I plant and grow rhododendrons?
Rhododendrons can be purchased from most nurseries. They are usually sold in containers. Each plant should be examined carefully to see if it is pot-bound. If it has been grown in one container for a long time,then the roots will have formed themselves into a tight hard mass and will be difficult to establish in the garden. Conversely, plants that have been growing in the container for a short time are also problematic, as they will not hold soil when they are removed from the container.
Rhododendrons can be planted at any time from spring to fall, but early fall is probably the best time. However, one advantage of buying a plant in the spring is that you can preview the flower before planting. Note that rhododendrons will often not re-bloom for a year or two after planting. The particular species or variety that you choose will depend on your hardiness zone and the design plan of your garden. Rhododendrons are available in a large variety of colors, ranging from white to purple to bright red and even yellow. Azaleas are usually deciduous and rhododendrons are usually evergreen.
Rhododendrons grow well in moist, well-drained soil with adequate fertility. They do not tolerate lime, and the preferred pH is between 4.5 to 6.5, i.e. slightly acidic. Even planting along cement foundations and concrete driveways and paths can be a problem if calcium leaches into the beds.
It is very common for newly planted rhododendrons to grow well for a while and then start dropping their foliage, become yellow, and die back. This can nearly always be traced back to root rot caused by poor drainage. You can check the drainage conditions of your site by digging a 20-inch-deep hole and filling it with water. If the water disappears within 4-5 hours, the drainage should be adequate. The bed can be enriched with peat, well-rotted leaf mold, or other organic material to ensure good drainage. The planting hole should be about the same depth as the root ball.
It is very important not to plant too deeply. The top of the root ball should be level or slightly above the existing soil level. In poorly draining beds, the rhododendron should be planted above the soil level. The soil placed around the plant should be firmed (but not too much) to prevent the plant from sinking later. Some slow-release fertilizer can be added to the soil, but soluble fertilizer should be avoided. Water the plant immediately after planting, and if necessary construct a berm to temporarily hold the water. Staking of rhododendrons is rarely necessary unless they are top-heavy. It is a good idea to mulch around the plant to retain moisture and to prevent freezing in the winter. Organic mulches are best. Only coarse material such as wood chips, shredded bark, coconut coir, rotted leaves or pine needles should be used. Subsequently, apply mulch each year in late fall or early winter to a depth of about 3 inches around the plant. It is important not to mound mulch against the main stems of large rhododendrons.
As discussed earlier, rhododendrons will not tolerate stagnant conditions, but they require a plentiful supply of water during the growing season. Newly planted plants require close attention to watering; they should not be allowed to wilt for any period of time. Fertilizing rhododendrons is rarely necessary, especially if mulch is applied regularly around the plants. Poorly flowering rhododendrons can be given sulfate of ammonia and superphosphate early in the spring to encourage the formation of flower buds. Rhododendrons rarely require pruning. Diseased or dead branches should be removed periodically. If necessary, pruning should be done in March or April, but this leads to the loss of one year’s flowers. Many gardeners prefer to wait until late spring, when the flowers have died. Deadheading, although tedious, will result in greater bud set and enhanced flowering the following year. Greatly overgrown plants that have become open and ungainly can be regenerated by cutting back the whole plant to within 1-2 feet of the soil. If this seems too drastic, you can cut back 1/3 of the plant each year for 3 years. Finally, if you don’t like the location of your rhododendrons they can easily be moved to another site, as rhododendrons tolerate transplanting very well.
For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service