Rhododendron Winter Damage and Prevention
Frost causes considerable damage to leaves, stems, buds, and flowers of some rhododendrons. On damaged plants leaves will be distorted, curled, and may grow only on one side of the main vein. Part or all of a winter-damaged leaf will be brown-colored. If too unsightly, remove the damaged leaves.
Some rhododendrons protect themselves from winter dehydration by drooping and curling their leaves as temperatures decrease. Leaf movement occurs rapidly and it is reversible as temperature warms.
Frost also will cause the bark of affected stems to split longitudinally near ground level...even peeling away. This typically happens in early fall or late spring when the plant is not fully dormant. Often these symptoms are overlooked and damage only noticed at a later date. If noticed recently after the injury and before the bark has had time to dry out the area can be wrapped with galvanized wire or a non-sticky wrapping. Check the wrapping monthly and remove the wire when the repair is successful.
Cold weather may cause branches to appear to be dead. Wait until late spring then scratch the bark on dead-looking branches. If there is green wood underneath, the branch is still alive, leave it in place. If it's brown underneath, the branch is dead and can be pruned off.
Cold weather can kill flower buds. Dead buds are brown-colored and easily break off. Often rhododendron buds are less hardy than other parts of the plant. Choose varieties that are bud cold-hardy for your area.
Because rhododendrons roots are very shallow, it's important to use a thick layer of mulch to provide protection from the cold. It'll also slow water evaporation from the ground, helping plants to stay hydrated. On warm days water your plants so they have a chance to survive cold periods.
A windbreak made from burlap, lattice or a snow fence can help prevent damage from winter's drying winds.
Growing Media pH
What is pH, and how do I obtain the proper pH for my rhododendrons?
The term "pH" refers to the acidity of a material. Technically, it is a measurement of the hydrogen ion content. The pH scale ranges from 0 to 14 and pHs of 0 to 7 are acidic - pHs of 7 to 14 are referred to as being basic or alkaline. A pH of 7 means the material is neutral.
The experts indicate rhododendrons prefer an acidic medium. The preferred pH should be between 5 and 6.5.
It is almost impossible for a layperson to determine the pH of the potting medium they use. There are pH meters on the market...but in my experience the ones that cost less than $100 are practically worthless. I have yet to try one that is better than plus or minus 1 pH.
But...all is not lost. It is actually fairly easy to get your pH in the desired range. Fir or hemlock bark is almost always in an acceptable range and, therefore, an ideal medium to use. It is best if the bark has decayed or mulched for six months prior to use.
The reason pH is important for plants has to do with the intake of minerals and nutrients. If the pH is too low, yes, soil can be too acidic, the plants have difficulty taking in the nitrogen and phosphorous they need for growth.
The foliage will not be the rich green that you expect. Adding lime to the medium or the soil will raise the pH and help this condition. Too low pH can occur when soil has been fertilized heavily for years. The fertilizer frequently increases the acidity of the soil...that is...it lowers the pH.
If the planting medium of soil is too alkaline, i.e. the pH is too high, it usually causes iron and/or manganese deficiencies. These deficiencies result in chlorosis - a condition where the veins may remain dark green - but the spaces between the veins will be yellow...the leaves are said to be chlorotic. To remedy this condition sulfur is often applied for a quick fix. Good mulching will also help in the long run. A decomposed mix that would not use up the nitrogen in your fertilizer is best.
In summary, pH is important but your plants will tell you if you have a problem. Generally, it is always best to use bark in pots and bark or pine needle mulch as an additive for your soil...and you will rarely have a problem.
Fertilizer Nutrient Elements
A bag of fertilizer has three numbers prominently featured on the label...something like...5-2-0. It's important to know what those numbers mean because the wrong combination can do more harm than good. The numbers indicates the amount of three major plant nutrients: nitrogen, phosphorus, and potassium, abbreviated NPK.
One way to remember what those things do, and keep them straight is the phrase "up, down, and all around." Nitrogen is needed for green, leafy growth...the up. Phosphorus helps produce healthy roots...the down. Potassium is important for overall plant health and resistance to water or insect stresses, so it's...the all around.
Plants need anywhere from 14 to 18 plant nutrients for best health and production. Magnesium and sulfur comprise the macronutrients along with NPK. The micronutrients include: iron, manganese, boron, zinc, copper, molybdenum, and maybe chlorine. Testing your soil is the way to determine what is needed.
Growing Rhododendrons From Seed
Rhododendrons and azaleas are easily grown from seed. Unless the parent plants are species from isolated areas, the resulting seedlings will exhibit much variability. Unless you are interested in hybridizing and selecting new cultivars, use of seeds as a means of propagation should thus be limited to species plants. Even then some physical differences will be evident.
- Obtain clean seeds.
- Prepare a sterile container at least three inches deep, with bottom drainage; size depending on how many seeds you wish to plant.
- Fill the container to within ½ inch of the top with a mixture of 40% perlite and 60% fine sphagnum peat moss. This mixture should be moist...but not wet.
- Level and firm the surface of the mix.
- To control fungal disease, spray the soil surface with fungicide Captan. Read and follow product instructions.
- Sprinkle seeds thinly on the surface...do not water again.
- Put plastic or glass over the container to make it moisture tight.
- Place in a warm dimly lit area until seeds germinate.
- Put under fluorescent light for 18 hours a day at 70-75°F.
