JARS v41n1 - Weed Control Around Rhododendrons

Weed Control Around Rhododendrons
Robert L. Ticknor, Ph.D.
Canby, Oregon

Nature abhors a vacuum and unfortunately fills it with plants we call weeds (plants out of place - plants for which a use has yet to be found). The vacuum that tends to be filled is the space between our choice azaleas and rhododendrons particularly in new plantings.
There are five methods of preventing weeds from taking over. Cultivation controls most weeds but will damage the shallow roots of azaleas and rhododendrons. Hand pulling of the occasional weed that escapes other control methods is not too laborious and is safer than chemicals when the weed is growing in the root ball of the plant. Ground covers or companion plants can fill the space between the rhododendrons and prevent weed growth by competition and shading of the soil surface. Mulching, particularly when renewed annually, controls most weeds. A depth of 2-3 inches (5-7.5 cm) initially and 1-2 inches (2.5-5 cm) annually is needed to reduce weed growth. In most plantings, mulching is the best method for weed control. Chemical control may be the only method for control of some perennial weeds and may be the most economical method of weed control in large planting areas.
Living mulches (companion plants) for use around azaleas and rhododendrons were discussed in the Summer 1983 issue of the ARS Journal by R.T. Johnson and R.E. Lyons. Mulching with organic materials is discussed in many articles and books. There are new weed controlling fabrics to be considered as an alternative. These are woven or non-woven fabrics which are laid on the soil surface before applying a mulch such as bark. Unlike black polyethylene they permit air and water to pass through but still restrict the penetration of weed roots. Mulch can be used in small amounts for aesthetic reasons in conjunction with the fabric. The disadvantages of the fabric are cost and difficulty in planting since the fabric must be cut and removed.
Chemical control of weeds is more complex than the other methods so most of this discussion will be on this method. Several terms are used when discussing chemical weed control and these are defined as follows:
Active ingredient or A.I.A. - The amount of chemical to apply most often is stated as the amount of active ingredient per acre or A.I.A. Herbicides are available in different concentrations and formulations but the amount of active ingredient to apply remains the same.
Selective - Kills only certain types of weeds. The grass killers Poast and Fusilade are examples.
Non Selective - Kills a broad spectrum of weeds regardless of type.
Sterilant - Kills all vegetation for an extended period.
Fumigant - Applied pre-planting usually under a plastic cover. Weed seeds and plant roots in the treated area are killed. Soil diseases and insects may also be controlled.
Preplanting - Applied before the crop is planted. Some volatile residual herbicides such as Eptam and Treflan are shallowly incorporated which is best done without the crop present. Killing perennial weeds is best done before planting with materials like 2,4D and Roundup which leave little or no soil residue.
Pre-emergence - Applied before weed seedlings germinate. Most herbicides used in landscape plantings and nurseries are used in this way.
Post-emergence or Contact - Herbicides absorbed by the leaves or roots of growing weeds.
Spray - Water or oil is used to dilute the active chemical which is applied with a small compressed air or large power sprayer. Products for spraying are labeled E.C. emulsifiable concentrate, W.D.C. water dispersible granule, W.P. wettable powder, S.P. soluble powder. For equipment without mechanical agitation E.C. or S.P., types are best.
Granular - The active ingredient is coated or impregnated on clay or sand granules and is applied without dilution.
Wipers - Applicators which are dragged across the weed to be killed. These are either a wax bar containing the herbicide or a rope or pad connected to a reservoir of concentrated herbicide solution.
Broadcast - Herbicide applied to the total area.
Band - Herbicide applied in strips next to the crop with the row middles untreated.
