- Why Are the Leaves on My Rhododendron Rolling Up and Wilting?
- Why Rhododendron Leaves Curl in Winter
- Treating Your Broad-Leaved Evergreens
- Yellow Leaves
- Brown Spots On The Leaves
- Stem Die Back
- Curled Up Leaves
- Buds Don’t Open
- Few (Or No) Flower Buds Form
- Growth On The Stems
- More Information
- Common Diseases That are Mostly Observed in Rhododendron Plants
- Stem Dieback
- Leaf Gall
- Powdery Mildew
- Garden detective: What’s eating her rhododendrons? | The Sacramento Bee
- JARS v38n3 – Pests of Rhododendrons
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.
Why Rhododendron Leaves Curl in Winter
Now that the weather has turned mighty cold, you may notice something strange happening to your Rhododendrons. The leaves on your plants are drooping down and curling up. Is something wrong with these plants?
You are seeing nature at its best. It’s cold, and when certain broad-leaved evergreens get cold, they take measures to protect themselves. They practice thermotropism. Much like a human wraps their arms around themselves or animals huddle when cold, rolling up their leaves offers these protection from cold winds. The inner part of the leaf, where a lot of moisture loss occurs, is hidden from the wind when rolled.
Gardeners claim they can tell the temperature by how their Rhododendron leaves appear; the more they roll and droop, the colder it is, until around zero they start to look like green beans hanging from the branches. As it warms again, the leaves unroll and stand up again. That is, unless it’s gotten so cold they have dried out completely and died, which can happen even with the hardiest rhododendrons. Moisture loss is what causes most winter injury in plants.
If you want your rhododendrons to look like this in the spring, you’ll want to protect them from the winter elements.
Treating Your Broad-Leaved Evergreens
Treating your broad-leaved evergreens an antitranspirant in fall will help. This water based product seals the leaves and stems so that moisture is not released from them. It’s something to remember for next fall. Also remember to plant broad-leaved evergreens in areas protected from north winds to help prevent leaf desiccation and plant loss.
If you’d like to learn more about tree and plant health, take advantage of a free consultation with a Carpenter Costin arborist.
Learn the common types of Rhododendron problems that may cause your normally care-free plant to be unhealthy and how to fix them.
Generally speaking, Rhododendrons are very care-free plants that don’t require much maintenance at all. (You can find more about caring for your Rhododendrons HERE).
However, occasionally you may run into an issue where your plants are just not looking up to par.
Since I’ve had rhododendrons in my garden for so long, I have run into a few of the common rhododendron problems over the years. So I thought I would share my experiences in case you run into the same problems and need some help.
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One potential Rhododendron problem is that the leaves are turning yellow.
According to the American Rhododendron Society, if the plant has older leaves that have turned yellow between green veins, you likely have a Magnesium deficiency. Sprinkle some Epsom Salts* around the plant to correct the issue.
If the same symptom appears in new leaves, then it is likely that the soil is too alkaline. The alkalinity prevents the plant from taking in the iron that it needs to remain healthy.
For a quick fix, you can try spraying the ground and leaves with an iron sulfate* solution. While this will provide quick relief to the plant, you will probably want to look into making your soil more acidic long term. Applying some peat moss and acidic mulching materials around the plant will eventually help to bring the soil pH down.
Brown Spots On The Leaves
A second type of Rhododendron issue is brown spots on the leaves. There are several types of fungus that can cause this.
With the humid summers here in SC, I always seem to have at least one Rhododendron with this problem.
To fix it, I usually clean up around the plant and try to cut back a few branches to reduce overcrowding and improve the air circulation.
Although the brown spots don’t look great, they don’t seem to bother the plant much.
Stem Die Back
Stem Die Back
Every once in a while I have a whole stem of a Rhododendron branch that dies in the middle of an otherwise healthy-looking plant. According to the Clemson University Cooperative Extension, this is caused by another kind of fungus that gets under the bark.
It usually attacks plants that are under stress due to drought, too much sun or late frosts. So the plan of attack is to cut off the dead branch as soon as possible and then try to remedy the source of the stress (if possible).
