Calvin Chong and Peter Purvis, Department of Plant Agriculture, University of Guelph Division: Horticultural Research Institute of Ontario, Vineland Station

It has been estimated that a typical mid-sized container nursery could spend in excess of $40,000 annually on weed control.

     Container production has increased rapidly and represents about 30 per cent of Canadian-grown nursery stock. Weeds are a major problem in container nurseries because they compete for the limited amount of water and nutrients within the container, thereby reducing plant growth, quality and saleability.

U.S. nurseries effectively control weeds in containers with a spectrum of licensed herbicides. In fact, on the recent Landscape Ontario Growers' tour of several nurseries in Lake County, Ohio, it was interesting to see how "clean" and weed-free the containers were, mainly due to their reliance on herbicides. However, our container nurseries have traditionally resorted to different means - primarily hand weeding, and the use of a limited number of herbicides for container weed control (OMAFRA 1997).

Weed Control Discs
Another option for weed control is the weed disc. The typical disc is round and has a slit so that it can be fitted around the stem of the plant. The effectiveness of the disc is reduced with multi-stemmed plants. It should be easy to apply; should fit snugly on top of the container mix; should not easily be dislodged or wind-blown; should allow penetration of water to the mix; should not support weed germination and growth on its surface; should be available in various sizes; and should be durable and cost effective (i.e. perhaps costing less than $.05 or $.10 for a #2 container).

Historical
In the early 1980s, Connon Nurseries (AVK, Rockton, Ontario) introduced the "Weed Guard" in Ontario. This disc is made of semi-rigid plastic similar to a 45 rpm record. Small holes allow water to penetrate - but also the escape of weeds. Two discs in off-set positions provide better control than a single disc.

     Chong et al. (1989) reported an 85 per cent reduction of container weeds using "Mori-Guard" discs made from geotextile fabric (1/16-inch thick, introduced by Mori Nurseries, Niagara-On-The-Lake, Ontario), or from foam (1/8-inch thick, similar in consistency to polyfoam used for winter storage protection and packaging). The foam disc tended to curl upward at the edges, partially exposing the surface of the container mix.

     It is noteworthy that, during the past 12 years, the Ornamental Nursery Program at the Horticultural Research Institute of Ontario has annually re-used the same fabric discs obtained originally from Mori Nurseries. Unfortunately, these discs are unavailable, most likely due to the high unit cost.

     Since then, several types of weed discs have "come and gone" because they did not meet one or more of the criteria listed above. However, there seems to be a renewed interest in weed discs (Mervosh 1999), due in part to limited choice and experience with container herbicide use, and also to their potential negative impact on both plant and environment. Nonchemical alternatives may also mitigate concerns of environmentalists that crops will become resistant to chemicals as we continue to use them in our crop production systems.

The Next Generation
Since 1998, we have been evaluating several "new-generation" weed discs fabricated from materials such as pressed peat moss, cardboard, fabric, and plastic (Table 1), and comparing them to mulches of pine sawdust and paper mill waste, to various herbicides, to selected "old-generation" weed discs, and to the Mori "Weed Bag." Mori Nurseries introduced this patented method of weed control in the early 1990s. Black polyethylene sleeves with pre-punched holes are placed onto the containers in the same fashion that florist plants are prepared for market.

     Although this research, supported jointly by Willowbrook Nurseries, Fenwick, Ontario and The National Research Council of Canada, Industrial Research Assistance Program (IRAP), is still in progress, we believe it interesting and worthy to draw further attention at this time to the various "nonchemical ways" of container weed control. Previously, we made and used weed discs fashioned from commonly-used synthetic weed fabrics. We suspect there may be other materials that are inexpensive and potentially worthy of similar use. There seems to be a good market for the right product.

     Nonchemical alternatives will remain important as long as herbicide use is restricted, and may become even more so should there be similar restrictions in other jurisdictions

Literature cited
Chong, C., B. Hamersma and D. Ponzo. 1989. In search of the ultimate weed control disc. Horticulture Review 7(17):8-11.
Mervosh, T. L. 1999. Weed patrol. American Nurseryman 190(5):32-38.
Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA). 1997. Nursery and landscape plant production. Publ. 383. OMAFRA, 1 Stone Road W., Guelph, Ont. 202 pp. + appendices.