QBARS - v11n3 Rhododendron Weed Control by Chemicals

Rhododendron Weed Control by Chemicals
By R. L. Ticknor and P. F. Bobula
University of Massachusetts, Waltham, Massachusetts

We realize that mulching is the best means of weed control in rhododendrons; however, mulching is not always feasible in large commercial plantings. Since cultivation destroys part of the shallow root system, weed control by chemicals may some day be a part of commercial rhododendron production.
For three years we have been conducting chemical weed control experiments in a group of Rhododendron catawbiense hybrid seedlings. Growth records have not been taken because of the extremely diverse growth habits. None of the ten chemicals that we have used has produced injury symptoms when the sprays were directed at the base of the plants so that only the basal foliage is contacted.

All of our experiments have been with pre-emergence weed control; that is, the chemicals were applied to weedfree soil to prevent weed growth rather than to destroy established weeds. We are emphasizing chemicals that have relatively low toxicity to established plants, but are active in destroying seeds as they germinate. Our soil, classified as a Gloucester fine sandy loam, might require more chemical to give satisfactory weed control than would a lighter soil.
Herbicide applications were made with a one-gallon jug fitted with a "Weedone Sprayer" nozzle. This method was used so that a separate sprayer could be used for each chemical to avoid mixing the chemicals. For general weed control work around limited areas of plants, we have found a knapsack sprayer equipped with a flat fan-type nozzle to be most satisfactory. Large commercial equipment may not be too adaptable to rhododendrons, since limbs break rather easily. Care was taken to wet a minimum amount of foliage so that only the lowest leaves of a plant were contacted by the chemicals.
Our weed control area consisted of ten 120-foot rows, each of which contained 30 plants. Each row was divided into three 2-foot by 40-foot plots so that chemical treatments could be randomized in the field.
In 1956, chemicals were applied twice during the season-June 14 and August 3. Chemicals used were Alanap-3 at 16 lbs./acre, a combination of Alanap-3 at 3 lbs./acre and Sesone (Crag Herbicide 1, SES) at 3 lbs./acre, Sesone at 10 lbs./acre, Monuron (Karmex W, CMU) at ½ and 1 lb./acre, Diuron (Karmex DW, Telvar DW) at 1/z and 1 lb./acre, Chloro IPC (CIPC) at 4 and 6 lbs./acre, and an untreated check. Each treatment was replicated three times. Other chemicals tried in previous years and found less effective as pre-emergence herbicides were potassium cyanate, Herbisan, Oktone and Pentachlorophenol.
Weed counts and fresh weight of tops were recorded on four one-square foot areas of each plot. In addition, a rating system of 0 to 5 was used, with 0 signifying four weeds or less to one square foot; 1, 20-percent of surface weed covered; 2, 40-percent coverage; 3, 60-percent coverage; 4, 80-percent coverage; and 5, 100-percent coverage. Final records for the June 14 application were taken on July 30, 1956, and for the August 3 application on September 26, 1956.
Contribution No. 1099 of University of Massachusetts, College of Agriculture, Experiment Station, Amherst, Mass.

Results and Observations
Table I lists the average number of weeds, the fresh weight of the weed tops of each plot, and a numerical estimate of the percentage of the soil surface covered by weeds.

TABLE 1. Weed cover ratings. average number of weeds, and fresh weight of the weed tops in grams in established rhododendrons after treatment with 9 chemicals during Summer of 1956.
Chemical Amt. /A Rating 1 (0-5) Rating 2 (0-5) Number
of weeds 2
per 4 sq. ft.
of weeds 2
per 4 sq. ft.
1. Alanap-3 6 lb. 2.58 3.68 251 276
2. Alanap-3
3 lb.
3 1b.
1.08 2.93 147 xx 255
3. SES 10 lb. 1.75 3.25 173 xx 205
4. Monuron ½ lb. 2.00 .75 xx 29 xx 37 x
5. Monuron 1 lb. .08 x .00 xx 0 x 0 xx
6. Diuron ½ lb. .93 .43 xx 16 xx 8 xx
7. Diuron 1 lb. .43 x .00 xx 0 xx 0 xx
8. CIPC 4 lb. .75 x 1.68 x 67 xx 119
9. CIPC 6 lb. .75 x 1.75 x 45 xx 199
10. Check 3.83 3.33 221 330
x - Significantly different from the checks at the 5% level.
xx - Significantly different from the checks at the 1 % level.
1 Period June 14 - July 30, 1956
2 Period 2-August 3 - September 26, 1956

Most consistent weed control has been obtained with Monuron and Diuron, with Chloro IPC being somewhat less effective. Chloro IPC has given good weed control except for one weed, Henbit- Lamium amplexicaule which tends to take over plots treated with this material. Between Monuron and Diuron, the latter would be preferable for rhododendron weed control since it is less soluble and thus less likely to cause plant injury by being absorbed through the roots. Plant injury in the form of chlorosis of the new growth of two-year old box leaf Japanese holly-Ilex crenata convexa plants was observed this year following the use of Monuron. In the same plots were two-year old plants of Japanese yew - Taxus cuspidata , Pfitzers Juniper - Juniperus chinensis pfitzeriana , and spreading Cotoneaster - Cotoneaster divaricata which gave no evidence of injury.

Rhododendron weed control
Fig. 32.  Rhododendron weed control.  Diuron applied at a rate of one pound per acre
on August 2, 1956 following clean cultivation.  Picture taken on October 16, 1956.
Rhododendron weed control
Fig. 33.  Rhododendron weed control.  Check plot following clean cultivation on August
3,  1956.  Picture taken October 16, 1956.

Alanap and Sesone and mixtures of the two chemicals have at times given excellent weed control but have not been consistent in performance. Adequate soil moisture at the time of application appears to be very necessary for satisfactory weed control with these materials. The combination of Alanap and Sesone seems to be most effective during cool weather.
If any reader wishes to set up limited trials for himself, he might try the following materials: Diuron (Karmex DW or Telvar DW) at ½ or 1 pound per acre rates, making no more than two applications during the year. Chloro IPC at 4 or 6 pounds per acre, which is particularly good against purslane - Portulaca oleracea and against annual grasses and chickweed in the fall. A mixture of Alanap and Sesone at a rate of 2 or 4 pounds of each per acre applied in April or May give very good weed control early in the season.

Future Work
Since the pre-emergence type of weed control is based on applying the herbicides to weed-free soil, some cultivation is necessary. To eliminate this cultivation, we will study the use of mixtures of contact and residual herbicides. This way the existing weeds would be killed by the contact herbicide and long-lasting weed control would be provided by the residual herbicide.
Recently a new method of applying herbicides has come into limited use. It is the application of herbicides in granular form. Inert materials such as clay, perlite, or vermiculite are impregnated with the herbicidal chemicals. The resulting material can be applied with a fertilizer spreader. Since the foliage is not wetted, the chance of damage to the plants should be much reduced. Granulars should never be applied when foliage is wet. This coming year we will be testing a wide variety of granular herbicides.