Protecting rhododendrons in winter

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.

Some of the branches on my rhododendrons were damaged over the winter. Can you explain why this happened and how it can be prevented in the future?

Over the last couple of weeks, there have been many reports of damaged rhododendrons. Most people have noticed that some of the branches on their plants have curled leaves that don’t open on warm days, indicating they have been injured in some way. In most cases much of the damage appears to have occurred at random. While a few scattered branches may have been injured on a plant, the rest are seemingly healthy. Even adjacent rhododendrons often fared differently, with one suffering significant damage, whereas the other is unscathed.

The most likely explanation for this phenomenon is winter injury. Broadleaf evergreens, such as rhododendrons, lose water through their leaves even in winter when the weather is relatively warm and sunny or during periods of high winds. When the ground is frozen, the roots aren’t able to take up enough water in the soil to make-up for what is lost through the leaves. The leaves curl and droop and may turn brown at the tips and edges. Curling their leaves is a protective mechanism against dehydration during cold periods, and normally the leaves unfurl when it warms up. More often than not, damage that occurred over the winter does not become apparent until the spring. This past winter was exceptionally windy by any standard, which likely precipitated the dieback noticed on many rhododendrons.

Explaining why winter injury often occurs in random patterns is difficult to explain. In many places there was very little protective snow cover on the ground. Snow acts as an insulator, protecting leaves that are underneath the snow, and helps moderate the soil temperature. The state of different rhododendrons may also have to do with variety. Many hybrid rhododendrons, which are very popular as landscape plants, are not as cold hardy as species types.

Repairing Damage

If you believe that your rhododendrons suffered from winter injury, the best thing you can do now is wait to see if any of the damaged branches survived. It’s possible to sustain leaf damage without the branch and buds being killed. If slightly scraping the outer layer of bark reveals green, the branch is still alive and likely to recover. Branches that don’t show any signs of life once growth resumes in the coming weeks and months should be pruned out. Buds are generally hardier than leaves and are much better at surviving extreme weather conditions. By the end of June, rhododendrons should have produced new leaves, at which point dead branches should be removed.

Prevention

In the future, you can try to prevent winter injury in a few ways. Proper care during the growing season is a crucial part of keeping rhododendrons alive through the winter. Providing adequate water is essential. For optimum growth, most rhododendrons require one inch of rainfall or supplemental irrigation every week. Fall watering is extremely important and should continue until the temperature drops below 40 degrees. Also avoid fertilizing after mid-September because it may delay dormancy.

Many winter injury issues can be solved by choosing appropriate plants. Hardiness is the first thing to consider. Rhododendrons should be hardy enough to survive in the zone they are planted without too much extra care. In most of New Hampshire, this means selecting plants which are hardy in zones 3-5.

Location is just as important as plant selection. Since harsh winter winds and sun can damage rhododendrons, they should be planted in partially shady areas where they are protected from prevailing winds. Generally speaking, avoid planting in dry soil, full sun, or on exposed windy sites. Avoid exposed southern or western sites where winter sun and wind will cause the most damage.

Other important measures include mulching or using physical barriers such as burlap to block the wind. Mulching rhododendrons, especially those that have been newly planted, insulates the soil and protects the plant’s roots. At least two inches of woodchips or straw should be applied over the root zone, taking care not to pile the mulch against the trunk. Creating windbreaks from materials such as burlap or canvas may also help.

Do you love learning about stuff like this?

Subscribe to NH Outside with Emma Erler

These experiments also revealed something else about the changes in leaf position when it comes to shape. As it turns out, curling made no difference in protecting the leaves from light damage. It would seem that drooping and curling are responses to two different types of environmental stress. So, why do the leaves curl?

The answer to this question is physical and one that has gained a lot of research attention in the field of cryogenics. When living tissues freeze, ice crystals build up to the point that they can rupture cell membranes. This is only exacerbated if the tissues thaw out quickly. Anyone that has ever tried to freeze and then thaw leafy vegetables knows what I am talking about.

To best preserve tissues via freezing, they must freeze quickly, which reduces the size of the ice crystals that can form, and then thaw out slowly. Researchers found that Rhododendron leaves freeze completely at temperatures below -8 degrees Celsius (17.6 degrees Fahrenheit), temperatures that occur regularly throughout the range of temperate Rhodo species. Again, experiments were able to demonstrate that flat leaves thaw much more rapidly than curled leaves. This is because a curled leaf exposes far less surface area to the warming sun than does a flat leaf. As such, curled leaves don’t thaw out as fast and thus are able to avoid much of the damaging effects of daily freeze-thaw cycles.

