Weed control in forestry
Managing weeds helps to protect your trees. You need to monitor your forest, so you can find and deal with invasive weeds before they become a problem. Learn about controlling weeds, and some of the weeds you might need to deal with.
What is a weed?
Weeds are plants that are growing well where they are not wanted. They spread quickly and easily. Weeds compete with seedlings and plants for sunlight, water, and nutrients.
Every region has different weeds to deal with. Plants that are weeds in your region may not be a problem in another region. It’s a good idea to learn about which weeds may be an issue for your planting project.
Regional pest and weed management plans
Many regional and district councils require landowners to manage and control certain pests, weeds, and diseases. Check with your council to find out what responsibilities you may have.
Controlling weeds
Controlling weeds is an important part of looking after your forest. Invasive weeds can:
- take nutrients and water needed by trees
- restrict the growth of young trees
- harbour pests that may be harmful to your trees
- smother seedlings
- kill mature trees by smothering them and competing for light.
Inspecting your forest regularly is the best way to find and deal with invasive weeds. It’s best to deal with them before they become a problem. It can be costly to replant seedlings that have been smothered by weeds.
The Department of Conservation has a guide on how to control weeds.
Forest & Bird have a weed control guide that includes photographs and specific ways to deal with different types of weeds.
Weed control methods
There are different ways to control weeds. Below are some of the common options. You may find it useful to use a few different methods.
Mechanical
Mechanical weed control involves using machines like weed eaters to remove weeds. This method removes the leaves from the weeds but doesn’t remove the root system. You’ll need to revisit the site to remove the weeds again if they regrow.
It’s best to use mechanical weed control before the weeds have any seeds. This will help to prevent them from spreading. You’ll also need to be careful to avoid damaging any seedlings.
Manual
Manually removing weeds involves removing them by hand or using hand tools. It can be helpful for dealing with small amounts of weeds that are easy to fully remove from the site.
If you have a lot of weeds on your site, or weeds with strong root systems, it might not be the best method.
Manual weed control can be time consuming, and you may need to revisit the site to do it often.
Herbicide
There are different types of herbicides to control weeds.
If you’re thinking about using herbicides, some things to consider include:
- what kind of herbicide is the best option for dealing with the weeds on your site
- will the herbicide damage any of your seedlings?
- will you spray before or after planting?
- will the chemicals have a residual effectiveness?
- how to apply the herbicide
- safety requirements.
If you decide to use herbicides to deal with invasive weeds, get advice first. There are health and safety requirements, and you may need to let your local council know.
We have a page about agrichemical spraying before and after planting your site.
Mulch
Putting mulch down after weeding helps to suppress weed growth. It also helps to keep moisture in the soil. If you use mulch, make sure it doesn’t touch the seedling stems.
Planting closely to reduce weeds
Planting seedlings at a higher density (closer together) is one way to reduce the risk of weeds. This reduces the amount of open space available for weeds to grow in. You’ll need to manage weeds while the seedlings grow, but after they’ve established there will be less weed maintenance.
Planting like this also helps you get canopy closure sooner. This is when the tops of the tree branches create a cover. When this happens there is less light, making it harder for weeds to grow.
Planting on grass sites
Many planting sites have grass on them. Grass can smother seedlings and take water and nutrients from the tree. If you’re planting on a grass site, you’ll need to manage grass and weeds before planting and while establishing seedlings.
Tāne’s Tree Trust have a guide about preparing grass sites.
Tips for dealing with weeds
- Identify the weeds
- If you have a lot of weeds, choose the priority weeds to deal with first
- Decide on the best way to control the weed, you might need a few different methods
- Prevent the weed from spreading by controlling it before they fruit or go to seed
- Check with your local council before disposing of weeds
- Get help if you need it.
Te Uru Rākau – New Zealand Forest Service offers free forest advice to landowners. Find out about the free service and how to talk with a regional adviser in your area.
Types of weeds
Below are some of the weeds you may need to deal with.
Vine weeds
Vine weeds are a big risk to the survival and establishment of seedlings. The vines smother trees, plants and forest canopies. They affect new seedlings and established trees, making it hard for them to survive.
Vine weeds include old man’s beard, blue morning glory and banana passionfruit.
Shrub weeds
Shrub weeds grow over or replace other plants. They include barberry, blackberry, boneseed, and gorse.
Blackberry
Blackberry is a scrambling shrub weed. It forms dense clumps and spreads out over the ground. Blackberry smothers low growing plants and can stop native seedlings from establishing.
The Weedbusters website has information to help you manage blackberry.
Gorse
Gorse is a shrub weed, originally introduced as a form of farm hedging. It is easily distinguished from other pest plants by its thorny leaves and bright yellow flowers.
Gorse can spread quickly over large distances. The seed pods explode, spreading seeds across many metres. They can also be picked up by the wind and by the fur or fleece of passing animals.
Gorse grows quickly and densely and can outcompete pasture and forestry for nutrients, water, light and space. It can regrow quickly from any remains left after burning or cutting, making it hard to control and even harder to eliminate.
In some regions gorse can be left on site to help seedlings grow. The gorse provides shade and shelter for the seedlings. It also reduces competition from other weeds.
If you have gorse on your site, be aware that the plants dry out over summer, increasing the risk of fires.
Willows
Willows are trees that grow well besides waterways. They are a common weed at riparian planting sites and compete with native species.
Wilding conifers
Conifers are trees which spread from cones, such as pine trees. A wilding conifer is an unwanted conifer that has spread naturally, instead of being planted by foresters. Wilding conifers have become a threat to New Zealand's native landscape, high country farming and biodiversity.
More information about weeds
The New Zealand Plant Conservation Network and Weedbusters can help you identify weeds.
Monitoring pests, weeds and diseases
Tell the Ministry for Primary Industries if you see any unusual weeds, pests or diseases.