QBARS - v21n4 Chemical Weed Control in Rhododendrons

Chemical Weed Control in Rhododendrons
R. L. Ticknor, Associate Professor of Horticulture
Oregon State University-North Willamette Experiment Station

Presented at the A.R.S. Annual fleeting, Asheville, N.C., published as Special Report No. 243, Agricultural Experiment Station, Oregon State University, Corvallis, Oregon.

The shallow root system of rhododendrons limits the use of cultivation for controlling weeds. Considerable research, therefore, has been devoted to the chemical control of weeds. Several herbicides have proved useful, but varieties react differently to them. Simazine at rates which have no effect on the typical elepidote rhododendrons causes deciduous azaleas to become chlorotic. To date no plants have died from a single low rate application of the pre-emergence herbicides mentioned in this report.


Before discussing chemical weed control in rhododendrons, mulching, the safest weed control method, should be considered. Mulching prevents soil erosion, provides for better soil moisture penetration and retention, and increases the availability of soil nutrients. Except for possible increased radiation frost damage to susceptible plants in the spring and fall, most of the disadvantages of mulches are related to using the wrong materials. Some finely ground materials, i.e., peat moss and some saw dusts, tend to crust and inhibit moisture penetration. Large leaves such as maple often pack so tightly that soil aeration is restricted. Chopped oak leaves, ground bark, pine needles, sugar cane and woodchips are suitable mulching materials. Mulching is usually too costly as a commercial practice.

A point often overlooked in mulching for weed control is the necessity of annual applications of at least one inch of fresh mulch material. A mulch this deep prevents weed seeds from germinating readily. Because decay of the mulch ties up nitrogen, which often results in yellow plants, it should be added at a rate of one pound of ammonium sulfate per one hundred square feet of organic mulch one inch deep. Most of this added nitrogen will be available to the plants later.

Black polyethylene covered by a regular mulch material has proved useful in new plantings. This technique combines the attractive appearance of the mulch with effective weed control by the plastic. The polyethylene is spread over the ground and cross shape slits for planting are cut and holes are dug. Part of the excess plastic is dropped into the holes to channel irrigation and rainwater into the root zone.

Chemicals for weed control can be applied at three times-pre-planting, pre-emergence, and post-emergence. Pre-planting treatment is the ideal time to begin a weed control program. Most chemicals used in this manner are soil fumigants that control soil diseases and insects as well as weeds. Pre-emergence herbicides must be applied before the weed seed germinates to be most effective, although some herbicides used in this manner will selectively control certain growing weeds. Post-emergence herbicides are absorbed by the foliage of the weeds and are usually applied when the weeds are growing actively.

Since soil type, rainfall, climatic conditions, and weed species influence the performance of a particular herbicide, the following suggestions are intended only as a guide for trials. With any unfamiliar practice, it is always well to begin on a small scale. Personnel of the state universities who work with herbicides and of companies supplying herbicides can often offer useful suggestions for weed control in ornamentals.

Pre-planting Treatments

Soil should be in seed bed condition, be moist, and be over 60°F for best results. If this condition is maintained for a week before treating, the weed control will be better.

Eptam . An effective herbicide when immediately tilled into the soil after application. Plants with a soil ball can be planted immediately. Used at rates of 6 to 10 pounds per acre or approximately 1¾ teaspoon to 3 teaspoons per 100 square feet particularly for perennial grass control.

Methyl Bromide . A fumigant which gives very good weed control at rates of one pound per 100 square feet. Better control of soil diseases is achieved at rates of 2 to 3 pounds per 100 square feet. This toxic material must be injected under a plastic cover that is left on for 24 to 48 hours. Planting is delayed for a week to allow the gas to escape.

Mylone . Not as effective as other fumigant material, but it is an easy-to-use granular material. The 50 percent material is applied at a rate of 1⅓ pounds per 100 square feet and is lightly raked into the surface. This is followed by a water seal of approximately ½ inch of water. Usually a 3 week delay before planting is necessary.

Vapam . A fumigant which controls weeds well when applied at a rate of 1 quart per 100 square feet with a water seal of ½ inch to 1 inch of water. A plastic cover increases weed control. Delay 10 days to 2 weeks before planting.

Pre-emergence Treatments

The soil surface should be weed free at the time of application. The amount of water used to apply this type of herbicide is not critical as long as uniform coverage of the soil surface is achieved. Water, either as natural rainfall or irrigation, should be applied following application. The comments made on plant safety refer to other woody plants which may be associated with rhododendrons in the landscape as well as to the rhododendrons themselves.

