Why Are the Leaves on My Rhododendron Rolling Up and Wilting?

Depending on the time of year, the rolling and wilting of your rhododendron leaves could be caused by cold temperatures or a disease called rhododendron wilt. During the winter, rhododendron leaves often droop and curl in response to cold temperatures. It’s their way of protecting themselves from dehydration. If you see drooping and rolling of foliage during the growing season, it’s likely that your rhododendron has a wilt disease caused by the soilborne fungus phytophthora. Phytophthora is most often a problem in poorly drained, wet soils (such as that near a downspout).

Symptoms include stunted growth, leaf yellowing, and drooping leaves. Infected roots are dark and mushy instead of light tan and firm. Plants may be killed by rhododendron wilt. Fungicide treatment is ineffective. A better solution is to change the growing conditions. Improve soil drainage and aeration if you want to continue growing the plant in the same location. Incorporate compost or other organic matter to loosen heavy soils. Consider installing a raised bed to improve drainage, and transplant your rhododendron into the amended raised bed.

Charlotta Wasteson / Flickr Creative Commons

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.

Heat Stress

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.

Winter Damage

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.

Stem Borers

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.

Watchful Waiting

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.

Top Causes of Death in Rhododendrons

Jan D. Kelley
Drain, Oregon

Our hopes soar with the coming of spring as we anticipate another excellent growing season for our rhododendrons with their exquisite flowers. As we ponder the fantastic new hybrids in the pages of the several catalogs that we receive our vision of being successful gardeners bursts forth. However, as you reflect upon last year’s plant losses a ray of doubt creeps into your consciousness, and the nagging question emerges: “Why did that plant die?”

For the past 15 years I have enjoyed raising rhododendrons. During that period of time I believe that I have killed rhododendrons in every conceivable way. In the remainder of this brief article I would like to identify some of the various ways that rhododendrons succumb in our yards and gardens. My experience indicates that most rhododendrons die from about seven causes.

To begin with, excessive water kills about 75 percent of all rhododendrons purchased. Rhododendrons are fibrous, shallow rooted plants that need good drainage to perform well. Historically, gardeners have been told to dig a hole twice as wide and twice as deep as the root ball. After the hole is completed put the plant in the hole and back-fill it with a mixture of peat, soil and other amendments. Many rhododendrons die from this guidance. The result of digging the hole and planting the rhododendron in it is nothing more than putting the plant in a bathtub that holds excessive water. The continual presence of water around the root ball prevents the roots from taking in vital oxygen as well as serving as an excellent incubation chamber for fungus diseases. It seems that most of the native soils around the country have an excessive amount of clay in them. The presence of clay in the soil prevents good drainage, which is vital to the growth of the rhododendrons. Anyone who has ever been to the several locations around the world where rhododendrons originate knows that rhododendrons grow in shallow beds of highly organic matter. The drainage is typically excellent.

Another cause of rhododendron death is lack of water. Rhododendrons do not have taproots like trees: their roots grow very near the surface. Therefore, they need frequent watering. The acquisition of new plants in the spring requires regular watering. During the first couple of years watering the plants at least twice a week is a must. As the time goes by and the plants increase in size and root development, watering less frequently works well. After about five or six years it is possible to water weekly or even bi-weekly. Frequently sunburned leaves are the result of the lack of water. For many varieties that have burned in the sun in the past, the cause was lack of water not too much sun. Burned tips on this year’s new growth is typically indicative of lack of water as the plant withdraws water from the tips of the new foliage first.

Another cause of rhododendron death is the excessive application of fertilizer. This is particularly true of applying fertilizer directly at the base of the trunk of the plant. A good rule of thumb is to fertilize more frequently with smaller amounts, rather than one large dose. This is especially true for small plants or newly transplanted plants.

A fourth reason for rhododendron death is planting too deep. As indicated earlier, rhododendrons are shallow rooted plants. Their roots grow just below the soil line. If they are placed too deep in the ground, the soil that covers the roots serves to smother them. I have found that planting too deep will basically stop the plant from growing. Eventually this leads to the death of the plant.