- Anytime after true leaves have formed, harden the seedlings off by gradually opening the cover over the period of at least one week. Water carefully as needed to keep moist. Watering through drainage holes in the bottom is safest.
- Transplant when ½ to 1 inch tall to flats using 50% sphagnum peat and 50% perlite. Lift under the roots and handle by a leaf. Plant at same depth. Water to settle in.
- Water to keep moist, but not wet. Fertilize with azalea food or other acid fertilizer once a month using ½ half strength. Always water at least once between fertilizer applications. To slow growth and harden off, stop fertilizing and water less frequently.
- Transplant when seedlings become crowded.
Keep the growing area clean to prevent damping off, water properly, and...success is almost assured!
Winter Protection For Rhododendrons
If you haven't planted your rhododendrons in a protected location, you may have to consider providing them with some winter protection! Rhododendrons, particularly large-leafed forms, are sensitive to winter sun as well as winter winds, and if not protected properly, you may have serious damage or lose the plant by spring.
Damage to plants is likely to happened if the plant did not receive ample moisture before in the Fall. During the winter, drying winds and frozen ground deprives plants of their natural moisture intake. The exposed portions of the leaf...usually the central portion when the leaf was curled...may become brown. This may also appear on the edges of some leaves.
To prevent scorch, plants should be well watered in November, especially if rainfall has been sparse; protected from drying winds; mulched well, and given some shade. New plant growth may not have a sufficient amount of time to become woodsy and harden off for the winter. Also, flower buds are the least cold hardy part of the plant.
Rhododendrons, boxwood, azaleas, hollies, and laurel will benefit from an application of an anti-desiccant, such as Wilt-Pruf. Read and follow all instructions. Spraying should occur in late Fall when temperatures are near 40 degrees F. Most anti-desiccants are composed of a "waxy" substance that can break down during winter's thaws necessitating a reapplication.
If you do not get a chance to apply an anti-desiccant, you may want to provide a wind-shield, such as burlap to these shrubs as well as some mulch protection around the base of the shrub. Young plants can be enclosed with chicken wire or dog fencing, and packed loosely with oak leaves. Remove the protection in the spring as buds start expanding. Mature plants can be mulched with several inches of wood chips at the base.
Evergreen boughs can be leaned or tied against plants to limit winter injury. A teepee-like structure constructed with three or four evergreen trees or branches with their points forced into the ground and tips tied together provides adequate protection...or use a burlap covering around individual plants. Snow fencing alone or with a polyethylene plastic sheeting attached to it also gives effective wind protection. Cover the plants just before freeze up in the Fall and uncover after all the frost is out of the ground in the spring.
As a Master Gardener, I am often asked about pruning rhododendrons. There are several reasons for pruning with different timing and methods. The main reasons are; 1.) removal of spent flowers (deadheading), 2.) removal of dead, diseased, or damaged branches, 3.) shaping of the plant, and 4.) plant rejuvenation.
Removing of the flower after it starts to fade not only improves the look of the shrub, but focuses the energy to making new growth versus producing seeds. On most rhododendrons deadheading is accomplished by pinching the truss at the base with a slight twist. It can also be accomplished with a small pruning shears such as a long nosed grape shear. There will be new buds emerging just below the truss, so be careful not to damage them. It will be easiest to deadhead early before the new growth has started to grow. If the plant doesn't set seeds, it is not necessary to dead head. Small-leafed rhododendrons rarely need deadheading.
Removal of dead, diseased or damaged branches can be done at any time, the sooner the better after it is recognized. Depending on the size of the branch, it may require the use of pruning shear, loppers or saw. Be sure to make the cut below the diseased part and burn or send it away in the garbage. Broken or dead branches should be cut just above a dormant bud. There may be inside branches with buds that do not see the sun that can also be removed because they will eventually die.
Pruning for shape enhances the rhododendrons form and habit. Depending on the landscape, the rhododendrons shape is important. For instance, tall rhododendrons should not be placed where they will obstruct the view or other landscape features. All rhododendrons can be pruned to maintain their natural habit, but to try to keep a tall growing rhododendron small or low growing is not practical. Pruning for shape should be accomplished in early spring to early summer, but not after July 1. Later pruning will encourage new growth which may not be hardened off prior to first frost and subject to damage. I prefer to prune for shape before the new growth emerges, in February or March. Early pruning will most likely remove some flowers the first year, but will assure more blooms the following year. Pruning after the flowers fade is another alternative, but reduces the flowers the next season.
As rhododendrons age they lose vigor and often become open and ungainly. As a consequence, they lose much of their value in the landscape. Rejuvenation pruning, sometimes major, can solve the problem. The larger-leafed rhododendrons have many dormant buds that can be forced into new growth. However, on very old wood, the dormant buds may have atrophied and are not viable. So there is some risk that drastic pruning may not be successful. But, by pruning just above viable dormant buds, new healthy growth can be directed to obtain the desired shape.
Some nurserymen recommend cutting back one third of the branches over a three year period. I prefer cutting the entire plant back to the height desired early in the season before new growth emerges. That may result in a loss of blooms the first year, but will result in a better shaped plant in less time and will have added blooms the next year. A word of caution however, the plant will have a large root system and may produce excessive new growth. To counteract this, I use a spade to chop off 50% of the roots at the drip line. This will balance the uptake of nutrients to the now reduced branch and leaf structure.
It may be difficult to see the dormant buds of some varieties. For these plants prune the branches back to the desired level and after the new growth emerges, prune off any stubs that remain.