Casoron . A very effective granular herbicide when applied in the fall or early spring. It is more effective if lightly incorporated into the soil or when lightly covered with a mulch. The normal rate of application is 100 pounds per acre of the 4 percent granular material (5 tablespoons per 100 square feet). If difficult perennial weeds such as wild chrysanthemum, Canada thistle, or quack grass are to be controlled, 150 pounds per acre (7 tablespoons plus 1½ teaspoons per 100 square feet) are often required. A 2 percent formulation in a graduated applicator is available in some areas. Residual effect usually not more than one growing season.

CIPC . Effective for the control of many winter annual weeds when ap-plied in the late fall. Short residual in warm weather. Dog fennel and possibly other weed species are resistant; safe on most plants. The 10 percent granular formulation is used at 60 to 80 pounds per acre or 4 tablespoons plus 2¼ teaspoons to 6 tablespoons plus 1 teaspoon per 100 square feet.

Diphenamid . Safe on most plants. Sometimes a few weeds develop immediately after application. Fairly long effective period. Can be applied any time of the year. Used at 8 to 12 pounds per acre of 50 percent wettable powder (5 to 6½ teaspoons per 100 square feet) or 5⅓ to 8 pounds per acre of 75 percent wettable powder (3 to 4½ teaspoons per 100 square feet).

Dacthal . Very safe; can be used immediately after planting. Short residual life. Very good on seedling grasses, but some broadleaved weeds are not controlled. Used at 8 to 12 pounds per acre of 75 percent wettable (3 tablespoons to 4 tablespoons plus 1½ teaspoons per 100 square feet).

Diuron . Very effective but may cause chlorosis. Rates of application 2 pounds per acre of 80 percent wettable powder (¾ teaspoon per 100 square feet). Long residual life.

Falone. Safe on most plants. Short residual life. Best used with other herbicides such as Dacthal or diphenamid to provide immediate control and to broaden the spectrum of weeds controlled. Used at a rate of 1 gallon or 4 pounds per acre of active ingredient (1¾ teaspoon per 100 square feet).

Herban . Effective when used for winter annual weed control. Does not control pigweed in summer at the safe levels of application, 2 to 3 pounds per acre of 80 percent wettable powder (1¼ to 2 teaspoons per 100 square feet).

Simazine . Probably the most widely used herbicide for nursery plant production. Long effective period. (In the Pacific Northwest used at rates of 1¼ pounds per acre in winter and 2½ pounds in summer.) Safe on most plants but has caused chlorosis of deciduous azaleas at 2 pounds per acre. This rate has been safe on other groups of rhododendrons. For small areas use ½ teaspoon per 100 square feet in winter and 1 teaspoon per 100 square feet in spring. If applied over an organic mulch, double these rates.

Post-emergence Treatments

These herbicides are sprayed over the top of the weeds and are absorbed by the tops and in some cases by the roots of the weeds. Several of them have to be applied so that the herbicide does not contact the crop foliage, otherwise damage will result.

Amino-triazole . Used as a spot treatment to control poison oak or ivy, Canada thistle, quack grass and other difficult perennials. Do not contact the foliage of rhododendrons or other crop plants. Toxicity persists in the soil. Use at rates of 8 pounds per acre or 4 teaspoons per 100 square feet of 50 percent wettable powder or 2 tablespoons plus 1 teaspoon of the 2 pound per gallon liquid formulation.

Casoron . Normally used as a pre-emergence treatment but will kill annual and some perennial grasses when applied to growing plants in the fall or early spring. Also suppresses the growth of some of the perennial broadleaved weeds such as chrysanthemum weed, Canada thistle, and sheep sorrel. Rates of application are 100 to 150 pounds per acre of the 4 percent granular formulation or 5 tablespoons to 7 tablespoons plus 1½ teaspoons per 100 square feet. This chemical has been effective even over an organic mulch. Does not have to be directed.

Chloro IPC . A winter application will control growing annual blue grass and chickweed when used at 60 to 80 pounds per acre of 10 percent granular material or 4 tablespoons plus 2¼ teaspoons to 6 tablespoons plus 1 teaspoon per 100 square feet. Does not have to be a directed application.

Paraquat . Excellent contact killer of most weeds. Non-residual in the soil. Must not contact the foliage but can contact bark of trees. Not widely available in the United States. It is used extensively in European nurseries. Suggested rate of application is ½ to 1 gallon per acre. An effective spray dilution is 1 tablespoon in a gallon and spraying to wet weed foliage. A wetting agent must he included in the spray.

2-4 D . Low volatile ester or amide used to control some of the perennial broadleaved weeds. Great care must be used in application. Do not use in hot weather. Many formulations available use one pound of active ingredient per acre. Follow directions on the package for spot or lawn treatments.

Weed control chemicals have greatly helped the commercial production of rhododendrons. As they become available in smaller packages, more people will be trying them in their home gardens. I hope the use of these materials will free people from staring at weed-infested ground so that they may appreciate more fully the beauty of rhododendrons.