Weed Management in New Tree Plantations

Dave Hall, WI DNR (12/30/98)

Weeds are some of the worst enemies of young tree plantations. Unchecked weed growth will weaken young trees, slow their growth and often kill them. Until recently, most growers of forest plantations relied on herbicides to keep their plantations weed-free. However, restrictions on the use of herbicides and worries about their environmental effects are causing tree planters to adopt a new approach to weed control, "integrated vegetation management."

New Tree Plantation: Treated bands in tree rows with alfalfa left between rows.

What is Integrated Vegetation Management?

Integrated Vegetation Management integrates several practices in combination to minimize weed competition. It relies on tilling, cover crops and mowing to control most of the weed growth; herbicides are used selectively around the tree seedlings themselves. Experiments are also underway to test the value of mulches and protective tubes.

How Does it Work?

Lets look at an example: in late summer, a grower plows down an old alfalfa/brome grass hay field and disks, drags and seeds the entire field to a perennial grass cover crop. The following spring, the farmer plants hardwood and conifer seedlings in rows and treats a two-foot band of soil in each row with a pre-emergent herbicide. The weed-free band is maintained and the grass between the rows is mowed once or twice per year until the crowns start to touch each other.

This integrated approach to weed control is what is meant by integrated vegetation management.

How Can I Control Weeds?

Tilling can often be used over an entire planting area on level ground, or in contour bands in erodible terrain. A single moldboard plowing will kill alfalfa and most perennial grasses. (Quack grass may require a year of summer fallowing.)

Disking between the tree rows can be an effective means of controlling weeds after planting. If the trees were originally planted in a square grid, disking can be done in both directions.

Cover Crops protect the soil and prevent invasion by weeds.

• Timothy is inexpensive to plant, stays reasonably short and can be mowed once a year, after it forms a seed head.
• Perennial ryegrass stays reasonably short and can be mowed often. However, it is expensive to plant, is subject to winter kill, and low, frequent mowing often allows invasion by dandelions.
• Orchard grass is inexpensive to plant, stays relatively short and is moderately shade tolerant.
• Tall fescue and red fescue are often used as cover crops but may be allelopathic to black walnut roots.

Mowing is a well-known weed control method and is effective between rows. However, because it does not kill the weeds, the grower must monitor weed growth regularly and mow often.

Mulches may be applied with varying success. Plastic mulches are difficult to keep in place and are not very long lasting. Shingles are long lasting but can be blown around by strong winds. Wood chips and sawdust are sometimes effective for a season or two if perennial weeds have been removed first. Protective tree tubes are in the demonstration stage, but are expensive.

Herbicide restrictions change frequently. For example, the forestry section has been removed from several simazine labels. Oust is now registered for preplanting and post-planting application in over the top of several tree species. Keep abreast of new products and new pesticide restrictions by checking with your local DNR forester or a private forester.

Using Integrated Vegetation Management, a grower can achieve effective and environmentally-safe weed control. It requires careful planning, but your efforts will pay off with a well-established, healthy tree plantation.

Of special note:

• Perennial grasses are among the worst enemies of young walnut trees, through the sapling stage. Heavy brome and quack grasses overtop and smother newly planted trees, and heavy bluegrass sod often causes stagnation of sapling walnut plantations.
• Alfalfa is difficult to kill after trees are planted. Once the trees are planted, the only control method available may be hand application of herbicides. A separate leaflet is available from the Forest Pest Management Unit, Wisconsin DNR that details alfalfa control.

Weeds  ||  Forest Health Protection