New shoots may grow back after the Rhododendron has been cut back
I had one plant last year that all of the stems eventually died back, and I thought it was dead. But then, much to my surprise, it started to grow some brand new shoots from the bottom. So there is a chance that your tough rhododendron may recover.
Curled Up Leaves
If you have evergreen rhododendrons and live in an area where it gets cold in the winter, you may see the leaves curl up and look pretty unhappy during a cold spell. Don’t be alarmed. The leaves are just trying to conserve water in the cold, dry air. They will open up and come back to life once it warms up.
If you see curled up leaves in the summer, though, that is a sign that your Rhododendron is much too dry. Water immediately!
Buds Don’t Open
There is nothing more disappointing than seeing your Rhododendron covered in buds and then not getting any flowers!
These are the 2 common causes of this that I’ve seen.
Bud Blast is a fungal infection that causes the buds on the Rhododendron to die and turn black before they bloom.
It isn’t a life-threatening condition for the plant, but it is really disappointing for the gardener!
It is thought to be spread by the Rhododendron Leaf Hopper, an insect that doesn’t do much damage to the plant other than spreading this fungus around.
You can get insecticides to kill the bugs, but I prefer to just pick off the infected blooms as soon as I see them (less detrimental to the environment). That decreases the number of fungus spores that are available to be transported and limits the spread of the issue.
If you have a late frost, it’s possible that the buds on your Rhododendrons were affected.
In this case, they usually turn a light brown color and get a little mushy (for lack of a better term). In other words, they look like they’re been frozen and thawed.
Other than covering the plant during the frost, there isn’t much you can do about this condition. But fortunately, it usually doesn’t cause any lasting damage either. Unless you are trying to grow Rhododendrons that aren’t zoned for your area…then you might want to look for some other varieties that won’t be affected so much by the cold.
Few (Or No) Flower Buds Form
Again, there are a number of reasons why you don’t have any buds on your Rhododendrons.
Incorrect Pruning Or Deadheading
The most common cause of not having any blooms is that the plant was pruned after flowering the previous year, or too much of the flower head was removed when deadheading. In this case, you’ll just have to wait it out until next year…and remember not to cut off the flowering stems 🙂
Too Much Shade
Too much shade can also cause fewer (or no) blooms. While most Rhododendrons will do fine in the shade, there are some varieties that like more sun. And almost all of them will produce more blooms if they get dappled shade, rather than total shade.
Too much fertilizer applied after the plant has finished blooming is another culprit of fewer blooms. First, since the Rhododendron roots are so shallow, too much fertilizer can stunt the growth of the whole plant. Second, fertilizer applied after the plant has finished blooming mostly promotes lots of leaves…so if you are going to fertilize, be sure to apply it at the correct time.
Not Enough Water
Letting the plant get too dried out or allowing the roots to get too hot will also affect the number of buds that form. Since the buds grow during the summer, making sure that you have a thick mulch cover and that your Rhododendrons are getting at least 1″ of water per week will help it to be as healthy as it can be.
Growth On The Stems
Lichens are a combination of algae and fungus forming a partnership to help each other grow. Generally, they don’t actually hurt the plant that they are living on, since they make their own food.
However, if your Rhododendrons have lichens growing on them, it usually means they are thinning because of poor growing conditions, disease or old age. So you may want to take a look at rejuvenating the plant, by pruning it or improving the area around it to make the plant happier.
I have seen this on plants that have become too crowded or are lacking in Magnesium (those yellow leaves from above).
If you want even more information about Rhododendron pests and problems, this guide from Washing State University has an extensive list of Rhododendron problems and how to fix them.
Have comments or questions on Rhododendron problems? Tell us in the section below.
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Common Diseases That are Mostly Observed in Rhododendron Plants
Like many other plants, rhododendrons are also prone to diseases. Here is a brief overview about the common diseases that affect these plants.
Rhododendrons are among the popular choices for landscaping, as they produce spectacular and showy flowers. Rhododendron is a genus of flowering plants, and it belongs to the family Ericaceae. Around 100 species of plants are included in this genus, and most of these plants produce showy flowers. Azaleas are among the most popular rhododendrons that are commonly used for landscaping. However, rhododendrons are prone to various diseases, and most of them can be prevented with proper care and treatment.