Though these are all components of the Rhodo leaf puzzle, there is still much work to be done. What we do know is that leaf drooping and leaf curling are two separate behaviors responding to different environmental pressures. Indeed, it appears that these two traits seem to be tied to cold hardiness in the genus Rhododendron. What the exact triggers are and how they produce the changes in shape and orientation, as well as the mechanics of winter survival at the cellular level are topics that are going to require further study. Until then, I think its safe to say that we can appreciate and, to some degree, rely on the spot forecasting abilities of these wonderful shrubs.

Photo Credits:

Further Reading:

Rhododendron P.J.M.

Size & Form

A hybrid evergreen rhododendron reaching 3 to 6 feet high and wide.

Tree & Plant Care

Best planted in spring.
Prefers a moist, acidic, organic-rich soil. Will tolerate a higher soil pH , but well-drained soil is a must.
Locate in part shade to full shade in a protected site out of winter winds.
Plants are shallow-rooted and benefits with a layer of organic mulch to conserve moisture and moderate soil temperature fluctuations.
Water in dry periods.
Pruning is rarely needed but faded flowers can be removed to improve appearance.
Avoid pruning after July when new buds are forming for next years flowers.

Disease, pests, and problems

Root rots and crown rots can occur in too wet a soil.
Black vine weevils

Attracts birds, pollinators, or wildlife

Spring flowers attract butterflies including the giant swallowtail, pipevine swallowtail, and spicebush swallowtail.

Bark color and texture

Light straw-colored brown

Leaf or needle arrangement, size, shape, and texture

Smooth, dark green, 1 to 2 1/2 inch, elliptical, leathery evergreen leaves turn a mahogany to wine-red in fall

Flower arrangement, shape, and size

Lavender-pink flowers bloom in clusters of 4-9 flowers in late spring.

Fruit, cone, nut, and seed descriptions

Plants set little or no seeds

Rhododendrons – Pruning, Winter Care and Fertilizing

These plants should be pruned in late spring, after they have flowered. However, very little pruning is required. Remove crossing or crowded side branches back to a main branch. Removal of spent flower heads is important for promoting next years bloom as well as a good overall shape. Spent flower heads should be taken back to a side branch.

Pay special attention to this plants fertilizing and winter care needs. This plant grows and develops best in acidic soil. Most of the soil, all of the rainwater and all of the ground water in southeastern Wisconsin is basic or alkaline. Therefore, for some plants to thrive, they need to have special care.

Initially, the soil the plant is installed into should be amended to make it more acidic and to increase the amount of organic matter in the soil. Over time, the soils acidity should be supplemented with an acid-based fertilizer. A liquid fertilizer, Miracid works well. This liquid fertilizer is mixed with water and applied the same as you would water the plant (see product for specific details). This type of fertilization should be done three or four times per year, starting in April and ending by the mid-July. Soil can also be acidified with soil sulfur, aluminum sulfate or cotton seed meal, however, these products will not fertilize the plant. Mulch is also important to acid loving plants. A 2-4″ thickness of bark or needle mulch should surround the plant at least as far out as its drip line.

PJM type rhododendrons in most years will not require extra winter care. For the other years protection from the winter sun and wind is the most important measure you can take. This is best accomplished by wrapping the plant in burlap, or enclosing it in a burlap tent. This form of protection will keep the wind and sun off, but will not cut off air flow or trap heat, which can be harmful to the plant. A burlap tent is made by setting three wooden stakes into the ground around the plant and stapling burlap to it on all three sides. The top of this tee-pee like structure should be left open. Non PJM rhododendrons do require extra protection on a regular basis.

These plants should be mulched at the base with 3-4″ of compost, bark or straw. Add winter protection for this plant in late November and remove it in mid April. Watering in late fall is very important for the winter health of any evergreen and should be done to young plants every year in early November. Even more mature plants can benefit from late season waterings, especially after a dry fall season.

Damage to rhododendron and azalea leaves and buds can occur at low temperatures. Exposure to cold can cause dry, brown areas on leaves and brown-colored buds. Sometimes new plant growth may not have had sufficient time to harden off before cold weather sets in and may be killed. Give the plant plenty of time to send out new growth as temperatures warm before pruning off the damage. Warm temperatures followed by a quick freeze, such as early freeze in the Fall or a late freeze in the Spring can be more damaging than a gradual drop in temperature.

