Rhododendron Winter Care: Preventing Cold Injury In Rhododendron Shrubs

Rhododendron Winter Care: Preventing Cold Injury In Rhododendron Shrubs

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By: Kristi Waterworth

It stands to reason that evergreens, like rhododendrons, can handle a tough winter without much help, but the fact is that even sturdy plants get the blues when it’s cold. Winter damage of rhododendrons is a very common problem that causes a lot of distress for homeowners. Luckily, it’s not too late for preventative rhododendron winter care.

Care of Rhododendrons in Winter

Caring for your rhododendrons through the cold season is easier if you understand how these plants are damaged to begin with. Cold injury in rhododendron is caused by too much water evaporating from the leaves at once, without anything to replace it.

When cold, dry winds blow across leaf surfaces, they tend to take a lot of extra fluid with them. Unfortunately, in the winter, it’s not uncommon for this to happen when the ground is frozen solid, limiting how much water can be brought back into the plant. Without adequate water levels in their cells, the tips and even entire leaves of rhododendrons will wither and die.

Preventing Rhododendron Cold Damage

Rhododendrons attempt to protect themselves from winter dehydration by curling their leaves, allowing them to hang down. This mechanism is often effective, but there’s even more you can do to help protect your rhodies from winter damage.

Because rhododendrons root much more shallowly than other plants, it’s extra important to keep a thick layer of mulch over this delicate system. Four inches of an organic mulch, like wood chips or pine needles, is often adequate protection from the cold. It’ll also slow water evaporation from the ground, helping your plant stay hydrated. Make sure to give your plants a long, deep drink on warmer days so they have a chance to recover from cold snaps.

A windbreak made from burlap, lattice or a snow fence can help slow those drying winds, but if your plant is already planted in a protected area, it should be safe enough from winter damage. A little bit of winter damage is ok; you’ll just want to cut out the damaged sections early in the spring so your rhododendron can get back into shape before the bleached leaves become an eyesore.

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Answer #2 · Maple Tree's Answer · ali-Great close up picture. The burning of the leaves is a definite indication of winter damage. Fortunately it does look as though a lot of new growth is appearing. I would wait a couple of weeks to see how much of the plant has survived. I'm thinking most of the plant will recover. In a couple of weeks I would cut out any of the old or damaged stems that have not sprouted new growth and remove any damaged leaves still on the plant. The plant looks as though it could use a heavier pruning to give it a more dense fuller appearance and clean out old stems that may be interferring with others. I think I would wait until next year after this years recovery to do any heavy pruning. There appears to be some spotting of leaves but not sure if this may be a fungal disease or not. Keep your eye on the leaves looking for black or brown spots and yellowing of the leaves appearing. This could be a fungal disease normally caused by to much moisture. Many have received a lot of rain this year so make sure the soil is moist but never wet or saturated. If spotting of the leaves appears a fungicide treatment may be needed. Fungal disease normally discolors the foliage but usually will not hurt or kill a mature plant. Let me know in a couple of weeks how the plant is doing.

Answer #1 · Maple Tree's Answer · Hi ali-I wouldn't assume your rhododendrons are dead yet. Unusually cold temperatures especially with some drying winds can do damage but not necessarily kill the plants. You can scatch small spots off the outer layer or bark of the stems to see if the underlying tissue is still green. If it is brown that portion of the plant is dead. I would only remove any browning leaves and dead stems. Moving down the stems cut out any dead growth until you reach areas of the stems that are still green. Always try and prune back to green wood cutting off the stem just above a dormant bud. If stems are still green and still living only the damaged leaves can be removed. The living stems will still sprout new leaves. Many times only the upper portion of the plants sustain most of the damage and the lower part of the plant will sprout new growth. The rhododendrons have shallow root systems therefore unusually cold temperatures can kill the entire plant if not mulched well or have a layer of snow as an insulator. Rhododendrons can also be cut back to within 6 to 8 inches of the ground if need be as in rejuvenation pruning. If you find only living tissue low to the ground I would most likely replace the plants. Many plants will recover from this drastic pruning but not all will survive. Drastic pruning such as this should be done in the winter when the plant is dormant. It may also be a couple of years before the plant will recover enough to produce flowers again if it needs this drastic pruning. At this time check to see how much of the plant is living and hold up on any fertilization at this time. Ferilization, forcing new growth, may possibly add to the plants stress. After new growth appears fertilization can be done.