Soil Incorporated - The herbicide has to be incorporated mechanically or with water into the top 2-3" of the soil to be effective. This prevents volatilization or photodecomposition on the soil surface.
All herbicides registered for use around azaleas and rhododendrons will give good control of many weeds but usually there are a few which they do not control even at higher than normal rates. If the weeds which are not controlled are not present, there is no problem. However some weeds which were rarely seen can become problems when there is no competition. In one nursery, Polygonum aviculare - Dooryard Knotweed, normally a prostrate weed growing in compacted soil areas, grew over the tops of young rhododendrons when Dichlobenil eliminated the competition. If a weed is only partially controlled by a herbicide, resistance to that herbicide may develop if the surviving weeds are permitted to go to seed. Tables have been prepared of the susceptibility of different weeds to individual herbicides.
Identification of the weeds present and those likely to develop at a different season helps determine which herbicides to use. If the weeds are not known, then a combination of two herbicides which control different weeds can be used. Alternating different herbicides is another good practice to reduce the resistance problem and the potential buildup of a herbicide in the soil.
Environmental, plant and soil factors influence herbicide activity. Rainfall or irrigation is necessary to activate most pre-planting herbicides by triggering germination of weed seeds. But if rain occurs too soon after the application of a foliage active herbicide it may reduce or eliminate the activity. High humidity by slowing drying time usually makes foliage applied herbicide more effective. Cool temperatures are needed to prevent the loss of CIPC, Dichlobenil, and Pronamide by volatilization so they are applied from October through February. Dichlobenil and EPTC can be lost with soil moisture as it evaporates from the soil surface.
There are several plant factors which influence the effects of herbicides on both the weeds and rhododendrons. Probably the most important of these is the age of the leaves and plant. Young leaves are not as waxy and chemicals penetrate them more readily. Also a large proportion of the root system of a young plant is near the surface where it is more likely to absorb a damaging amount of a soil applied herbicide. Some plants, including weeds, are not affected by a particular herbicide. To kill a plant, the herbicide must be absorbed, translocated to a site of activity, and cause disruption of the normal metabolism.
Soil type may be responsible for the success or failure of a herbicide application. Organic matter, including mulching materials, can reduce the effectiveness of some herbicides by absorbing them. Conversely, a very sandy soil may not absorb the herbicide and it washes into the root zone causing injury.
Technically, a herbicide should not be applied to a plant unless the plant's name appears on the label. Practically this is impossible since listing all the cultivars grown even of azaleas and rhododendrons requires a book, so registration is usually for the genus Rhododendron , but in some cases specific cultivars are mentioned on the label.
Most often a new herbicide is tested on evergreen azaleas for a couple of reasons. Plants are available in quantity at relatively low cost and much of the test work is done in southern areas where azaleas grow very well.
When starting to use herbicides do so on a small scale. These products should be used at the label rate - a little more could be a lot worse. Dichlobenil used at a 2X rate killed 'Hino-crimson' azaleas and 'Vulcan' rhododendrons in containers (it is not registered for container use). There were no symptoms to the tops at 1X but no roots grew in the top 1.5 inch of media. Cultivars of evergreen azaleas can vary markedly in injury from the same herbicide - Simazine. Because of the variable reaction, azaleas and rhododendrons are no longer listed on the label of Simazine.
The usual result of using a herbicide labeled for use on azaleas and rhododendrons on a particularly susceptible cultivar is injury not dead plants. Most plants will recover if application of the injurious herbicide is not repeated.