Another reason that rhododendrons die is from cold winter temperatures. Most rhododendron sources indicate the lowest temperature range in which rhododendrons can be successfully grown. This hardiness rating is a guide not an absolute! In general, the lowest temperature during the past five years is a good guide for making selections based on hardiness. Years ago there were very few plants that were hardy in -25°F for the extreme climates. Now we have over 100 varieties that will survive those winter temperatures. Gardeners in the East should select hardy varieties in the beginning. With time and experience less hardy varieties can be successfully tried. A rhododendron rated hardy to 5°F, no matter how beautiful it is, planted in Green Bay, Wisconsin, will not survive.

As more and more home gardeners in the Southern and Midwestern states begin to grow rhododendrons, increased attention must be paid to the hot summer sun. Most varieties exposed to unprotected all-day sun are doomed. However, there are available rhododendron varieties that can stand direct sun. In general rhododendrons in extreme climates benefit from filtered light and partial shade. Planting in a southern exposure without any protection from the sun nearly guarantees plant death.

Finally, if you create the right conditions most rhododendrons will be subject to fungus diseases. Typically we combine several fungus diseases into a general category of “die-back.” The results of the disease are seen during the late spring when the plant is just beginning to grow and all of a sudden it drops dead. It is also seen during the summer when a branch turns brown and dies. Frequently the ailing plant will be lost. These phytophthora-type diseases are generally the result of conditions created by the gardener, as it is believed that the disease spores are present in the soil all over the country. Some of the ways that we promote these organisms is by planting the rhododendron too deep, thus providing a water culture for the development of the disease organisms. Puddles of water that remain more than an hour after watering also harbor disease. Watering in the late afternoon or evening encourages disease development. Finally, failure to use fungicides during the late spring and summer encourages the development of fungus.

In conclusion, you are not alone if you have lost plants to any of the above mentioned causes of rhododendron death. Most of the causes can be overcome with the intelligent selection of plants that are suited to your geographical area. Finally, think about where and how you planted your rhododendrons and what you did to promote their death.

Rust

As you might guess, the collection of plant diseases known as rust take their collective name from colorful infections that happen when rust fungi infect plants. Many different fungal species cause rust, with varying modes of operation. Some rust diseases target a single, specific type of plant. Others, such as cedar-apple rust, alternate between two plant hosts.

Moist weather and cool to moderate temperatures provide ideal conditions for rust infections. The windborne spores can’t germinate or infect plant tissue without water. But once infection occurs, rusts diseases can persist even in drought. Though rust rarely kills plants, it lowers flower and crop production, increases vulnerability to insects and other diseases, and makes previously healthy plants unsightly.

Rust Identification/Symptoms: Rust diseases go through different stages, but they all eventually form blister-like, powdery pustules. The pustules typically occur on the undersides of leaves, but stems and upper leaf surfaces can develop pustules, too. Most affected edibles and ornamentals show rusty yellow, orange and brown pustules, but some types of rust appear purple-brown or even black.

Affected leaf areas turn yellow and brown as infections expand. Distorted leaves turn dry and fall from plants. In hollyhocks, heavy infestations leave entire plants rusty orange. Geraniums develop cinnamon-colored pustules underneath leaves, with circular yellow spots on top. Hydrangea foliage shows dark, coppery leaf spots and red-orange pustules on leaf undersides.

How to Control Rust: In the early stages, rust diseases often sneak by unnoticed on greenhouse cuttings and transplants. Successful control combines good cultural practices to limit moisture on foliage with effective preventive treatment to protect healthy plant tissue from rust infections.

Daconil® fungicides from GardenTech® brand offer highly effective, three-way protection to stop, control, and prevent rust and more than 65 other fungal diseases. Start preventive treatment whenever moist, moderate conditions favor the disease’s development or at the first hint that rust is present. These products treat edible and flower gardens as well as shrubs and trees:

  • Daconil® Fungicide Ready-to-Use simplifies precision treatments for small-scale gardens, including containers and individual plants. Use the convenient, grab-and-go spray bottle to treat all upper and lower plant surfaces until thoroughly wet.
  • Daconil® Fungicide Concentrate provides an economical treatment option for larger garden areas. Pour the recommended amount into a hand-held, hose-end or tank-style sprayer with the easy-to-use measuring cap. Then add the recommended amount of water, and mix well. Spray all plant surfaces, upper and lower, to the point of runoff.

