Innovations for container weed control

By Ian T. Roul and M.A. Lemay

Handweeding container grown stock is not economically feasible. While recent work on costs of handweeding is not readily available, a 1974 study placed the value at $3600/acre (Gilliam et al. 1990). As a result, nurseries have employed pre-emergent and post-emergent herbicides as their primary source of weed control. The most common form of application is cyclonic broadcast of granular herbicide, which is associated with significant non-target herbicide loss. It has been found that, depending on pot size, anywhere from 20 to 80 per cent of applied herbicide never makes it into the pots, but falls in the spaces between pots (Gilliam et al., 1992). At $1398/ha for an average season of herbicide treatment, the amount to be saved ranges from $279/ha to $1118/ha (Table 1).

     Recent research has identified two new methods that combat both the economic and environmental problems associated with non-target herbicide loss: slow release herbicide tablets and recycled newspaper pellet mulch.

Slow release herbicide tablets
Slow release tablets were developed in response to the high amounts of herbicide lost from container grown stock. The premise is a simple one, concentrate and slow down the release of the herbicide - and make the tablets large enough that each can be hand placed easily (Gorski, S.F., 1993).

     Earlier work with slow release tablets was less successful due to the inability of tablets to maintain shape and the low water solubility of most common herbicides. These issues were resolved with the use of dicalcium phosphate as filler, and a one to two per cent surfactant (Smith and Treaster, 1990).

     If tablets are punched, each will weigh 1.25 g. When placed on the soil surface, each tablet produces a mean area of inhibition of 10.9 cm2 (Horowitz et al., 1990). For each one-gallon pot, three tablets provided excellent weed control, ranging from 66 per cent to 100 per cent (Smith and Treaster, 1990).

Recycled waste paper pellets
Field nurseries have had success in managing weeds with recycled newspaper mulch. The process involves spreading a mixture of newspaper strips and water over the field. This concept, however, is not practical for container grown stock. Recently, recycled waste paper pellets have been developed, which enable container nurseries to utilize the same strategy. Recycled newspaper pellets are made by grinding waste paper, and then compressing the pulp into 3-1/6 in. x 1 in. pellets. These pellets are available from Tascon, Inc., Houston, TX.

     Recycled waste paper pellets act as a mulch and barrier to germination of weed plants. They are more effective than strips of newspaper or news­paper granules because they are denser. Research results indicate that when applied at a depth of 25 mm, pellets provided excellent control of spurge (Table 2). Comparisons to Rout 3G herbicide were favourable, with both processes being classified as excellent (Table 3). Growth indices (Height + 2 Perpendicular Widths/3) were similar among treatment types but pellets applied at a depth of 25 mm resulted in lower indices. This is believed to be caused by excess moisture retained in the pellet mat. To compensate, an altered watering plan could be put into place.

     Recycled newspaper pellets have shown to increase aluminum levels in the soil, which can have a negative impact on the growth of the plants. It was found, however, that aluminum levels could be reduced if triple superphosphate was added at 7.5 mg/l (Table 3) (Smith et al., 1998).

Secondary measures
Research has shown that both slow release tablets and newspaper pellet mulch are effective for weed control and do not affect the growth or health of crops. These primary techniques can be augmented with additional low cost to no cost techniques for even greater weed prevention.

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

Solarization or steam
In cases where there is a significant weed seedbed, a pre-treatment with heat can provide the necessary starting point for effective use of either newspaper pellets or slow release tablets. 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 55°C or greater. This works well in warm areas or summer months of more northern areas, but it is not always practical in northern areas of Ontario. Steaming soil has a similar effect on weed seeds and pathogens because it raises temperatures to even greater levels (90°C) (Quarles, 1997).

Since both newspaper pellets and slow release tablets provide highly effective weed control and reduce the amount of herbicide used, it becomes a question of priorities. Slow release tablets are the most economical form of weed control once the pressing equipment is acquired. It is the most appropriate option for areas that are not located near sensitive groundwater or other natural features whose contamination could result in liability situations. As the cost of recycled newspaper products decreases, the attractiveness of this non-chemical treatment should increase (Table 4). Either method reduces non-target herbicide loss, resulting in lower costs and reducing negative environmental impacts.

     It should be noted that the use of herbicide tablets may require regulatory approval. The Pest Management Regulatory Agency should be consulted for advice before proceeding.


Slow release tablets:
Elton M. Smith, Hort & Crop Science, Ohio State University, Howlett Hall, Columbus, OH 43210

Newspaper pellets:
Charles H. Gilliam, Auburn University, Auburn, AL 36849 e-mail:

Gilliam, C.H., Fare, D.C. and A. Beasley. 1992. Nontarget Herbicide Losses from Application of Granular Ronstar to Container Nurseries. J. Environ. Hort. 10(3): 175-176
Gilliam, C.H., Foster, W.J., Adrian J.L., and R.L. Shumack. 1990. A Survey of Weed Control Costs and Strategies in Container Production Nurseries. J. Environ. Hort. 8(3) : 133-135
Gorski, S.F. 1993. Slow-Release Delivery System for Herbicides in Container-Grown Stock. Weed Technology. 7(4) : 894-899
Horowitz, M., Smith, E.M. and S.F. Gorski. 1990. Feasibility of Adding Surfactants to Slow-Release Herbicide Tablets for Container-Grown Landscape Plants. J. Environ. Hort. 8(1) : 36-41
Quarles, W. 1997. Alternatives to Methyl Bromide in Forest Nurseries. The IPM Practitioner. XIX(3) : 1-14
Smith, D.R., Gilliam, C.H., Edwards, J.H., and J.M. Olive. 1997. Recycled Waste Paper Pellets Provide Weed Control in Container Production. Highlights of Agricultural Research. 44(4) : 11-13
Smith, D.R., Gilliam, C.H., Edwards, J.H., Olive, J.W., Eakes, D.J., and J. Williams. 1998. Recycled Waste Paper as a Non-Chemical Alternative for Weed Control in Container Production. J. Environ. Hort. 16(2) : 69-75
Smith, E.M. and S.A. Treaster. 1990. Increasing Efficiency of Slow Release Herbicide Tablets. Ornamental Plants: A Summary of Research. 1990. The Ohio State University, Ohio Agricultural Research and Development Centre. SC 135 : 39-44
Swanson, B.T. and J.B. Calkins. 1996. Weed Control Strategies for Field and Container Grown Herbaceous Perennials. J. Environ. Hort. 14(4) : 221-227

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.