A common definition of a weed is ‘a plant growing in the wrong place’. This is a personal and subjective definition dependent on who is defining the plant. One person’s weed is another’s cup of healing tea. Weeds are plants and doing what comes naturally to them, growing wherever there is space to do so. It is said that nature abhors a vacuum, and moves to fill that space. This is how weeds act. Weeds see an empty or neglected space and so move in to fill it. Weeds will grow where no other plants can. They fill in the cracks in our pathways, roadsides covered in gravel or asphalt, empty and neglected construction sites waiting for work to begin. Weeds are natures’ gap fillers and agents of transformation. Weeds take over where there is neglect. If a road is left neglected, weeds will move in and dissolve the unused asphalt breaking it up and eventually transforming the space back to Nature. However many see weeds as an enemy that must be destroyed at all costs.
Volunteer plants are often called ‘weeds’ simply because they were not deliberately planted. That alone, does not justify efforts to control those plants. To determine their real ‘weed’ status, you need to consider the qualities of plants in relation to your objectives as farm manager. Weed management becomes easier if you classify plants as ‘real’ weeds requiring attention, and ‘other plants’ that you can safely ignore. The Victorian Department of Primary Industries recommends that before you classify and target plants as weeds, weigh their potential negative impacts against any benefits they may provide, which can include:
Soil protection and enhancement. Most plants protect otherwise bare soil from the effects of sun, wind and rain; produce organic matter and improve soil biological activity and water infiltration rates.
Food source for beneficial insects. Many flowering plants produce pollen or nectar that is useful food for natural enemies of crop pests. Others carry alternative prey like aphids that help to maintain populations of natural enemies on the farm.
Nutrient recycling. Deep-rooted plants absorb nutrients from lower in the soil profile and redistribute them when the plants decompose.
Weed suppression. Heavy growth of volunteer plants helps to suppress the development of undesirable ‘real’ weeds like Emex (three-cornered jack) and caltrop.
Soil indicators. Unlike crop plants that grow where we force them to grow, volunteer plants tend to thrive under conditions to which they are naturally adapted. They may therefore be used to some degree as indicators of underlying soil conditions (problems) such as compaction, acidity or salinity. When those problems are corrected, the soil environment becomes less favourable for the ‘weeds’, and the competitiveness of crop or cover crop plants is increased. Source
2. Where weeds come from
The question needs to put into context. Where agricultural and urban weeds come from is different from where environmental weeds come. For more information on where agricultural and urban weeds come from refer to the (Weed prevention and minimisation section of this guide). Most of Australia’s environmental weeds come from places with similar climates such as Europe, South America, South Africa, North America, Asia and New Zealand. An increasing number of weeds are Australian natives that grow outside their natural range.
3. How do weeds get here?
Weed seeds can hitchhike on camping equipment, bags, shoes and clothing, in food and grain, on cars, machines and animals; they may also be brought here deliberately as ornamental plants or for their commercial value.
4. Why do weeds go weedy?
Weeds are opportunistic. They will grow anywhere there is space for them, even cracks in pavements. They grow quickly and reproduce rapidly. Often people help weeds grow by creating conditions that favour them in natural areas and on farms, such as through the use of herbicides thereby creating bare and barren spaces which are ideal habitat for weeds.
Many weeds also have protective features such as thorns, spines and poisons which browsing animals sometimes know to avoid. Most weeds are also free of natural predators or parasites in the new areas they invade.
5. Weeds are important indicator plants
Weeds can tell us a lot about the environment they inhabit. It is well know that weeds are often the first plants to grow in areas that have been disturbed. In order to manage weeds in a sustainable way, it is vital to first understand what they are telling us about the area they have moved into. Weeds will grow in degraded and damaged soils and can work to restore the health and structure of the soil. For these reasons it is important to enter into a dialogue with weeds – we need to know what they are trying to communicate to us. Trying to ‘shut weeds up’ through killing them with herbicides closes off an important avenue of information in how to re-instate a healthy ecosystem.