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4 Ways To Keep Weeds From Trees

If you are beyond the point of installing rolls of plastic mulch, another option is to purchase “Brush Blankets.” The one-metre-

square pieces of plastic can be fitted and anchored around individual

trees to provide a weed-free zone.

Trees grow slow enough on the Prairies. Without a weed-free environment in the first three years, establishing a new row of trees will take many years longer.

Before planting, you should either chemically or culturally control existing vegetation on the planting site as well as ensuring the soil is adequately worked to a depth of 15 to 20 cm (six to eight inches). After that, you will still require a weed control action plan. This will help your trees thrive and survive this summer and for many years to come.

To be successful in controlling weeds, you should assess the type of weed control method that best meets your requirements for time management, budget and overall expectation of success.

The following information outlines four areas of weed control that can be implemented individually or used in an integrated approach for shelterbelt weed control.

1. ORGANIC AND INORGANIC MULCHES

Organic mulches include sawdust, wood chips and flax shives. Flax shives are the byproduct of a process that extracts the fibre from flax straw to manufacture paper products. Natural mulch fibres are an excellent weed control option for in-row weed control when properly applied. Organic mulches form a compact, dense layer that suppresses weed growth by smothering weed seedlings and blocking out sunlight.

You can apply them by hand or with a modified manure spreader or a bunk feeder. Do not apply more than 10 cm (four inches) as a settled depth on the soil.

On nutrient-poor soils, decomposing organic mulches may cause nitrogen deficiency. This will be evident from pale green leaves and slower than expected growth. If this occurs, apply nitrogen fertilizer in the spring or early summer

(do not fertilize late in the season). If using organic mulches, make sure to pull the mulch back 10 to 15 cm (four to six inches) from the main stem. This will discourage rodent damage and reduce the chance of stem rot.

Over the past 10 years or so, plastic mulch has become a preferred method of weed control for anyone establishing a new shelterbelt. Plastic mulch is non-perforated, 2.7 mm polyethylene plastic (perforated mulches are available) with built-in UV inhibitor properties.

The plastic comes in rolls or as single squares. You put the roll directly over newly planted tree seedlings with a specialized mulch applicator. Rolls are four feet wide and 1,500 feet long. The plastic allows trees to maximize their growth and get a head start due to elimination of competing weeds. In addition, soil moisture loss due to surface evaporation is greatly reduced under the plastic within the tree row.

Under plastic mulch, the soil will warm sooner and stay warmer longer during the growing season promoting and stimulating tree growth and root development. Most shelterbelt tree species under plastic mulch experience a 25 per cent increase in growth compared to trees planted without plastic mulch.

In order to effectively install this weed control technology, you should have your tree row centred down an eight-foot-wide strip of well worked soil. This will allow the mulch applicator to properly apply and anchor the plastic. Mulching can be done up to a month after the trees have been planted providing the soil is worked to a minimum depth of six to eight inches. With conifers, you can actually mulch up to a year after planting because they are much slower growing.

If you are beyond the point of installing rolls of plastic mulch or your site is unsuitable, another option is to purchase “Brush Blankets.” The one-metre-square pieces of plastic can be fitted and anchored around individual trees to provide a weed free zone. If you are planting in an area where little or no soil preparation is feasible, such as a riparian area, then Brush Blankets would be an excellent option.

2. INTER-ROW GRASS STRIPS

Sowing grass between tree rows is an alternative to cultural or herbicide weed control. The grass also provides a clean surface and easy access for equipment following rain. It also eliminates soil erosion in the plantation, especially if integrated with mulches.

Main disadvantages are that grass requires mowing and, if not controlled,

can compete with tree seedlings for water, nutrients and light. Grass should not be allowed to grow immediately alongside the tree row. If plastic mulch is used, grass can be allowed to grow up to the edges of the plastic. If you are using herbicides or mechanical weed control, an area of approximately two feet on each side of the tree row should be kept free of grass.

The choice of grass is very important. Choose a hardy, noncompetitive and drought-tolerant species. A tall, vigorous grass that requires regular mowing is not desirable. In general, low growing bunchgrasses are less competitive than rhizomatous grasses. The Shelterbelt Centre at Indian Head, Sask., has tested these species and blends: sheep fescue, Alpine bluegrass, hard fescue, Parkland mix, and crested wheat grass.

Sheep fescue is a bunchgrass that performed the best in the studies and appears to be the best choice for the Prairies. It forms a dense ground cover up to a maximum height of 10 cm. The sheep fescue did not require mowing during the growing season whereas crested wheatgrass was mowed three times. Sheep fescue also tolerates a certain level of shade, which is helpful once the tree canopy matures and starts to fill in.

3. HERBICIDES

You have two strategies for using herbicides to help control weeds in a tree row. You can apply a preplant soil incorporated herbicide or choose from a relatively limited herbicide arsenal for ongoing herbicide weed control after the trees are planted.

Currently there are two registered chemicals that can be used as a preplant herbicide treatment: trifluralin (Treflan, Rival) or a mixture of trifluralin and metribuzin (Sencor). Both treatments require tillage for incorporation and can be applied in the fall or in the spring, prior to planting. The treatment you choose will depend on the tree species to be planted. Check the labels.

Several herbicides are registered for post-planting weed control. Sethoxydim (Poast Ultra) can be applied for control of annual grasses and suppression of quack grass. Paraquat (Gramoxone) and glyphosate (Roundup Original) can be used — but targe the weed areas only. AVOID contact with tree leaves or green, immature bark. Glyphosate provides control of annual and perennial weeds while Gramoxone gives control of annual weeds and top growth of perennial weeds.

Three herbicides are available to provide residual control (control of flushing weeds) through soil application. Application in the fall of the first year, or the early spring of the second year, should be restricted to linuron (Linuron, Lorox, and Afolan). In the year after planting and in subsequent years, fall linuron application can be continued or simazine (Simazine, princep) or dichlobenil (Casoron) can be used instead. For all these soil-applied herbicides, selection should be based on the weed species to be controlled, soil texture and the tree species. Linuron, simazine and dichlobenil should be applied to a trash-free soil surface and in some cases may require some form of precipitation to activate the chemical in the soil. For best results, foliar-applied herbicides should be applied with a shrouded sprayer to better target the application and minimize drift that can harm trees. Some herbicides can SERIOUSLY harm trees if applied incorrectly. Consult the herbicide label for detailed information and contact your local retailer for further product information, availability and current prices. Also refer to the brochure “Controlling Weeds in Your Agroforestry Planting,” which is available online at www.agr.gc.ca/shelterbeltor by

calling 866-766-2284.

4. CULTURAL WEED CONTROL

Cultural methods were once the only tool in the arsenal of shelterbelt weed control options. Now they should be considered part of an integrated weed control management strategy. Mechanical weed control methods such as cultivating, rotovating and hand hoeing can be successful, but they take a lot of time. Mechanical weed control is best used as one component of a multifaceted approach to weed control in your shelterbelt.

Also note that tilling too close and too deep beside trees can cause severe root pruning leading to damage, poor performance and in some cases death. It is advised to till only to a depth of two or three inches, otherwise you risk damaging important tree feeder roots.

The success of your Prairie shelterbelt planting depends on many variables, some of which can be influenced, such as in the case of weed control. Controlling weed competition is especially critical during the first three years after planting and can lead to greater success in growth and survival of your trees.

For more information on the care and maintenance of trees in shelterbelts, call 866-766-2284, e-mail at [email protected]or visit the website at www.agr.gc.ca/shelterbelt

Blair English is an agroforestry specialist with the Agriculture and Agri-Food Canada Agroforestry Division. He is located in Brandon, Man.

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