By Kevin Wilcox – Rhododendrons have had a tough year. This spring, many have turned completely brown or else the leaves of specific branches have turned brown even though the rest of the shrub looks healthy. Why the sudden appearance of damaged leaves? The evergreen leafed species of Rhododendron are just that, they remain green throughout the winter months. Winter’s colder temperatures keep the leaves’ green chloroplasts from turning brown even after they are damaged. But as temperatures increase in early spring, the damaged cells try to continue their assigned biological processes only to find they cannot, and subsequently, they die and turn brown.
The damage can be traced to one or more of three problems: heat stress from last summer, infestations of Rhododendron stem borers, and/or our harsh winter weather from these past few months. So, before you prune or hack your rhodies to the ground, try to assess the problem. In many cases, the plants can be saved, though they may be set back some.
Rhododendron are shallow rooted, with their roots mostly growing in the organic-rich layer on top of the soil to maybe two or three inches down into the soil, so they are highly susceptible to damage from extremes of heat, cold, rain, and drought. This past year we saw all four extremes, with one after another increasing the stress on our plants. June was a deluge, followed by a hot, dry July. This heat killed many small roots in the topmost layer of soil, preventing rhododendrons from adequately absorbing water and nutrients. It is during July and August when rhododendron are finishing their yearly growth, forming flower buds for spring, and getting ready for winter. The excess of heat may not have prevented these shrubs from completing their biological preparedness for winter, but it did ensure that many plants started winter with unresolved stress-related problems. Those plants with the highest degree of stress are the plants that are now dead.
Frigid temperatures, drying winds, heavy snow loads, and intense sun light can all impact a plant. And this past winter we had it all. Cold temperatures resulted in the snow staying around and not melting. Last but not least, we had many days with bright sunlight that reflected off the snow and burned the leaves of rhododendrons. Had the snow melted between storms, the damage to rhododendrons would not be bad, or nearly as bad as what we are now seeing across the state.
Rhododendron are often planted in afternoon shade so hot summer sun won’t burn the leaves. These plants are therefore more susceptible to winter leaf burn because the sun reflecting off the snow reaches foliage that is not usually exposed to such intense light, resulting in a light to medium browning of their leaves, especially on the south facing side. The damage was principally to the leaves and not stems of the plants, so when it is time for new growth to emerge, it will. Old, browned leaves will drop off and be replaced with fresh green leaves. To test this theory, you can check the stems to see if they look full and plump, or wrinkled and dry. Plump stems will also have growth buds that will easily snap off. Those buds are still alive. Dry looking stems will have buds that will take some effort to break off the stems. Those buds are dead. You may also find flower buds dried and brown in their center, but if you’re lucky, they will still be green. In some cases, plants with a single branch of browned leaves may have been damaged by the weight of snow, which could bend the stems enough to cause vascular damage.
The other cause for dead branches on an otherwise healthy looking plant could be an infestation of Rhododendron stem borers. Let’s assume you are seeing damage from stem borers. The adult borer is a moth, which lays eggs typically at the base of the shrub or at the bottom of the v-crotch of two branches. The eggs hatch and the immature caterpillars bore to the center of the stem and tunnel their way up the inside of the stem. Once you have the insect inside the stem, there is no chemical control, but you can snake a thin piece of wire into the entry hole and skewer the insect. If you search for and find bore holes, complete with saw dust-like material, it would be best to contact the Conn. Agricultural Experiment Station, either in Windsor, or New Haven. You can find them at www.ct.gov/caes/site/default.asp. The staff at the Experiment Stations is extremely helpful and will explain how to find and treat the plants with insect damage. Rhododendron stem borers are not typically deadly, but an untreated infestation can be troublesome.
For now, be patient. Check to see if the stem tips of your rhododendron are still alive, look for physical damage and remove any broken branches, and keep an eye out for stem borers. Most rhododendrons will begin to grow in the next few weeks, showing you where they may need to be pruned, or that they don’t need to be pruned at all. Any brown leaves will drop off as the new growth emerges. If you feel the need to fertilize, do so sparingly and with something organic instead of the blue colored liquid soluble fertilizers. Placing a layer of mulch or compost 2-3 inches thick under your rhododendron will help keep its roots cool and moist this summer. And, don’t forget about your rhododendrons when the weather turns hot and dry; if nature doesn’t provide any rain, a little bit of water each week will reduce their stress.
Kevin Wilcox is the owner of Silver Spring Nursery in Bloomfield, and is a member of the CHS Board.