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Planting to Reduce the Spread of Weeds


Weeds are unwanted plants that compete for moisture, nutrients, sunlight and growing space needed by trees, crops and native vegetation. Weeds are generally difficult or expensive to control because they grow rapidly and produce large numbers of seeds that are easily and widely dispersed. Weeds that are not native to an area can be particularly menacing, as they often don’t have any natural predators that provide natural controls to their growth and distribution. Farm forestry can be used to suppress infestations and control the spread of weeds. However, many introduced tree species are also potentially weeds themselves and some types of farm forestry make controlling them more expensive.


In some large areas infested with weeds it is often too expensive or impossible to remove weeds. One solution is to plant the affected area entirely with trees to suppress the weeds. The most appropriate species are usually those that can form a dense canopy that quickly shades the weeds and suppresses growth and reproduction. For example, dense plantations of pines have been used to control the flowering and spread of serrated tussock in Victoria. After just 5 years the weeds were under control, thereby reducing the risk of spread out into adjacent farmland. Depending on what is present, a site may become clear of weeds after just a few years. But many weed seeds can stay dormant in the soil for many years.


Trees may also be able to stop the spread of weeds by slowing the wind and capturing wind-blown seeds. Trees can also attract a variety of plant-eating insects that may help in controlling weeds.


As pasture restricts weed growth, trees can indirectly encourage the spread of weeds by reducing pasture quality and vigour. Trees also prevent the use of cheap control options like grazing or broadacre spraying and hiding weeds from view.


Many introduced commercial tree species are themselves weeds. For example, pines are able to invade native forest due to their tolerance of shade and tagasaste, a promising fodder tree is a serious weed of native forests on sandy soils in Victoria. Farmers need to be aware of the risks of introducing new species to an area that might be spread by stock, birds, wind, water or other means. Once established, like willows in streams, the cost of control can be very much more than the contribution these trees might make to agriculture.

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