USDA hardiness zones (and similar maps in other countries) can be consulted to determine annual minimum temperatures at your location. Where your garden is situated in a hardiness zone and the garden’s specific details are important, as local microclimates may be different than the general hardiness zone you live in. Cold hardiness estimates provided by the ARS for selected rhododendron and azalea species and hybrids can be used to find plants suitable for your garden. The ratings give an indication of minimum temperatures that a well-established plant can be expected to survive without damage. Certain rhododendrons and deciduous azaleas can survive temperatures as low as -35°F (-37°C).

It is normal for some rhododendrons to exhibit leaf droop and curl at around 32°F (0°C).; the lower the temperature the tighter the curl. Leaf movement occurs rapidly and it is reversible as temperature warms.

Rhododendrons and evergreen azaleas lose some of their leaves each year. Typically leaves are retained for one to three years dependent on the variety. Leaves may turn yellow, red, or purple before they fall off. For some rhododendrons and azaleas the retained leaves turn red or bronze-colored in the cold months. In some azaleas the only leaves remaining are those that surround the flower buds at the tips of the branches. The degree of leaf coloration or loss is determined by a plant’s genetics. In cold areas, certain evergreen azaleas may lose more leaves than they would in milder areas.

Drying winds and frozen ground deprives plants of their natural moisture intake. A good soaking in the late fall before freezing and a good mulch will greatly help a plant’s survival. In cold climates, rhododendrons and evergreen azaleas can benefit from an application of an anti-desiccant, such as Wilt-Pruf. Read and follow carefully the manufacturer’s instructions. Spray in late Fall when temperatures are near 40°F.

Even with recommended varieties, plant performance will be improved with reasonable protection from drying winds. In some windy areas gardeners protect rhododendron plants by building a windbreak around them or screening them with burlap or other protective material during the worst part of the cold season. Rhododendron or azalea especially valuable to you can be protected with a mesh enclosure filled with oak leaves, or with a teepee-like structure constructed using three or four evergreen branches with the points forced into the ground and other ends tied together, or by snow fencing alone or with a polyethylene plastic sheeting attached to it. Protect the plants just before freeze occurs and remove the protection after all the frost is out of the ground.

Among the major types of rhododendrons (big leaf and small leaf) and azaleas (evergreen and deciduous) there are differences in their tolerance to various weather conditions. Generally large-leaf rhododendrons are less tolerant of sun and wind than small-leaf rhododendrons and evergreen and deciduous azaleas. Planting locations with early morning or late afternoon sun or dappled sun throughout the day from an overhead canopy or a shade structure, protection from high winds and proper watering can minimize leaf sunburn and wind damage problems. Generally, the east and north sides of the house are better locations than the west and south. Some varieties will not tolerate full sun, developing quite yellowish leaves under such conditions. There are others that become a better shaped plant if grown in a location with lots of light. Deciduous azalea species are very heat and humidity tolerant, and are widely grown by gardeners in the mid Atlantic and southeastern regions of the U.S.

There are many exceptions to the above so an awareness of what specific rhododendron or azalea you want to grow and and attention to where you want to plant them are important if you want to be successful. With hundreds of different rhododendrons and azaleas to choose from you are sure to find plants suitable for most climatic conditions.

For further information on protection of rhododendrons and azaleas consult the following Journal ARS articles:
Winter Desiccation Injury of Rhododendron by John R. Havis
Flower Bud Hardiness Of Rhododendron Taxa by Harold Pellett, Susan Moe and Wayne Mezitt
Cold Hardiness Ranking of Rhododendrons By Means of Flower Bud Damage by Russell Gilkey
Notes on Winter Hardening Rhododendrons
Causes and Significance of Winter Leaf Movements in Rhododendrons by Erik Tallak Nilsen
Rhododendrons and Hot Weather by George W. Ring
Growing Rhododendrons In The Gulf South by John T. Thornton
Rhododendron Species Hardy in Southern New England by J. Powell Huie
Azaleas Suitable for Different Areas in Connecticut by By J. W. Oliver
Growing Azaleas In New England by Charles W. Findlay, Jr.

Index of Contents

Classification

Plant Selection

Landscape Use

What To
Plant Where

Soil
Conditions

Planting

Irrigation

Fertilization

Pruning & Spent
Flower Removal

Propagation & Hybridizing

Transplanting

Insect & Disease Control

Leave a Reply

Your email address will not be published. Required fields are marked *