Woody Weed Plant Controls
Weed Herbicide Time of Application
Blackberry, Evergreen
Rubus laciniatus
Roundup September-October
Krenite September
Blackberry, Himalaya
Rubus procerus
Roundup September-October
Krenite September
Garlon mid summer
Gaultheria shallon
Garlon summer
Blackberry, trailing
Rubus vitifolius
Garlon mid summer
Poison Oak
Rhus diversiloba

Table 1 lists the herbicides registered for use on azaleas and/or rhododendrons and the weeds which are controlled by these herbicides. This information is based on research at Oregon State University, published reports, and manufacturer's labels. A performance code of G-good, F-fair, P-poor, or N-no control and blank for no information is used. In some cases the designation is P-G which indicates variable control which could be the result of many factors.
There are some woody weed species which can be problems in rhododendron plantings that are not listed in Table 1. Unfortunately most of the chemicals which will control these plants will also cause injury to rhododendrons or are only available to licensed applicators. Chemical control should be done before planting into any area infested with noxious weeds using chemicals and rates suggested by the manufacturer for your problem weed.

Table 1.  Herbicides Registered For Use on Rhododendrons and Weeds Controlled by Them.
Chemical Name Bensulide Chlora-miben Chlor-propham DCPA Dichlo-benil Diph-enamid EPTC
Fluazi-fopbutyl Glypho-sate Meto-lachlor Napro-pamide Napta-lam Oryzalin Oxadiazon Oxyfluorfen
+ oryzalin
+ pendi-methalin
Prona-mide Seth-oxydim Tri-fluralin
Trade Name Betasan, Betamec Ornamental Weeder Furloe, CIPC Dacthal Casoron, Norosac Enide Eptam Fusilade Roundup Dual Devrinol Alanap Surflan Ronstar Rout Ornamental Herbicide II Kerb Poast Treflan
Annual Bluegrass
Poa annua
G N G F G P G G G G F - G G G G G
Barnyard Grass
Echinochloa crusgalli
G G G F P - G P - G G G G G F - G G G F - G G G F G G
Bedstraw, Catchweed
Galium aparine
Bittercress, Little Western
Cardamine oligosperma
G G N G P - G P - G G G G N P - N
Canada Thistle
Cirsium arvense
Mollugo verticillata
Malva parviflora
Chickweed, Common
Stellaria media
Chickweed, Mouse-eared
Cerastium triviale
Clover, Crimson
Trifolium incarnatum
Clover, Subterranean
Trifolium subterranean
Spergula arvensis
Digitaria sp.
G G G F - G G G G G G G G G F - G G G F G G
Taxaxacum officinale
Rumex sp.
Dogfennel, Mayweed
Anthemis cotula
G P G N P - G C G P N P
Fireweed, Willowherb
Epilobium angustion-folium
F - G P P G
Senacio vulgaris
G P G P P - G G G G P G G G N P - N
Equisetum arvense
Henbit (Deadnettle)
Lamium amplexicaule
G G P G G P - N F G G G G F - G
Knotweed, Common
Polygonum aviculare
G G P - G G G G F - G N P - G G G G G
Knotweed, Spotted
Polygonum persicaria
Chenododium album
Petty Spurge (Milkweed)
Euphorbia peplus
Miner's Lettuce
Montia perfoliata
G G N - F G G G G G

Table 1.  Herbicides Registered For Use on Rhododendrons and Weeds Controlled by Them.
Common name Bensulide Chlora-miben Chlor-propham DCPA Dichlo-benil Diph-enamid EPTC Fluazi-fopbutyl Glypho-sate Meto-lachlor Napro-pamide Napta-lam Oryzalin Oxadiazon Oxyfluorfen
+ oryzalin
+ pendi-methalin
Prona-mide Seth-oxydim Tri-fluralin
Trade Name Betasan, Betamec Ornamental Weeder Furloe, CIPC Dacthal Casoron, Norosac Enide Eptam Fusilade Roundup Dual Devrinol Alanap Surflan Ronstar Rout Ornamental Herbicide II Kerb Poast Treflan
Morning Glory (Field)Convolvulus arvenis F N F - G P - N F P P P P P P - F
Mustard Brassica sp. G G N F P N N - P G N G G G F N
Nightshade, Black Solanum nigrum G G N G G P P G G G F P
Nutsedge, Yellow Cyperus esculentus G N G F F - G P P N P - N P - N N N
Oxalis Oxalis sp. G N N G G G G P - F
Pearlwort Sagina procumbens
Pigweed, Redroot Amaranthus retroflexus G G F G G P - G G G F - G F - G G G G F G
Pineapple Weed Metricaria matricarioides P G G G G P - N G N P - N
Plantain, Narrowleaf Plantago lanceolate G G F G
Prickly Lettuce Latuca scariola G P - N G F - G P - N F - P F F P - N
Purslane Portulaca oleracea G C G F - G G G G F G G G G G G G G
Quackgrass Agropyron repens G G G P P F
Ryegrass Lolium sp. F G G P - F P - G N P - F P - F G P - F
Scarlet Pimpernel Anagallis arvensis N N
Sheepsorrel (Red Sorrel) Rumex acetosella G F
Shepherds purse Capsella bursa-pastoris G G N P - G F - G G G P G P - G G G G F P - G
Sowthistle, Annual Sonchus oleraceus G F G G P G G G P - N
Smooth Cat's Ear Hypochoeris glabra N G N G G G N
Speedwell (Veronica) Veronica sp. G G F G G G F - G
Thistle, Bull Cirsium vulgare G G N
Vetch Vicia sativa G N G
Wild Carrot Daucus carota G G G

