Chemical Weed Control in Rhododendron Plantings
By David G. Leach
The graphic account by Mr. George D. Grace of his battle with weeds in his rhododendron plantings, as described in the January issue of the "Bulletin," must have received a sympathetic response from many enthusiasts with sizable collections. I believe I can contribute at least a partial solution to the problem and I hope that it will be useful to Mr. Grace and to others, as it has been to me.
A few years ago I experimented with a variety of chemicals in an effort to induce the setting of flower buds on hybrid seedlings intended for further breeding. Among the chemicals used were 2,4-D and 2,4,5-TP, the active ingredients in commercial weed killers, and I noticed that the tolerance of rhododendrons to these chemicals increases more rapidly as the season advances than does the resistance of woody weeds. Applying this observation to my own plantings has saved an enormous amount of labor.
My hybridizing and trial grounds are in an open woodland where wild Cherry, Maple, Oak and brambles together with both annual and perennial herbaceous weeds spring up quickly, especially where the soil has been disturbed. Quack Grass was brought in, concealed in the earth balls of purchased plants, and has spread in some areas. After several years of cautious, limited experiment both weeds and grass have been controlled almost exclusively by chemical means.
About the middle of July the weeds are sprayed with a water solution containing 1% of "Weedone Brush Killer 64," made by the American Chemical Paint Company, Ambler, Pa. In areas where Quack or other grass is a problem, "Dalapon", made by Dow Chemical Company, is added to the solution at the rate recommended in the manufacturers directions for spot treatment. In early September late-germinating weeds and grass are sprayed once again. The following spring the weeds are cut, not pulled, and the July spray kills them in the course of their second growth.
The results have been gratifying. No rhododendron has yet been injured, either in the rows of hybrid seedling progeny from my crosses, or in the plantings of larger specimens undergoing evaluation elsewhere in the woodland. Avoiding disturbance of the soil has brought about a phenomenal reduction of weed germination. It is apparent that many of the weeds which previously emerged through the shallow leaf mulch sprouted where the soil had been disturbed by pulling their predecessors.
The spray should be rather coarse, applied at low pressure from a knapsack sprayer with a hose long enough for the nozzle to be held close to the weeds. On a still day there is very little drift.
Where "Dalapon" is added to the solution or applied separately to kill grasses, there is moderate discoloration of the leaf tips of scaly leaved rhododendrons, but not of the elepidotes, if it is used directly over the root area. There appears to be no permanent injury and since one application when the ground is moist usually kills Quack and other grasses, this discoloration from a September spray is not too serious. It is not ordinarily necessary to use "Dalapon" around the same plants the following season.
Readers should note carefully that this program depends for its success on the marked increase of resistance to 2,4-D and 2,4,5-TP by rhododendrons as the season progresses. A spray when these plants are producing succulent new spring growth could be extremely hazardous.
Inasmuch as climatic and growing conditions vary so greatly throughout the regions of the country where rhododendrons are cultivated, this program should be applied cautiously, on a limited scale at first, by those who wish to try it, adapting the timing of the first spray to the growth cycles of the plants as they appear locally. In a season of heavy rain, with late growth, the maturity of the foliage rather than a fixed date should govern application of the chemicals.
I believe the time is at hand when the maintenance of large rhododendron plantings can be enormously reduced by chemical control of weeds. Mechanical cultivation, so destructive to the surface roots of these shrubs, need no longer be the economical alternate to hand weeding in nurseries.