 |
Weed Information |
 |
| Biodiversity | Weed control | Physical control | Chemical control | Biological control | How long to control? | Successful biocontrol | 'Useful' weeds | Finding biocontrol agents | Biocontrol agents in NZ | After control |
Chemical Control
| Chemical
herbicides ("chemicals that are toxic to plants") are the most
common form of weed control. They can be applied to relatively large areas
quickly with relatively little labour.
woolly nightshade |
|
Herbicides can be harmful to all plant species in an area. Chemicals are not specific to killing only the weed species, any plant that gets a dose of herbicide (including our native species) may be harmed. This is called a non-target effect or impact. Herbicides must be used with great care along waterways (rivers, lakes, etc.) to avoid contamination, and where there are people and animals that might be subjected to herbicide spray drift. Herbicides are also expensive and repeat applications are necessary to keep the weed under control. This may cause weeds to become resistant to particular herbicides.
Types of herbicides
Selective herbicides only harm a narrow range of plant species.
Broad spectrum herbicides can damage or kill many different plants, so care must be taken to only apply the herbicides on the weed!
Contact herbicides kill plant tissue at or near the point of contact with it (they do not spread around the plant). To kill weeds using contact herbicides you need good, even herbicide coverage.
Systemic herbicides move through the plant tissues and these can be injected into the plant circulation system at one spot.
Residual herbicides can be applied to the soil to kill weeds through the roots. They remain active in the ground for some time (depending on the chemical) and can control germinating seedlings. Residual herbicides in the soil can also kill nearby native plants.
Herbicide selectivity - Selectivity is where herbicides harm or kill some plants but not others.
Herbicides can be selective because of:
- Biological
differences between plants
- For example, some herbicides such as haloxyfop (products such as Gallant) only kill grasses (some damage can occur to other monocots such as flax and rushes, etc.). These are useful chemicals for controlling weeds such as pampas when there is a risk of accidentally spraying nearby trees and shrubs.
- How the
herbicide is applied
- The way in which herbicides are applied can reduce the risk of harming native plants nearby. By applying herbicide directly on or in a weed, rather than spraying, the chances of damaging plants other than the weed are much lower.
Herbicide application
All herbicide application must be done by people who have been trained in the use of herbicides and know how to use herbicides safely. For example, brightly coloured dye may be added to the herbicide solution so that you can see where the herbicides ends up.
This method sprays herbicides onto the leaves of the weed. This can be done by a technician with a herbicide knapsack, or with a spray gun and hose from a tank on a vehicle, or by blanket spraying with a ground (tractor) or aerial (helicopter) boom sprayer. |
|
Foliar spraying is not very selective; herbicide spray can drift onto plants other than the weed. Knapsack spraying is the most selective of foliar spraying, the technician using the knapsack can target the spray onto the weeds. All plants in the area will be sprayed if blanket spraying is used. Spray gun and hose is not very useful in areas where there is not vehicle access (e.g. in a forest).
These methods involve drilling or cutting through the bark into the sapwood tissue in the trunks of woody weeds and trees (e.g. willow, woolly nightshade). Herbicide is immediately placed/injected into the hole or cut. The aim is to reach the sapwood layer just under the bark (the cambium growth layer), which will transport the chemical throughout the plant. Making cuts with an axe and filling cuts with herbicide is called frilling. By using a battery drill to make the holes (drill and inject), there is less chance of the herbicide spilling out onto other plants or into the soil. This is a very selective way of applying herbicides.
-
Cut and paint (or stump swab)
For this method, the weed is cut off completely at its base (no higher than 15cm from the ground) using a chainsaw or an axe. Herbicide is then immediately painted or swabbed onto the cut stump to kill the stump and the root system. This control method can't be used for trees that resprout from cut stems and branches (e.g., willows).
For more information on the use of herbicides for controlling weeds, please contact your regional council
Biodiversity Weed
control Physical control Chemical
control Biological control How
long to control? Successful biocontrol 'Useful'
weeds Finding biocontrol agents Biocontrol
agents in NZ After control 