The most common rhododendron disease is stem dieback, which is characterized by death of the branches, in an otherwise healthy plant. This condition is caused by a fungus called Botryosphaeria dothidea. It has been observed that the inner parts of the bark of the dying branches develop a reddish brown discoloration. If you notice any symptom of dying stems, then scrape the bark with a knife, and check for reddish discoloration. This disease can be controlled by pruning. In other words, the infected (dying) branches have to be removed at the earliest. Such pruned branches have to be disposed of in a distant location, and the pruning tools have to be disinfected with bleach solution.
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Commonly seen in azaleas, leaf gall is a springtime disease, that may affect other rhododendron species. In this case, the leaves, buds, or stems, develop swollen growths with distorted shapes. Sometimes, these growths may get covered with white spots. This condition is caused by the fungus Exobasidium, and it can be controlled by removal of swollen parts, that have to be burned.
Usually, this condition develops during the later part of winter, especially when the weather is cold, and the soil remains warm. This condition is characterized by the presence of water blisters on the leaves and other parts. It is caused by the over absorption of water by the roots. As the water pressure in the cells increase, blisters are formed on the leaves. In severe cases, such blisters can be found on the flowers and other parts too. These blisters may also turn hard, and resemble warts in appearance. This condition may eventually affect the growth of the plants. This can be controlled by avoiding over watering, and by spacing the plants, so that they get sufficient air circulation.
Chlorosis is a disease characterized by yellowing of rhododendron leaves. This condition is caused by non availability of micronutrients, that are necessary for the healthy growth of these plants. These micronutrients include iron, manganese, and zinc. So, it is always recommended not to plant rhododendrons in highly alkaline soil. Such yellowing can be caused by pollutants too. So, it is better to plant them away from locations, where they can be exposed to such chemicals. If you want to treat the condition, test the soil to find out the missing nutrient.
This is a fungal disease that is characterized by white or gray blotches or coating on the leaves and other parts. You may also notice the affected leaves dropping off. In severe cases, the growth of the plant may get stunted. This disease is mostly found in humid climates, and very rarely in cold weather. So, rhododendrons in the coastal areas are more prone to powdery mildew. This disease can be controlled by collecting and burning the affected leaves, and spraying the plant with a fungicide.
There are various other rhododendron diseases, like leaf spots, and root and crown rot. While leaf spots can be controlled with a fungicide, root and crown rot can be prevented to some extent by planting rhododendrons in well drained soil. Even pests like aphids, bark beetles, spider mites, scales, weevils, and caterpillars can attack this plant. So, provide these plants with healthy growing conditions, so that most of these diseases can be prevented.
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Issues with rhododendrons are rare as they are relatively low maintenance plants. Rhododendron pests and disease harm plants that are stressed due to environmental conditions or injury. Common problems of rhododendron bushes can be avoided by providing the best growing environment possible and maintaining a consistent pruning, mulching and fertilizing program.
There are several diseases that could cause black spots on your rhododendron, but one of the most common is Cercospora leaf spot. A fungal disease that causes irregular brown spots on the leaves, usually attacking lower leaves first. The spots turn lighter tan in the center and you may see small dark pimples about the size of a pinprick inside the larger spots on close inspection. These are the fruiting bodies of the fungus that will produce spores that spread the infection to healthy leaves. Rake and destroy fallen leaves to remove as much of the source of infection as possible.
Apply a fungicide in early spring as new growth appears to protect new growth from infection. Begin fungicide application when new growth starts and make repeat applications as directed on the label. We suggest using Bonide’s Fung-onil multi-purpose fungicide spray or Bayer’s 3-in-1 Insect, Disease and Mite Control spray.
If any of the stems are wilting in connection to the leaf spotting and yellowing, there are two other diseases that may be responsible: Botryosphaeria canker or phytophthora root rot. Botryosphaeria canker is a fungal disease characterized by sunken, dark brown cankers on the stems. The leaves on affected stems wilt as this causal fungus destroys the vascular tissue in the stem. You can see tiny black spots, which are the fruiting bodies of the fungus, in the dead bark over the cankers.
