Why manage weeds?: Rapid canopy establishment and early cropping are keys to profitability in any orchard, particularly modern capital-intensive systems.
Weeds compete with trees for moisture and nutrients and can also create a favourable micro-climate for pests and disease.
Young orchard
Research shows that competition from weeds in young, developing orchards can result in slower canopy establishment and delayed productivity.
In high-density orchards, the risks from weed competition are increased due to shallow-rooted dwarfing rootstocks that place tree roots in direct competition with weeds.
Established orchard
A poor weed management strategy will also have a negative effect on yields in established orchards.
Having an effective weed management strategy in place will help growers achieve their goals for orchard establishment, early yields and hygiene.
Start with orchard hygiene
Good orchard hygiene is the first step in any weed management strategy.
Movement of weeds on and throughout your property and the appearance of new weed species are largely determined by the degree of weed hygiene employed.
Be aware of new weeds appearing on your property, have them identified if necessary, and work towards eradicating them or reducing their spread.
Moving machinery from non-crop areas to the orchard and between blocks is the most likely method for spreading new weeds.
Reduce the spread of new weeds by periodically cleaning orchard equipment.
Strategies & control options
The most appropriate weed management strategy will vary from site to site and will depend on factors including orchard size, tree age, weed spectrum and density, soil type, available moisture and choice of under-tree management (i.e. bare earth, mulched or sod culture).
Over time, strategies need to respond to changes in the weed spectrum and growing conditions.
Weed management methods can be grouped as either physical or chemical, or can incorporate elements of both.
Physical weed control
Cultivation
Cultivation was once a common commercial practice in orchards and it does reduce competition from weeds, but at some cost.
Disturbing the topsoil is now known to negatively affect soil structure and organic matter levels.
Cultivation also increases erosion risk and can result in some root damage to trees, especially in blocks on dwarf stocks.
Spot cultivation using a hoe is labour intensive, but might be an option for smaller orchards as an alternative to broadscale cultivation or spot spraying.
Thermal weeding
Research shows that flame or thermal weeding using propane burners, hot air or hot water can be effective on small seedlings, but is less effective against larger annuals or perennial weeds.
There are health and safety and fire risks associated with these methods. Do not use thermal weeding near trees less than three years old, as severe crop damage can occur.
Grazing animals
Grazing with animals such as sheep, geese and fowls can be used to suppress weed growth and reduce seed load in the orchard.
Geese are heavy feeders of weeds such as grasses and they also help to clean up windfall fruit. Sheep can cause damage to trees if other feed is scarce.
If orchard-grazing animals are intended for sale, be aware of chemical residue issues. Consult chemical labels for information on stock withholding periods.
Mulching
If done correctly, mulching represents the most effective alternative to chemical weed control.
Mulching the under-tree row with large quantities of organic materials such as straw, old hay or bark chips, has multiple benefits including:
• moisture retention
• soil temperature regulation
• organic matter and soil microbe build-up
• weed control.
To be effective as weed control, mulch must be applied at sufficient thickness to act as a physical barrier to sunlight and weed growth. This depth will depend on the type of mulch being applied.
Side-cast mowers deposit slashings along the tree row and this can help suppress weeds and build up organic matter, however, this is not effective as a stand alone mulch treatment if the aim is to achieve a weed-free strip.
Growers should also be aware of the possibility of nitrogen drawdown effects when using some raw, non-composted mulches.
Chemical weed control
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?
(cont next issue)
Download the Orchard plant protection guide 2020-21
See this article in Tree Fruit May 2021