Consult with your local extension office for information on the control of problem weeds in your area.
Table 2 lists the rates normally used for the registered herbicide. It also gives the weight in grams or milliliters for a 100 square feet area and the number of tablespoons or teaspoons for 100 square feet. Amounts for small plots are provided as a convenience to users. It is illegal and not safe to transfer pesticides from the original container except for immediate use. Be sure to read the label before using any herbicide as well as other pesticides.

Table 2. Herbicides Registered on Azaleas, Deciduous Azaleas, and/or Rhododendrons.

Normal rates of active ingredient per acre (A.I.A.) and the amounts required to treat 100 square feet with different formulations and the number of cups (c), Tablespoons (T) and teaspoons (t) to approximate that amount.
Chemical Name Trade Names Lbs. A.I.A. & formulation Grams or c.c.
per 100 sq. ft.
Spoons per
100 sq. ft.
Bensulide Betasan, Betamec, Prefar 7.5 4 EC 16.3 cc
3.6 G 217 G *
Chloropropham Furloe 6 4 EC 13.2 cc
20 G 31.3 G 2T+ 2t
Chloramiben Ornamental Weeder 4 2 S.C. 17.4 cc
10 G 41.7 G *
DCPA Dacthal 6 75 W 8.3 G 1T
5 G 125 G c + 2T + 2t
Dichlobenil Casoron, Norosac 4 50 W 8.3 G 1T + t
4 G 104 G 5T
Diphenamid Enide 6 90 W 7.0 G 1T + t
50 W 12.5 G 1T + t
5 G 125 G C + 3T + 2t
EPTC Eptam 6 6 EC 8.7 cc
10 G 62.5 G c+ 1T + 2t
Fluaziflop Fusilade 0.5 4 EC 1.1 cc
Glyphosate Roundup, Ortho, Kleenup 1 4 SC 2.2 cc
Metolachlor Dual 2 8 EC 2.2 cc
25 G 8.3 G *
Napropamide Devrinol 4 50 W 8.3 G 1T+ 1t
10 G 41.7 G 3T + t
Naptalam Alanap 4 2 S.C. 17.4 cc
Oryzalin Surflan 2 75 W 2.8 G 2t
4 FL 4.3 cc
Oxadiazon Ronstar 2 50 W 4.2 G 2t
2 EC 8.7 cc
2 G 104.2 G c + 1T + 2t
Pronamide Kerb 1 50 W 2.1 G 1t
Sethoxydim Poast 0.29 1.5 EC 1.7 cc
Oxyfluorfen +
Ornamental Herbicide II 2+1 2 + 1 G 104.2 G c+ 1t
Oxyfluorfen +
Rout 2+1 2 + 1 G 104.2 G c+ 1T + 2t
* Tablespoons and teaspoons of this product not weighed
1 Tablespoon (T) 15cc
1 Teaspoon (t) 5cc
Formulation Types Units
EC = Emulsifiable concentrate lbs/Gal. Active ingredient
FL = Flowable = powder dispersed in liquid lbs/Gal. Active ingredient
G = Granular formulation Percent Active ingredient
SC = Soluble concentrate lbs/Gal. Active ingredient
W = Wettable powder Percent Active ingredient

Bob Ticknor, professor of horticulture at Oregon State University's North Willamette Experiment Station, presented a workshop on herbicides at the Western Regional Meeting, fall 1985. This article is based on that presentation.