Drought stress predisposes susceptible rhododendrons to this destructive disease. Although you have been watering this year, dry weather in previous years may cause an accumulated stress that made them more susceptible. Continue to water as long as the weather is dry, but stop when we are getting sufficient rain.
Prune out and destroy dead stems, and make sure you cut back to sets of leaves where there are buds that will begin to grow. Do not leave large dead stubs that invite infection by this disease. There is no chemical control for botryosphaeria canker.
Phytophthora root rot causes rhododendrons to wilt, and affected plants may be stunted. The wilted leaves may be marked with dark brown spots and usually turn a dull yellow-green color before becoming entirely brown. The shoots appear to die from the tip back as the fine feeder roots are killed and can no longer take up water. Phytophthora is more likely to occur in rhododendrons planted in heavy soil that drains poorly.
You can check to see if phytophthora is to blame by examining an affected stem at ground level. Peel back some of the bark and look at the underlying tissue. You will see a distinct boundary between healthy white tissue and diseased brown tissue. While fungicide applications can protect uninfected plants, plants that are already infected inevitably die. If phytophthora is confirmed, infected rhododendrons should be removed.
Too much water and overhead watering can lead to plant distress –
Rhododendrons are shallow-rooted plants and appreciate extra water when we are not receiving sufficient rainfall. However, drip irrigation and soaker hoses that keep water on the ground and off the plant are best ways to water. If that is not likely, at least use a watering wand and direct the water to the soil as much as possible. Although plants get wet when it rains or when there is heavy dew, there is no need to make the problem worse by overhead watering.
It is best to water first thing in the morning so that plants dry as quickly as possible when the sun comes up; overhead watering in the evening guarantees that leaves will stay wet all night, increasing the chance of disease development. Too much water however can also be bad as it can cause root rot. Wilting leaves can be an indication of both too little and too much water.
Yellow leaves on your Rhododendrons? – Chlorosis – Chlorosis, an iron deficiency, is common in rhododendrons and causes leaves to turn from a rich dark green to a light green or even yellow. New leaves may even emerge completely yellow. Chlorosis becomes a problem when the soil pH is 7.0 or higher. Amending the soil with sulfur and providing an iron fertilizer will help correct the problem
Read more at Gardening Know How: Issues With Rhododendrons: Dealing With Rhododendron Insect Problems And Diseases –
Contributing authors – Sandy Feather, Susan Patterson
Garden detective: What’s eating her rhododendrons? | The Sacramento Bee
Two of our rhododendron plants in large planters have parts eaten out of the leaves. We have never seen any insects on or around the plant or planters. Do you have any idea what might be doing the damage and how to prevent any more leaves from being eaten in the future?
– Ramona Bryant, Grass Valley
According to UC master gardener Carol Rogala, a variety of pests enjoy rhododendrons including snails, foliage-feeding caterpillars, cutworms and black vine weevils (the latter pictured here).
A sample of the foliage and pest damage are necessary for an exact identification. However, the photos you supplied look like black vine weevil as the culprit. This pest feeds on many landscape plants such as azalea, rhododendron, euonymus and liquidambar.
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Adult weevils generally feed on foliage. Leaves or flowers appear notched or ragged like yours. The most serious damage is done by larvae, which feed on roots and can kill or weaken some plants, especially azalea and rhododendron.
Destroy adults to prevent more serious damage. For rhododendrons, plant less-susceptible species. Provide cultural care to keep plants vigorous and better able to tolerate damage.
Check roots before planting to make sure they are free from larvae. Trim branches that provide a bridge to other plants or the ground and apply a 6-inch band of sticky material to trunks to prevent flightless beetles from feeding on foliage. Trapping may help.
Parasitic nematodes also may be effective in controlling larvae. Timed insecticides applied to leaves can control adult weevils.
I have two fruiting mulberry trees in my back yard. The previous owners of the property planted one that is directly over my back deck and another that is over my chicken coop. Walking through dropped berries is worse than walking in mud; they stick to the boots and are impossible to clean off completely. Despite our best efforts, we get stains on our deck and on carpets and tile in the house. Short of cutting down the trees, how can I keep them from fruiting?
