Monrovia - Horticultural Craftsmen

Maintaining Your Rhododendrons

Proper planting & maintenance is necessary to enjoy some spectacular blooms.

Rhododendrons

Rhododendrons offer some of the most spectacular blooms and foliage in all of nature. It is a diverse genus with nearly one thousand species found all over the globe. There are varieties found naturally in many extreme environments–from Malayasia to the Arctic Circle, from the Himalaya Mountains to the Florida sand dunes, from the Appalachian Mountains to the cultivated gardens of Europe. Although no rhododendrons are found natively in Michigan, they are very adaptable to our landscape. With a little planning on location and an analysis of the garden soil (making necessary adjustments), rhodies do very well in our state.

Drainage and pH Levels

Proper planting is essential for rhododendrons. That includes proper drainage and soil pH of 5.0 to 6.0 as well as a suitable exposure. To test your soil for good drainage, simply dig a small hole and run some water into it. If it does not disappear in a few minutes, you have poor drainage. You may plant a rhododendron on that site by making the following adjustment per plant.

1. Loosen the existing soil in an area 30 inches wide by 6 inches deep. Do not remove the soil.
2. Add organic material using one bushel of sphagnum peat moss and one bushel of shredded pine bark.
3. Mix the peat and bark with the existing soil, adding one cup of iron sulfate and mixing throughly.
4. Plant the rhododendron in the center of the mixture, taking care to plant it at the same level it was previously grown. Do not plant it deeper.
5. Mulch the mount with shredded bark.

With this extra care, you will ensure that the plant has adequate drainage and an acidic pH. Consequently, the rhodie will remain healthy and much less susceptible to insect and disease problems. If your soil is naturally well drained and acidic, you may add some peat moss and begin planting. Be sure to mulch.

Exposure

Another aspect to proper planting id exposure. Generally, rhododendrons to best on the north side of a building or under a grouping a mature evergreens, protected from the winter sun and wind. However, there are rhododendrons tolerate exposed locations. But please consult with your garden center staff for specific recommendations.

Choosing the Proper Plant

It is important to select hybrids hardy to your particular area. Each hybrid is hardiness rated by the American Rhododendron Society. Most growers' price list include these ratings. For Michigan, you should use H-1 plants (hardy to -25º F) and H-2 (hardy to -15ºF) in more protected locations. Consider the various microclimates in the landscape area near water. When comparing minimum temperatures under mature pines versus the open cold, there is a microclimate consistently 4-6 degrees Fahrenheit warmer nightly in the pines. These areas are a good place to plant the H-2s, and they are worth the extra effort. H-2s are used by a knowledgeable landscaper on the Michigan-Wisconsin border of the protected areas, the rhodies are thriving.

Low Maintenance

Rhododendrons do not need as much water and fertilizer as you would think. As long as they have been mulched, watering the rhododendrons once (sometimes twice) a week very thoroughly and letting them dry out in between waterings should be plenty in even the driest weather. Even in the best-drained soil, watering every night for a short period of time will almost certainly lead to phytopthora problems. As for fertilizing, we recommend a slow-release rhododendron fertilizer applied yearly in the spring at the lowest rate in cases. Sulfur-coated fertilizers give the additional benefit of acidifying the soil.

Pruning

After a rhododendron blooms, the old blossoms should be removed so the plant's energy is spent on growth rather them seed production. Shaping for a fuller plant can be accomplished at this time by removing the terminal bud of each branch with the result being multiple branching with shorter growths. A more severe trimming method should be done after June 15.

Pests

Black vine weevils are the most destructive insect pest. A saw-toothed chewing on leaf margins indicate the presence of adult weevils. While the leaf damage is unsightly, it is the larva of the weevils feeding on roots that is deadly. A new biological control, a nematode, is a available for homeowners. See your garden center for details.

An occasional caterpillar can chew holes in soft new leaves. BT (Bacillus thuringeinsis) or insecticidal soaps will usually take care of the problem.

Remember, after proper installation, rhododendrons thrive better on a little care rather than a great deal of care. By following these guidelines, plants, once established, become self sustaining and among the easiest to maintain in the landscape.