Weed strategies

If you inquire into weed control you will often read or hear terms such as the war on weeds, weed control campaign, plan of attack or weed control strategy. A close look at a book or pamphlet will reveal much more weed control imagery or language inspired by military terms or activities.

Of course it is a war we will never win. Although we may win an occasional battle, weeds are the ultimate guerilla force. They can disappear underground for months or years, or fade into the vegetation with clever camouflage disguises. They can even change their shape to avoid or search and destroy operation, develop resistance to our chemical warfare agents and regroup quickly, damaging our productivity before we know they are back. If we vanquish one lot, another appears to take its place. To utterly destroy them, we would have to better than concrete the entire surface of the earth - because every little crack would grow a weed.

So we know that weeds will always be there, that total control is only a fantasy or pipe dream. Therefore we should have a strategy for controlling them, or a plan of attack. If we don't, much effort can be wasted in controlling weeds. The control action may cause collateral damage, such as with injudicious use of cultivation; or resources may be wasted, such as:

where the weeds will not cause economic damage, or

where control gives rise to a secondary weed problem" (ie control of one weed allows another, which had been suppressed or out-competed by the first, to become established), or incorrect timing of control will result in less effective outcomes.

I moved to a new small property 12 months ago. It was very weedy, with some large trees (radiata pines, ash) which we regarded as invasive, numerous garden weeds (sorrel couch, kikuyu etc) and major "environmental weed problems" (mainly gorse and European broom).

Although the property less than one hectare, it will be very intensively used.

Priorities applying to the site are:

We will try to manage the property entirely organically from the start. This is an option that we would not always recommend to others. Sometimes it is sensible to treat a major weed with herbicide, and then to establish a management regime which will prevent the re-establishment of the weed, without recourse to repeated applications.

The pine trees occupy a large area (root run, shading and needle litter). They are expensive and labour intensive to remove, but they severely limit horticultural potential, due to shading.

We want to see early establishment of perennial food bearing or otherwise productive trees.

We want to be responsible neighbours, (we don't want to cause anyone else weed problems, we want to assist with rejuvenation of the roadside vegetation, we like to encourage our indigenous vegetation and wildlife).

Time and money are both limited.

Strategies to limit the spread of weeds:

Prioritise all weed control. Approach each area with a coordinated effort, so that the maximum benefit for effort is achieved.

Take care not to spread weed seeds around the work property with soil, compost or other materials.

Minimise the opportunity for weeds to establish or multiply by minimising disturbance.

Quarantine any materials from off the property, until they are shown to be weed free. If possible, put all organic materials through the compost. Use only clean materials as mulch.

Clean the build up of mud and weed waste from vehicles and equipment prior to returning home from trips away.

Environmental weeds are targeted for careful hand control using the Bradley method.

For all weeds, start the control where infestation is lowest. Work towards the densest growth. This allows you to get control and establish plants which can compete against weeds.

Paths and driveways are maintained with a gas flame weeder.

Low priority areas are mown to control seed set and root run until more intensive control methods can be applied.

Mulching with black plastic is acceptable as a short-term measure for weeds growing from stolons or rhizomes only.

Organic mulches are preferred, but won't work at all with some weeds.

Unfortunately digging is necessary for many weeds. Some bulbous weeds will survive composting - they are fermented in drums of water first, then applied to the aerobic compost.

In developing this property, we will use large quantities of compost - and what better ingredient than lots of fresh, green, recently pulled weeds. We try to change the species of weeds which grow, to those which provide some benefit, such as attracting beneficial insects into the garden, attracting pests away from the crop, soil cover, or a harvest of food and medicine. If these weeds are easily pulled when we need the space, then we can tolerate many more weeds. The compost and mulch, which we apply liberally to the soil, and which is made from weeds and other ingredients, is changing the structure and moisture holding capacity of the soil rapidly, with the result that weeds are more easily pulled.


easy to use.

permit kill of large populations economically.

can kill weeds which are very difficult to eradicate with other techniques.

access to weeds in difficult places (such as under fence lines).

usually a relatively cheap form of weed control.


incorrect use may damage non-target plants nearby.

may result in secondary weeds appearing, to fill the ecological niche occupied by the first.

sometimes leads to development of genetic resistance to the herbicide.

may cause harmful residues in soil, water or wildlife.

timely use may be critical for good results.

Non-chemical weed control options

Cultural techniques:

Hygiene - avoid transfer of weed propagules (clean equipment etc), quarantine, avoid seed set, prevent establishment, decrease weed seed reservoir.

Ecosystem manipulation - uses changes in soil fertility (nutrition, pH), drainage, irrigation (eg use drippers to restrict water), planting density and depth, planting date etc to control weeds.

Physical barriers - mulching-synthetic and natural.

Materials brought in from other sites are quarantined.

Mechanical control:

Hand pulling - slow but very selective and can prevent damage to vulnerable plants nearby.

Slashing or mowing - may not be suitable for some weeds eg creeping oxalis (Oxalis corniculata).

Brushcutting - expensive, but can give good control, especially suited to linear features such as fences.

Cultivation - may cause soil erosion or create seed bed for new weeds to establish.

Thermal weeding:

Burning will break dormancy of many weeds - which can then be destroyed (eg Apple of Sodom Solanum hermanni or Gorse Ulex europaeus). 55 minimum (80 - 100 for persistent weeds or thick seed coat).

Can stimulate germination of some weeds leading to new problems - may be dangerous - restricted to some times of the year.

Solarisation - cover with clear polyethylene sheet (40 -100  thick) and seal.

secure the edges.

soil must be moist.

can kill weed seeds.

give a minimum four weeks treatment, depending on daytime temperatures.

Use of vegetative cover:

The best way to exclude weeds is to use dense plantings of hardy competitive species that will suppress weed growth. Once established, they should require little maintenance. Closely planted trees may achieve this effect once the canopy closes over, but the technique is more often used for groundcovers.

Use of vegetative cover for weed control

Use of vegetative cover is the most desirable form of weed control, fro an ecological perspective. Vegetative cover has the following advantages:

It prevents new weeds from germinating or smothers weed seedlings.

It keeps the soil covered and protected from wind and water erosion.

It protects the soil ecology, by insulation, temperature control and continuous addition of small quantities of organic matter.

It provides an opportunity to locate native plants on the road reserve

It provides food sources or habitat for small animals, birds, insects and other organisms.

If appropriate species are selected and well grown they will require little maintenance.

Factors which influence the economics of this choice include the following:

Location and seasonal variation that will influence the number of times per year that grasses need to be mown.

Aesthetic requirements of the site may influence the number of times per year that grass must be mown.

Slopes where machinery access is difficult.

The ratio of edge to planting area (ie the length of boundary that needs to be maintained against weed infestation).

Weed resistance of the species chosen for groundcover or shrub massing.

The weed species present (eg rhizomatous or stoloniferous perennial weeds such as couch or kikuyu may be very difficult to remove from groundcover or massed shrubs).

The growth rate of species used.

The requirement of species used for additional inputs (fertiliser, irrigation etc).

The quality of planting stock.

Longevity of groundcovers.

Generally, the groundcover species used should be selected to be self-replacing, or to have a long life expectancy, for the technique to be competitive with mowing.

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