– Steve Liddick, Sloughhouse
Mulberry trees are planted because they grow relatively fast, need little pruning once established, require minimal fertilization and are generally free of pests and diseases, said master gardener Rogala. The fruiting varieties offer both shade and fruit.
The problem is the messy fruit. One possible solution is to knock the flowers off with a strong jet of water before they set fruit. If it doesn’t completely eliminate the fruit, it may decrease the amount of fruit so that the problem is tolerable.
A product called Florel Brand Growth Regulator and Fruit Eliminator by Monterey is effective on a fruiting mulberry tree. The active ingredient is ethephon and is registered for use on ornamental and landscape trees. The manufacturer recommends that the chickens be removed from the coop before spraying and not returned until the Florel has dried. Monterey indicates that it is not hazardous even when the ground is re-wetted by chicken urine, rain or water.
Look for it in nurseries and home improvement stores. Please read the label carefully and follow all instructions carefully.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&[email protected] Please put “Garden Detective” in the subject field and include your postal address.
To read past Garden Detectives, go to www.sacbee.com/gardendetective
JARS v38n3 – Pests of Rhododendrons
The mountain beaver is an unusual pest of rhododendron and is rare to nonexistent in many landscapes. However, in new housing projects that pervade forest lands, the mountain beaver may be one of the first real nuisances the homeowner encounters. This animal includes the terminals as food items along with other plant species. They will climb several feet into a bush to get at the tops of branches, and “snip” them off. This, of course, is unacceptable; so control efforts must be implemented. Trapping is very effective and success is much easier to achieve than it is for moles.
Pest control or pest prevention in rhododendrons is not necessarily difficult. All one has to remember to do an effective job is to properly diagnose the pest problem or have it done by an expert. Then, learn enough about the biology and life cycle of the pests, where they are known, so that management techniques can be effectively applied. Unfortunately, we lack adequate knowledge on certain pests that are minor or sporadic in their occurrence. There are university publications available that can help you diagnose and eliminate many of your specific problems. Seek them out and use them. A publication exists that deals with a number of rhododendron problems. (Antonelli, et al., 1984). If chemicals are used, proper timing is almost always essential. Additionally, if a chemical is used, remember to seek local expert advice. Be sure it is registered for rhododendrons, and follow the label directions and precautions.
Pest Management in the Rhododendron Garden
Weeds, insects, animals, even people can sometimes be pests. We may each define pests differently, but I call anything that works against my wishes as a pest. Some people think of the common dandelion, Taraxacum officinale, as an edible plant with beautiful golden flowers, but most of you will agree if it is growing profusely in our garden or lawn, it is a weed. The bunny rabbit may be a beautiful creature to some, but in my garden it is a pest. Other organisms that are commonly defined as pests are fungi that deform or kill our ornamental or vegetable plants, insects that devour or injure our gardens, and bacteria that are harmful to plants and animals.
Okay, so now we have defined the enemy, how do we wipe them out? But wait a minute, before we declare war, let’s look at the potential consequence. All of these creatures we call pests have evolved and survived as long, if not longer, than man. Perhaps they too have a place in the total scheme of things. Each of them serves a purpose, but perhaps not the one we desire. So, maybe we should negotiate with them, keep them in their place and out of our garden. I like to call it pest management.
Let’s look back in time before the advent of chemical pesticides. Our forefathers survived along with these things we call pests. They were still pests, but there was a balance: the good bugs ate the bad bugs, the good fungi controlled the bad fungi, and even good bacteria kept the bad bacteria in control. I am not advocating going back to the horse and buggy days, and I realize modern medicine has made our life easier and increased our lifespan, but I do think there were some advantages to the pre-chemical pesticide days. When DDT was first developed by the Swiss, it was touted as the miracle insecticide. It promised to wipe out the insects that spread deadly diseases that killed thousands in third world countries. The world jumped on the bandwagon and dusted or sprayed every nook and cranny where insects were hiding. In communities where mosquitoes were a problem, local jurisdictions spread DDT with machines that filled the air, so thick it was like fog. Children ran through this fog playing games. This application was very effective at wiping out the mosquitoes, and every other insect it fell on, perhaps even injuring the children.