Rust Tips: Use drip irrigation or water plants close to the ground to limit wet leaves. Water early in the day so excess moisture dries quickly. Immediately remove and dispose of rust-affected foliage or plants during the growing season. And don’t skimp on thorough fall cleanups—dormant rust fungi overwinter on fallen leaves.

Always read product labels thoroughly and follow instructions, including guidelines for treatable plants, application rates and frequencies, and pre-harvest intervals (PHI) for edible crops.

GardenTech is a registered trademark of Gulfstream Home and Garden, Inc.

Daconil is a registered trademark of GB Biosciences Corp.

Photo Credit:

Whitney Cranshaw, Colorado State University, Bugwood.org (CC BY 3.0 US)

Whitney Cranshaw, Colorado State University, Bugwood.org (CC BY 3.0 US)

Cesar Calderon, Cesar Calderon Pathology Collection, USDA APHIS ITP, Bugwood.org (CC BY 3.0 US)

Richard Webb, Bugwood.org (CC BY 3.0 US)

Symptoms

All rust fungi produce powdery masses of spores, typically on leaf undersides that are yellow, orange, purple, black or brown. Some rust fungi produce pustules on upper leaf surfaces as well. Spores are easily spread on air or with splashing water.

What will happen to my landscape?

Lesions may occur resulting in large areas of leaf distortion and defoliation may follow.

What can I do about rust?

Periodically prune all rust-infected leaves and dispose. At the end of the growing season, carefully clean up and destroy all leaves that have dropped. Space plants far enough apart to allow for good air circulation. A combination of cultural and chemical control is often required. Most local hardware stores carry fungicides that will help to control Rust. Because these sprays can cause injury to some plants, read the label thoroughly before using and apply according to the directions. Depending on the severity of the damage, more than one fungicide application may be required for complete control. When combating Rust its best to think of it as treating allergies, something that will never go away, but having fewer symptoms each year by having regular treatments. Combating Rust can be a timely and unpleasant weekend task.

How can Natural Way help?

Prevention is the key in controlling Rust. Here at Natural Way we have certified professionals who are used to preventing and controlling diseases like Rust. Our 6 Point Protection Program will provide several critically timed sprays to the landscape that will help control and prevent Rust, extend the life of the landscape and minimize overall damage.

Even though rust will not generally kill a plant by itself, the plant or tree will become more susceptible to other problems and its appearance will be unsightly.

Call to schedule a free inspection for Rust today!

My Rhododendrons Are Losing Leaves This Fall – Is This Normal?

Rhododendrons and Azaleas, what is the difference anyway?

All Rhododendrons are evergreen which means that they will hold leaves all winter long. Most Azaleas on the other hand (under the same Genus of Rhododendron) will lose their leaves at the end of the season. The other difference is that true Rhododendrons have ten or more stamens, 2 per lobe and Azaleas have only five stamens – one per lobe and 5 lobes in a flower.

The above picture of a Rhododendron showing its nice red-purple fall color. Notice the older leaves will color and drop even though it is an evergreen. The newer leaves are at the tips of the branches and will be retained all winter long.

Azaleas typically have nice fall as well and many will lose their leaves later in the season. Sometimes Azaleas in warmer climates hold their leaves.

Rhododendrons appreciate good fall moisture and love to have a nice 3” layer of mulch over their roots. The older, more interior red-purple leaves will drop from the plant and it is a very normal occurrence.

In colder climates and where your plants are exposed to winter winds you may want to consider some wind protection or maybe a spray on anti-transpirant later in the season to prevent the leaves from drying out.

Both Rhododendrons and Azaleas are best pruned right after they bloom in spring only so hold off pruning this fall.

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