JARS v56n1 - Pest Management in the Rhododendron Garden

Pest Management in the Rhododendron Garden
Bill Stipe
Greenbank, Washington

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 weed 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 nontoxic to humans and does not require EPA registration.

Animal Pests
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 ARS District 2 Director and owner of Glynneden Gardens on Whidbey Island, Washington.