Poor weed control accounts for most tree-planting failures, due to their competition for light, moisture and nutrients. Weeds can reduce a plants early growth rates by up to 70% compared to weed free sites and can decrease survival from an expected 90% of trees planted to as little as 10% survival rate.
It is therefore vital that some form of weed control is undertaken. Eliminate weeds early before they use up stored water. The following describes some of the more widely used forms of weed control.
Cultivation for weed control is more successful in lighter, well drained and friable soils. It increases water infiltration and stimulates germination of the soil’s weed seed bank by exposing the seeds to light and water. This technique can exhaust much of the soil stored seed before the plantings begin. It is effective when using pre-planting herbicide when undertaken in the season before planting.
In heavier soils cultivation for weed control can be expensive and may allow other weeds to invade. It may also result in windthrow during wet periods in the next year due to decreased soil structure and rainfall infiltration.
Mouldboarding can give effective weed control by simply burying weed seed and is a simple method for forming mounds on wet or saline areas. Plough back along the rip line from the other side to throw up the soil to form a raised bed. Do this in autumn to let the soil settle before planting.
By planting on mounds in wet or saline sites some of the trees roots are removed from the adverse soil conditions, instead growing in the mounded earth. This can substantially improve tree establishment and growth.
In waterlogged areas the riplines can be made on a slight fall to assist in drainage of the area to be planted.
Weed mats are and effective way of suppressing weed growth, particularly in areas where herbicides are undesired or inappropriate. They also have the advantage as acting as a mulch, improving moisture content in the soil. However it can be expensive when revegetating a large area.
Weed mats are usually made out of jute, recycled cotton, paper or other biodegradable materials. Mats break down at varying rates depending on the thickness and grade of the materials. They can be purchased long rolls for a line of plantings, or small squares for individual seedlings.
Alternative materials are being developed for their effectiveness and cost efficiency. For example, cardboard pizza trays can now be purchased for individual trees, making their use easy and affordable.
Larger weed mats can be laid out prior to plantings, or smaller mats put in place at the time of planting. Seedlings are planting through the mats, which can now be purchased with pre-cut holes, making planting easier.
Chemical control (before planting)
Use herbicides only in accord with the manufacturer’s recommendations. Handle any chemicals safely at all times. Always thoroughly read the label of the chemical being used.
In many cases it is more environmentally sensitive to us non-chemical weed control. This is particularly the case when planting near water courses or other sensitive environments. Prolonged use of chemicals can lead to resistance.
If using chemical weed control, apply a knockdown herbicide well before planting when the pasture is actively growing. Apply residual herbicides just before planting. It may be used in conjunction with a knockdown herbicide if weeds have emerged since the first spray.
When using residual herbicides (such as simazine or atrazine) it is important to remember that when you are planting your seedlings that their roots should do come into contact with the residual affected ground, as it may damage or kill the seedlings. This can be avoided by using a tree planter or by scalping the soil. Be particularly careful with high rates of the triazines (eg. simazine and atrazine) because leaching can occur on sandy soils.
Apply herbicide in spots 1.2 – 1.5 metres in diameter for individual planting spots, or 1.5 metre wide strips. Knapsack sprayers can be used for spot spraying. Knapsack sprayers or micro applicators can be used for strip spraying.
Chemical control (after planting)
The area within one metre of the tree must be kept weed-free for at least the first year to maximise survival and growth. Good pre-planting weed control minimises the need for post planting spraying. Some chemicals can be applied as an overspray or as a directed or shielded spray in the following autumn.
Where grasses are dominant the trees may be over-sprayed with a selective herbicide which does not damage broadleaved plants. Both grasses and broadleaved weeds can be controlled by using a shielded or directed spray of a knockdown herbicide. See manufactures recommendations on the label. An upturned bucket can be used to protect the tree.