QBARS - v24n4 Combinations of Herbicides and Mulches in Rhododendrons

Combinations of Herbicides and Mulches in Rhododendrons
Dr. Garvin Crabtree, Dept. of Horticulture,
Oregon State University, Corvallis, Oregon

In the Pacific Northwest, mulching with saw dust or bark dust is a common practice in the culture of rhododendrons in ornamental plantings. Often mulching alone does not control all the weeds so it would be desirable to use some herbicide in addition to the mulching material. The experiment reported here was designed to study interactions that may occur as a result of using a mulch and a herbicide in combination.
Two cultivars, 'Cynthia' and 'Pink Pearl', grown in 6" pots, since being rooted the previous year, were set in the field in April, 1966. A randomized complete block design was used with four plants in each plot and four replications of each cultivar which were pooled for analytical purposes as eight replications. The various herbicide and mulch treatments are listed in the table that follows. A four percent granular formulation of both herbicides was used at the rates of two pounds active ingredient per acre of Princep, simazine (2-chloro-4,6-bis (ethylamino)- s-triazine), and four pounds active ingredient per acre of Casoron, dichlobinil (2.6-dichlorobin-zonitrile). Medium sized particles of fir sawdust and finely ground fir bark mulch were applied at an unpacked depth of three to four inches in the mulch treatments. The herbicide and mulch treatments were applied during the first week in May, approximately four weeks after the plants were set in the field. Rainfall provided adequate soil moisture for activation of the herbicides. Weed control was provided by hand weeding in check plots and in any other plots as needed, after evaluation of efficacy of the applied treatments. Weed control evaluations were made in late July. 1966, and in May, 1967. Visual evaluation of rhododendron plant response to the mulch and herbicide treatments was also made July 1966, and September. 1967, but no significant effects of the applied treatments could be determined. Final evaluation of crop response was made in 1968 by making flower counts and measuring plant size. Individual plant size was designated as the product of plant height times plant width.
The weed control and plant response data are summarized in the table. These data would indicate that the mulches used in this trial were more effective than the herbicide treatments for controlling weeds, particularly when evaluated more than a year after time of application. There was no important difference between herbicides or type of mulch, however, there may he a slight advantage from the standpoint of weed control to apply the herbicide before the mulch is put down.
Analysis of the plant response data shows that none of the herbicide and mulch treatments affected by the average number of flowers per plant. Plant size was significantly influenced by mulching but not by the herbicide treatments. Larger plants were produced in mulched plots with no difference being exhibited between sawdust or bark mulch.
It can be concluded from the experiment reported here that mulching is beneficial to rhododendron plant growth and in aiding in the control of weeds and that mulching can be done safely and effectively either before or after the application of simazine (Princep) or dichlobenil (Casoron).

Table-Weed control and crop response obtained from herbicide and mulch treatments
in rhododendrons.

Herbicide Time of
Type of
Average Weed
Control Rating*
Plant Size**
1966 1967
none - none 1 3 428
none - sawdust 7 6 588
none - bark 7 6 605
simazine - none 6 4 451
simazine before mulching sawdust 9 7 544
simazine after mulching sawdust 8 6 558
simazine before mulching bark 8 7 518
simazine after mulching bark 8 7 660
dichlobenil - none 7 2 395
dichlobenil before mulching sawdust 10 7 636
dichlobenil after mulching sawdust 8 7 560
dichlobenil before mulching bark 9 7 502
dichlobenil after mulching bark 8 6 585

*Weed control rating: 0 to10 with 0 = no control, 10 = complete control of weeds.
**Plant size: product of maximum height (inches) X maximum width (inches).