Review and analysis:
Innovations for weed control in nursery operations

By Ian T. Roul and M.A. Lemay, AgTIS (Agriculture Technical Information Service)

One of the primary forms of weed control in field operations is the use of pre-emergent and post-emergent herbicides. With increasing economic, environmental and pro­duction concerns associated with the continued use of herbicides, research efforts have concentrated on alter­native methods for weed control. The ultimate goal is to develop systems that require fewer inputs, decrease production costs and reduce negative environmental impacts.

     Current research focuses on mulches as the most effective means of providing weed management in field nurseries. In particular, the use of recycled newsprint as a mulch and the concept of a companion crop or living mulch show significant promise. Although mulching is not a new concept, these techniques are modifications on existing practices (bark mulches and cover crops), which reduce some of the negative aspects.

     It is not the intention of this report to suggest that one of these techniques alone could provide total weed control. Instead, it is suggested that either practice could serve as the primary component of an integrated weed control strategy, although further research and field testing would be necessary.

Integrated weed control
Herbicide use alone can become costly. As well, there is a growing public sense of environmental impacts. Nurseries that employ less expensive and environmentally safer alternatives gain two benefits: costs savings and perceived environmental stewardship. Recent studies have shown that living mulches can provide effective weed control in field nurseries. Further, there has been recent work with the use of newspaper fiber as mulch. Either of these mulching techniques can be the primary source of weed control, especially when combined with secondary practices such as sanitation or targeted applications of herbicide, when necessary.

Living mulches
Biological controls are often overlooked as not being active enough. Outlined below is an active technique for using living material as a soil supplement and weed control, as well as techniques that reduce the need for responsive action.

     Living mulches are known by several names such as cover crops or green manure, but all work on the same principle. Living mulches out-compete weeds, prevent erosion and add nutrients to the soil upon death. Current literature suggests that rye is the most appropriate cover crop. It has demonstrated the ability to inhibit germination and growth of many weed species. It is an annual and contributes significant amounts of organic matter to the soil (Pritts, 1992).

     A three-season study of the competitive effect of cover crops of tree species showed that growth was similar for Acer, Fraxinus, Gleditsia and Tilia, whether there was a cover crop or clean cultivation (Table 1) (Lumis et al, 1996). In a seven-year study completed in 1996, it was found that, with the exception of Fraxinus, growth was not significantly different between rye treatment and herbicide treatment, based on branching density and an independent qualitative rating (1-5) (Table 2).

Newspaper mulching
Mulches have proven effective in controlling weeds when applied in one- to two-inch thick mats. Mulches can be made up of sawdust, sphagnum, hardwood bark chips, pine needles or old newspaper. Recent work found that wet, shredded newspaper spread at a depth of four inches provided excellent weed control for more than one season. Of the mulching options, four-inch paper mulch provided the best combination of weed suppression with no noticeable negative effect of the growth of plants. Four-inch bark mulch was found to reduce growth of G. x grandifolia (Table 3) (Pellett and Heleba, 1996). A side benefit of paper mulching is reduced soil temperature. Soil temperatures in the control reached 97oF, while mulched soils ranged from 73-75oF.

     When using a newspaper mulch, it becomes important to use a high nitrogen fertilizer. The paper has a high carbon:nitrogen ratio and causes the soil to use large amounts of nitrogen in its decomposition.

Secondary measures
Research has shown that both living mulch and newspaper mulch are effective for weed control and do not effect the growth of crops. These primary techniques can be augmented with other low-cost to no-cost techniques for even greater weed management.

Sanitation is crucial for the success of alternative weed control strategies. Weed seeds contaminate the nursery at two main points of entry - soil and water. Sanitation techniques involve filtering water before crops are irrigated and the careful use of "soils of unknown origin."

Solarization or steam
In cases where this is a significant weed seedbed, a pre-treatment with heat can provide the necessary starting point for effective weed management. Solarization and steam work on the basis that seeds and pathogens are killed by excessive heat. Solarization is the use of plastic coverings to trap heat and raise the soil temperature to 55oC or greater. This works well in warm areas or the summer months of more northern areas, but it is not always practical in the northern regions of Ontario. Steaming soil has a similar effect on the weed seeds and pathogens because it raises temperatures to even greater levels (90oC) (Quarles, 1997.)

Recycled newspaper fiber mulch provides multi-season control for the same cost as a single season of low-end herbicide. Further, it does not have significant negative impact on growth. It can be considered unsuitable for some locations due to aesthetics. In this case, a companion crop should be used. Both of these treatments provide the added benefit of reduced erosion and do not exhibit significant competitive effects.

If more information is required about the above techniques, readers should contact the following:

Rye cover crop
Dr. James B. Calkins, University of Minnesota, Dept. of Horticultural Science
St. Paul, MN 55108
Tel: [612] 624-9331, Fax: [612] 624-4941

Newspaper mulch
David A. Heleba, Research Technician, University of Vermont, Burlington

Calkins, J.B. and B.T. Swanson. 1996. Comparison of Conventional and Alternative Nursery Field Management Systems: Tree Growth and Performance. Journal of Environmental Horticulture 14(3) : 142-149
Lumis, G., Purvis, P., Ali, A. and L. Taurins, 1996. Cover Crops for Nursery Tree Production. Landscape Trades 18(7) : 11-14.
McCargo, H. 1996. Plants the Organic Way. American Nurseryman. 184(10) : 30-35.
Pellett, N.E. and D. Heleba. 1996. Yesterday's News, Today's Mulch. American Nurseryman. (183)1 : 42-44.
Pritts, M.P. 1992. Cover Crops Inhibit Weed. Fruit Grower. 112(5) : 28 - 29.
Quarles, W. 1997. Alternatives to Methyl Bromide in Forest Nurseries. The IPM Practitioner. XIX(3) : 1-14.

After completing a B.Sc. in physical geography at McMaster University, Ian Roul went on to complete a post graduate certificate at Niagara College in ecosystem restoration. Currently, he is working on the restoration of a mined peat bog near the Quebec/New Brunswick border.

Amy Lemay, M.Sc. is a research analyst with AgTIS, the Agriculture Technical Information Service.

The above is one of five research reports prepared by AgTIS under contract to the Landscape Ontario Growers' Group.