At that same time I was growing up on a wheat and cattle ranch in Eastern Washington State. We milked cows and sold their milk as an added source of income. Insects, particularly flies, were an annoyance to the animals and the people that tended them. We had a bag of DDT in the milk barn that I used to dust the backs of the cows before I milked them. The flies would roll off their backs the minute they landed. With the flies gone the cows didn’t have to swing their tails to fend them off. I thought this was a good alternative to getting swacked on the head with the cow’s tail. We also used DDT on the windowsills of the farmhouse to kill any houseflies that sneaked past the screen door. We never gave a second thought to what else the DDT was killing or the effects on our own health.
It wasn’t long before 2,4-D became available to kill unwanted weeds. It was selective in that it only killed the broad leaf weeds. It was common practice for me and my siblings (I had four brothers and eight sisters) to walk the wheat fields and rogue out the Jim Hill mustard, Russian thistle, and any other weed that did not belong in the wheat field. Some weeds, however, did not lend themselves to hand pulling. When the arsenal of chemical weed killers first appeared on the scene, we enthusiastically sprayed everything. Our family garden was located in the middle of a barley field. Not realizing that the 2,4-D ester formulation would vaporize, I killed the entire garden in short order. The entire farm community utilized this new chemical, some applying it with airplanes. It wasn’t long before most of the deciduous trees in the county were dying. The farmers that chose not to use the chemical herbicides suffered loses due to their neighbors’ actions. It wasn’t long before the ester formulation of 2,4-D was banned in the farming communities.
I was the youngest son in our family and I was expected to continue the farming tradition. However, I disappointed my family and moved off the farm. I sought an education at the University of Washington while working at the Boeing Co. Even though I gave up farming, I never lost my love for plants and Nature. I am now growing rhododendrons commercially and learning how to live with nature. I have recently been diagnosed with cancer, and cannot help but wonder if my exposure to chemical pesticides did not contribute to my condition. I am much smarter now and believe I can live with the natural entities I used to consider pests. I am evaluating many pest management techniques that will keep these pests under control and let me cohabitate in an ecologically friendly environment.
With all the advertisements for garden pesticides touting their merits, it may be difficult for the casual gardener to try the Integrated Pest Management (IPM) approach. But if we continue to upset the balance of nature, pollute our streams and water supplies, we will not only suffer the effects of the carcinogenic chemicals but we will be addicted to the ever increasing need to control pests through chemical processes.
If you have not lost interest yet, stick with me while I try to explain how to get your garden back in balance. At the 2001 ARS Annual Convention in Eugene, Oregon, I attended a seminar on “Soil Building” by Dr. Elaine R. Ingham. Dr. Ingham explained that every living thing has a predator. The soil food web is kept in balance when the good bugs are sufficient to keep the bad bugs in check, the same thing with bacteria and fungi. If you have used pesticides, you have upset the natural balance which must be restored. That balance may eventually be restored naturally, but will take some time. Compost is nature’s way to increase the good bugs, the good bacteria, and the good fungi. By using natural compost to make a “compost tea” the good organisms can be re-introduced into the soil to help regain the natural balance. Compost is the main ingredient in compost tea but it may also include extracts of plant materiel, molasses or other sugars, proteins, carbohydrates, kelp, rock powders, rock dust, humic and fulvic acids, sources of nitrogen, etc., as additional food for the microbes. Several firms in the Northwest are set up to brew compost tea. For more information on obtaining and using compost tea, visit the website at www.soilfoodweb.com.
I am using compost tea to combat the fungus that infects rhododendron leaves, Microspaera ssp., commonly called powdery mildew. An application of compost tea on the foliage leaves no place for the bad fungi to establish themselves. I have two large rhododendrons, ‘Unique’ and ‘Bruce Brectbill’, that were so badly infected with the mildew that they completely defoliated. After the new leaves emerged this spring, I obtained a gallon of compost tea brewed in accordance with Dr. Ingham’s recommendations, and sprayed both plants, assuring complete coverage of the leaves. I have seen a drastic reduction in the amount of mildew present. I believe I would have completely eliminated the mildew if I had applied another application after the leaves had completely matured. I am encouraged by the results and intend to try again next year.
