Granular pre-emergence herbicides preferred
The most effective method of weed control in a container nursery is the use of an appropriate granular pre-emergence herbicide.
While pre-emergence herbicides are an integral part of an effective weed management program, users should remember that most products are not labeled for use inside enclosed structures such as greenhouses and hoop houses. Pre-emergence herbicides are applied to the media before weed seed germination.
After being activated by rainfall or irrigation, these herbicides form a barrier at or just below the media surface. When the roots or shoots of germinating seeds come in contact with the herbicide barrier, their growth is inhibited.
Most pre-emergence herbicides are cell division inhibitors affecting the emerging root and shoot, which are sites of rapid cell division. Weeds that have already emerged (visible) are not consistently controlled because their growing point has escaped contact with the herbicide.
When selecting the proper herbicide, a grower must consider the weeds to be controlled (e.g., grassy weeds versus broadleaf weeds) and the crop tolerance to the herbicide.
Proper weed identification is critical to a successful management program. Once proper weed identification has been completed, growers can match selected weeds with the appropriate herbicide. Read and follow the label directions for the specific herbicide used. Calibration trays are available at the dealer for Snapshot, Rout and OH2.
In general, granular formulations are used more frequently than sprayable formulations. While granular formulations tend to be more expensive than sprayable formulations, they tend to be better suited for container growers.
The most common granular herbicides used in container woody plant production are Ronstar, OH2, Rout and Snapshot. Repeated use of the same herbicide or herbicide in the same chemical family will lead to a shift in weed populations.
One study in 2001 reported average herbicide cost per three-gallon pot per year for OH2, RegalKade G (1.5 pounds/1,000 square feet), Regal O-O, Snapshot TG and Treflan at $0.05, $0.05, $0.05, $0.08 and $0.01, respectively.
In 2001, North Carolina State University reported that hand weeding costs averaged $420/1,000 three-gallon pots over four months.
Read and follow label directions carefully as some products may recommend a waiting period before applying the chemical following potting. Two to three applications are required during a growing season.
To be most effective, pre-emergence herbicides should be applied to a media surface that is 100% free of weeds. Producers are seldom satisfied with any herbicide applied after weeds have gotten a head start, even after hand pulling. Tiny weeds are always missed.
Dr. James Atland, Oregon Ext. Weed Specialist, said during his talk 1-06 in Ohio that the 200 lbs/Acre rate of Snapshot is effective in preventing regrowth of woodsorrel or oxalis after hand pulling.
This information came from the Univ of Arkansas publication FSA6123, ?Weed Control in Container Nurseries? written by Drs. Jim Robins and John Boyd, Nov, 2005. I modified it slightly to fit Tennessee conditions.