Soil disturbance

Development and many other activities, both human and natural, result in disturbance of soil. Disturbance can result in:

• the loss of native seed source;
• weed invasion;
• the disruption of drainage and of the natural soil processes - often topsoil is removed or buried.

Urban areas are subject to huge amounts of disturbance as development and the movement of people continually impact on the environment.

Weed species are better adapted to colonising and surviving in disturbed environments than most native species. Weeds always occur in disturbed sites in urban areas due to:

• the availability of weed seeds and other propagules;
• the mobilisation of nutrients that occurs when soil is disturbed;
• an increase in light;
• the ability of weeds to spread easily and set seed in a very short time.

Invasion of bushland by weeds is most likely after disturbance, and the greater the degree of disturbance the greater the degree of invasion.Disturbance is often greatest and continuous on the edges of bushland and along creeks and drainage lines carrying stormwater and this is where most weeds are found.

Exposure of the soil
Disturbance usually causes exposure of the soil which:

• increases the amount of light which favours the growth of new seedlings with the fastest-growing (usually the weeds) winning the battle for supremacy;
• activates the nutrients in the soil again encouraging rapid growth;
• removes leaf litter enabling small seeds such as annuals which rapidly develop roots to penetrate the soil easily;
• eliminates competition for light, water or nutrients;
• some native seeds require these conditions for germination so the weeding of disturbed sites must be carried out carefully to ensure that it is weed species that are being removed and not native seedlings. Marking native seedlings with three sticks in a triangle will protect them from being trampled or pulled out inadvertently.

Disturbance of the soil by bush restoration practices - planting, weeding or mechanical disturbance also provides good growing conditions for weeds particularly annuals and other weeds which spread by wind, water or on clothes. After any bush restoration work we can expect an invasion of weeds to immediately follow. These are often weeds not seen on the site before and frequently weeds that are more invasive and damaging than the original weeds that were removed. So it is vital that maintenance regimes are followed, in order to eliminate the new waves of weeds and to avoid creating worse problems than the original ones. Mulch from off-site may also introduce weeds to the area.

So whenever you disturb the soil you must be aware that what will come back will mainly be weeds until there is a thick native ground cover and the canopy and space excludes the light that weeds need in order to germinate.

Always consider the potential impacts resulting from your actions:

• make minimal disturbance;
• return any soil you disturb to its original layers;
• protect bare soil with a mulch of the pulled weeds if they have no seeds and are not likely to re-sprout.