Competition for moisture and nutrients from grasses and other plants adjacent to newly planted seeds or trees is the major cause of failure in revegetation. Young seedlings need time to develop a vigorous and deep root system that can tap into reliable sources of soil moisture. Therefore it is vital that new seedlings have access to a weed free volume of soil until well established.
The experience of many revegetators around Australia shows that the best results are achieved by keeping the planting zone weed-free for two years prior to planting (Andrews, 2000). Satisfactory results are achieved by controlling weeds for at least one full year before planting. This can be achieved by a range of chemical and non-chemical methods, and depends on the technique used for planting, the soil type, the weed burden and the desired outcome. The two-year preparation allows soil moisture to build up creating an additional reserve for the new seedlings.
Repeated sprays with a knockdown herbicide (such as glyphosate) are effective in that they exhaust the soil weed seedbank, resulting in less weeds germinating after the planting. Residual herbicides prevent the weed seeds in the soil from germinating until the effect of the herbicide diminishes over time. Scalping (removing some of the surface soil, removes the majority of weed seeds and is very effective in a range of soils, although it may expose subsoils that are prone to cracking as they dry. Non-chemical methods include mulching with newspaper, straw, sawdust or similar; flaming; repeated cultivation and hand-chipping. Mowing reduces the vigour of the competing plants but is not as effective as complete removal. Many direct seeding machines have a built in scalping blade or disc to do weed control in a single pass. Most sites will still benefit from two-years of weed control prior to direct seeding.
Great care should be taken before exposing highly-erodible soils. Weed control should be in strips approximately one metre wide with a grassy strip retained between rows, or in spots one metre in diameter around each planting location.
When using herbicides it is essential that you read the label on the container and follow the manufacturers instructions.