Written by  NSW DPI

Manage weeds in the orchard (part 2)

Weeds compete with trees for moisture and nutrients and can also create a favourable micro-climate for pests and disease. Chemical herbicides have been the mainstay of weed management in orchards since the mid 1940s. 

Chemical weed control (cont from last issue)
Chemical herbicides have been the mainstay of weed management in orchards since the mid 1940s.
Using herbicides remains the most cost-effective and reliable approach to managing weeds in commercial orchards.
Types of herbicide and when to spray?
Generally, the best time to spray for weeds is either just before (pre-emergent) or just after (post-emergent) germination.
Most weeds germinate in either spring or autumn, and small weeds are easier to control than older, more mature weeds.
Orchard herbicides can be grouped into three broad categories:
1 pre-emergent residual herbicides
These perform best if applied to bare soil that is totally free of weeds and debris.
Any material that prevents the herbicide from contacting and penetrating the soil surface will reduce its effectiveness on germinating weeds. Most pre-emergent herbicides will provide effective control for a wide range of annual broadleaf weeds and grasses if applied correctly.
Established perennials such as paspalum will not be controlled without using a post-emergent herbicide.
2 post-emergent selective grass herbicides
These are useful where the predominant weed species is grass.
The three active ingredients with registrations for use in NSW as selective grass herbicides are all members of the Group A herbicide mode of action (MOA). This means they are considered highly prone to developing resistance and should be used in accordance with resistance-management principles.
3 post-emergent non-selective knockdown herbicides
These perform best when applied to young, actively growing broadleaf weeds and some grasses.
As these herbicides are non-selective, some can be harmful to fruit trees. Young trees are particularly prone to injury if not protected from knockdown herbicides. Consult product labels for specific recommendations.

IMPORTANT: Please read the product labels thoroughly before applying any herbicide in your orchard. Failure to do so could result in poor product performance or damage to your trees.

Should I be concerned about resistance?
Yes. Ryegrass resistant to glyphosate is present in orchards and vineyards across Australia because of an over-reliance on Group M herbicides.
Herbicides work by interfering with specific processes in plants, known as the mode of action (MOA). All herbicides have been classified into groups from A to Z according to their MOA.
Some groups are more likely to develop resistance and are considered high risk. The earlier the group is in the alphabet, the higher the likely susceptibility to resistance. Refer to product labels to determine the MOA group.
To minimise the risk of herbicide resistance developing in your orchard:
know your herbicide groups
do not rely on chemicals from the same group for every spray
use a lower risk herbicide in preference to a high risk one; for example, never use a group A herbicide when an L or M group herbicide will do the job
look for surviving weeds after spraying and prevent these from setting seed
use as many weed control techniques as practical and do not rely solely on herbicides.
Herbicide sprayer setup (cont next issue)

Download the Orchard plant protection guide 2020-21

See this article in Tree Fruit June 2021