Another pest on rhododendrons is the root weevil. There are several species of this insect that damage rhododendron leaves and roots, black vine weevil, Ortiorhynchus sulcatus, strawberry root weevil, Otiorhynchus ovatus, and obscure root weevil, Sciopithes obscurus. Several of these weevils are prevalent wherever conifers grow. My garden is surrounded by conifer forest so I am subject to weevil damage. The adults chew on the leaf margins and the grubs chew on the roots. On occasion I have found where the root weevil grubs had girdled the trunk just below the root surface, killing the plant. Several natural predators of the root weevil exist but apparently not in large enough numbers to keep them under control. The adult root weevil is nocturnal, eating on the leaves at night and returning to the duff under the plant in the day. If you go out at night with a flashlight, you can find them eating on the leaf margins. They can be hand picked at this time and destroyed. Another control is to put a sticky substance around the trunk that will trap the adult on his way to the leaves.
I have found the introduction of beneficial nematodes to be an effective control. There are two nematode species that are commercially available, Steinernema feltiae and Heterorhabditis heliothidis. Nematodes are tiny, microscopic roundworms that live in the soil. They kill the weevil larvae by infecting them with bacteria. They search for insect hosts and swim on a film of moisture to invade the host’s body. Although these nematodes are found in most locations, few will survive in soil colder than 55°F (13°C). In our location in the Pacific Northwest USA, they must be reintroduced each spring to obtain efficient control of root weevil. I have been using them for ten years with satisfactory control. I realize that the root weevil occupies the forest surrounding my garden, and that I will never eliminate their entire population. I only apply the nematodes at the base of rhododendrons that show signs of leaf damage. I know they will only kill the grubs, so I still get some damage from itinerant adults. Because these nematodes are living organisms, care must be taken to keep them alive prior to being placed in the vicinity of the root weevil. Nematodes are sensitive to ultraviolet and must not be exposed to bright sunlight. They also need moisture, so should be applied on a cloudy day with moist soil conditions, after the soil temperature has reached 55°F (13°C).
There are several chemical pesticides registered for control of the root weevil, but I choose not to use them. If you grow rhododendrons in containers, or if you have a nursery that sells rhododendrons in plastic containers, chemical control may be your only alternative. For more information on root weevil, refer to the fall 2001 issue of the ARS Journal.
There are several other insects that are troublesome pests in my garden, but none that require chemical warfare. Insecticidal soap is effective on aphids and other small insects and is not harmful to the environment or people.
There may be other insects or diseases that attack rhododendrons in your location, but I would encourage you to look at non-chemical solutions before using any chemical, and then follow the instructions explicitly.
There is a saying I learned in high school physics class: “Nature abhors a vacuum.” I have learned that Nature also abhors bare ground. Whenever I clear new ground, several species of weeds seem to immediately germinate. The seeds were already in the soil, may have been for decades, and sprout as soon as the sunlight gets to them. Besides the dormant seeds, Nature has other means of seeding open ground with her persistent ground covers. Some come floating in on the wind (dandelion, thistles, milkweed, etc.), the birds bring in all kinds of berry seeds they eat (blackberry, elder berry, salmon berry etc.), animals (deer, dogs, cats) bring in seeds that cling to their hair (bedstraw, beggar ticks, cocklebur, etc.), and some spread by propelling their seed like a shot. It is obvious that Nature will have her way unless we intervene.
The chemical companies would have you believe products such as Weed and Feed or Weed Begone are the simple answer. Well, when you try to outsmart Nature, there is no simple answer. Prevention should be considered the first line of defense. All weeds proliferate by spreading their seeds. Some weeds produce literally thousands of seeds per plant. Physically removing each plant will reduce the number of seeds that will germinate the next season. You probably have a favorite tool for removing weeds; mine is the Winged Weeder. It is a V-shaped hoe that cuts off weeds with either a push or a pull. Sometimes it is easier to just bend over and pull the weed; that way you will get the root system also. Perennial weeds will re-grow if the entire root is not removed, so just removing the top is not sufficient. When I’m walking through my garden, I’ll pull any weed I see. If it has seed about to mature, I’ll stick it in my pocket and later dispose of it in the garbage. The compost pile is not a good place to dispose of weeds with seeds or perennial weeds that could continue to grow.