Let me know what you find when checking the branching for living green tissue. You can also upload a picture of the plants if you would like. Above this answer and to the right of your name below your question you will see where you can upload any picture you have saved on your computer.

Plants→Rhododendrons→Rhododendron 'Firestorm'

General Plant Information (Edit)
Plant Habit: Shrub
Life cycle: Perennial
Sun Requirements: Full Sun to Partial Shade
Water Preferences: Mesic
Soil pH Preferences: Strongly acid (5.1 – 5.5)
Minimum cold hardiness: Zone 5a -28.9 °C (-20 °F) to -26.1 °C (-15 °F)
Maximum recommended zone: Zone 8b
Plant Height : 3 - 4 feet
Plant Spread : 3 - 4 feet
Leaves: Evergreen
Flowers: Showy
Flower Color: Red
Other: Deep red
Bloom Size: 3"-4"
Flower Time: Spring
Late spring or early summer
Uses: Windbreak or Hedge
Provides winter interest
Cut Flower
Wildlife Attractant: Bees
Resistances: Rabbit Resistant
Propagation: Other methods: Cuttings: Tip
Other: Tissue Culture
Parentage : Vulcan x Chocolate Soldier

» Search the Rhododendrons Database: by characteristics or by cultivar name

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Care for Rhododendrons and Azaleas (rhodazal)

Care for Rhododendrons & Azaleas
by Don Janssen, Extension Educator

Rhododendrons and azaleas require little care once they are established. A good heavy mulch will conserve moisture and keep down weeds. No cultivation should be done because the shallow, fine roots are easily damaged.

Avoid using general garden fertilizers for rhododendrons, azaleas and other acid-loving plants. Use those specially formulated for acid-loving plants and follow directions. Fertilizers supplying the ammonium form of nitrogen are best.

Fertilization should be done carefully, or the fine, delicate roots close to the soil surface will be damaged. A fertilizer analysis similar to 6-10-4 applied at 2 pounds per 100 square feet to the soil surface is usually adequate.

Fertilizing should be done in May, but do not fertilize after July 1. Late summer fertilization may force new growth that will be killed by the winter.

Soil acidity must be maintained to ensure good growth. If the soil pH is above 5.5, apply iron sulfate or agricultural sulfur to the surface of the soil. The amount to apply will depend on the existing pH, but in all cases apply only a small amount at any one time.

The thickness of the mulch (2-3 inches) must be maintained. As the old mulch decomposes, add new mulch. This is best done in the late fall.

Many rhododendrons have been killed by overwatering in sites where drainage is bad. Check the soil before watering. Avoid excessive irrigation in fall. Plants kept dry in September will tend to harden off and be better prepared for the winter. If the fall has been excessively dry, watering should be done after the first killing frost.

There is little need for pruning azaleas and rhododendrons. If growth becomes excessive, reduce the size with light pruning. It is important to remove the flower stems on rhododendrons as soon as flowering is complete. Failure to do this will reduce flowering the following year. Break out only the dead flower cluster, not the young buds clustered at its base.

The two winter enemies of evergreen rhododendrons and azaleas are sun and cold wind. If hardy types are selected and proper planting locations are chosen, little or no winter protection is needed. If existing varieties show winter damage, provide some protection. Don�t be alarmed when leaves curl and droop on cold days that is normal.

(This resource was added May 28, 2006 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement)

University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office

When And How To Prune Rhododendrons And Azaleas

Rhododendrons don’t need regular pruning unless it is to reduce the size, reshape or remove deadwood. Although a little pruning every two or three years helps keep any leggy stems in check. This will aid in developing a bushy growth habit with younger plants and encourage more flower production. If pruning is required, it should be done immediately after flowers have faded. If pruning is left to be done later in summer, you will likely lose next year’s flowers.

Deciduous azaleas are easy to maintain by trimming to the height you wish after the flowers have faded. Mature deciduous azaleas may need a few old stems to be removed to encourage new stem growth.

What Happened to My Rhododendrons?

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.

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 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.