Because my garden is planted on ground that was originally forested, I have had lots of experience managing Nature’s ground covers (pronounced weeds). After the trees and stumps are removed, I try to cultivate the ground for several years. This exposes the dormant seeds and allows them to germinate. Then I use glyphosate (Roundup) to kill most of the weeds before they go to seed. Some perennial weeds are not killed by glyphosate and require a different approach. After using this “chemical warfare” for several years, I then plant a heavy cover crop of crimson clover and Austrian peas. Before planting the garden, I turn this cover crop under. This helps to rejuvenate the beneficial microbes in the soil and crowds out most of the weeds.
The battle against weeds is not over. After planting the ground to rhododendrons, I spread a 2- to 3-inch (5-7.5 cm) layer of mulch. This stops further germination of the native seeds because it blocks the sunlight from reaching the soil. However, weeds that drift in or are carried in by the animals or birds will still germinate in the top layer of the mulch. These invaders will require pulling by hand, but the mulch makes it easy to remove them.
Another source of weeds is the soil on the plants you bring into your garden. As a nurseryman, I strive to keep the garden soil weed free, but that is next to impossible. Some nurseries grow their potted plants in artificial soil, thereby eliminating weed seeds. But if these potted plants are grown outside, weeds have a way of entering nursery pots. I’m not advocating not buying nursery plants, just warning you to watch for and eliminate any weeds that show up around a newly purchased plant.
Canada thistle (Cirsium arvense), horsetail (Equisetum), and wild morning glory (Convolvulus arvensis) are perennial weeds that require extreme measures. The roots of these weeds are widespread and go deep. There are several herbicides that claim they will kill them, but they only knock down the top growth and the weed will come back from the root. I have been trying to kill a stand of horsetail for eight years using all of the chemicals registered. The plants are somewhat weaker each year, but they still come back. A new product has just come on the market called Blackberry and Brush Block. It is a highly concentrated wine vinegar, 1000 times stronger than that used on salads (5% citric acid and 95% acetic acid). When applied to the soil, this concoction lowers the pH of the soil to 3 or lower, souring it to such a degree that nothing can grow. The great part of this plan is that all vegetation dies but the beneficial organisms in the soil simply go dormant. When the weeds are all dead, the soil pH can be brought back with the application of a fast acting lime. This is not a procedure you will want to use in existing plantings, but if these weeds are a problem, you may want to move your rhododendrons and give it a try. Blackberry and Brush Block is non-toxic to humans and does not require EPA registration.
My garden is located in a rural area surrounded by forest so it is not unusual to have wildlife present. Deer, rabbits, raccoons, and coyotes are common, but deer and rabbits cause the most damage. Deer will eat the leaves off of most azaleas and some lepidote rhododendrons, but usually will not eat the larger leaved rhododendrons. In the fall, however, male deer will pick out several mature rhododendrons and attack them with their horns. They will also damage any tree with a trunk caliper under 3 inches (7.5 cm). They will continue this attack until there is nothing left but a few leafless stubs. Some experts say the deer are trying to rub the velvet off their horns, but I have a different notion. I believe the bucks are trying to impress the does. This only happens during the mating season. I have tried all of the deer repellants on the market and none of them are very long lasting. I have had a German shepherd on duty, but the deer seem to come around when she is asleep. The only solution I have found is an 8-foot (2.5 m) high fence.
There is a black polypropylene fence with 2½ inch X 2 inch (6.25 X 5 cm) spacing by 8 feet (2.5 m) that is reasonably priced and doesn’t look too bad. This fence, if installed close to the ground, will also keep out rabbits, raccoons, coyotes and the neighbor’s dog.
Nobody said it would be easy to raise a garden on Whidbey Island, but I am determined to continue and improve it despite the many pests we have.
Bill Stipe is the owner of Glynneden Gardens on Whidbey Island, Washington.