Orchard Weed Control

Weeds compete for water and nutrients, and can also create humid microclimates around the base of a tree, increasing the risk of attack by diseases such as Phytophthora. Some species can act as alternate hosts, harbouring unwanted orchard pests.

A lot of research has been carried out around the world to quantify the effect of weed competition on orchard performance. Suffice to say that tree growth in the first growing season can easily be halved by excessive weed competition and even on mature trees, yield and vigour can be significantly impaired by excessive weed competition.

The most critical period to maintain good control over weeds is during the spring and early summer growth flush however when moisture is not a limiting factor competition during the later part of the growing season can still be significant.

Herbicides still represent the most efficient and cost effective way of controlling weeds in during the critical tree establishment phase.

Spring provides the ideal opportunity to apply a soil active herbicide to control weeds through the critical spring flush and depending on product choice, much of the summer period. As a general rule of thumb, these products are less effective on soils containing high levels of clay or organic matter. It is critical when contemplating the use of soil active herbicides that the herbicide strip is free of established weeds or any other material likely to prevent an even application of herbicide to the soil surface. If there are a lot of weeds or trash present in the herbicide strip it may well be pointless attempting to use a soil active herbicide.

These products are best applied during the early spring period when the soil is still relatively cool and moist and prior to the main germination of spring weeds. Rain or irrigation is generally required within a week or so to ensure they are carried into the soil for maximum effectiveness. In areas such as Hawke's Bay, August/September is often considered the ideal time to be applying these soil active herbicides. Minimal spring weed germination has occurred and there are generally good prospects of rainfall to wash any residual applications into the soil on orchards without fixed irrigation systems.

If seedling weeds, or larger, are present and you are contemplating the use of residual herbicide with no knockdown activity, then the addition of a knockdown herbicide to control these weeds will be required. Product choice will depend on weed spectrum, I've always found one of the amitrole products to be particularly useful at this time of the year, especially if you have used glyphosate during your autumn clean up.

Amitrole's weed spectrum compliments that of glyphosate and in addition can have a soil active life of up to six weeks. Other possible spring knockdown treatments include the desiccants Preeglone or Buster.

In young tree situations the safest residual herbicides to use are the dinitroanilines "Surflan" and "Stomp". These products provide good residual control of most annual grasses and a number of broadleaf species. In mature plantings they are often used in combination with triazine compounds such as simazine to broaden the spectrum of weed species controlled.

Triazine herbicides have proved to be extremely useful herbicides for orchard use over the years, providing extremely cost effective residual control. To maintain their effectiveness however it is important that their use be rotated or that they be used in combination with herbicides from an alternative chemical group. This use pattern will help prevent build up in weed species which are not susceptible to the triazine group. A "Stomp" or "Surflan"/simazine mix as mentioned earlier, "Boundary" and "Fenican" are all examples of residual herbicide applications using a combination of alternative herbicide chemistry. Tall willow herb would be a good example of a weed species' that has become more prominent in Hawke's Bay orchards over the last ten years or so as a result of fairly heavy reliance on glyphosate and triazines such as simazine and "Caragard" for weed control. It is as a response to this situation that products such as "Boundary" and "Fenican" which contain alternatives to our standard triazine products have become more popular in recent years.

In any event most orchardists find themselves applying at least one knockdown herbicide product during the growing season, if not more.

In pipfruit orchards amitrole products should not be used after fruit set however glyphosate, "Preeglone", "Buster" and the selective grass killers such as "Gallant" and "Fusilade" are all options for use over this period.

Extreme caution needs to be exercised when using any of the broad spectrum knockdown herbicides around plantings of young trees as plant uptake via young immature bark on the trunks of these trees can potentially result in tree death. Whilst the use of tree guards can eliminate this problem, they can also present problems of their own by providing an ideal environment for pests and weeds to flourish. I've also seen situations where the humid conditions created around the base of a young tree by these sleeves have encouraged the development of burr knots and resulted in increased incidence of Phytophthora.

In mature tree situations, all of these products can be used with relatively safety providing, in the case of glyphosate care is taken to avoid any spray drift onto low branches. This is particularly important in stonefruit and Pear blocks which appear to be extremely sensitive to glyphosate damage. In fact a number of stonefruit orchardists avoid the use of glyphosate right throughout the growing season. During the mid-season period, timing of your knock down herbicide application is critical. With increasing fruit weight bringing lower branches down and weed growth continually heading upwards, often a very narrow window of opportunity exists in which to apply a herbicide before weeds are into the lower tier of fruit and good coverage with any herbicide application without damaging the tree becomes impossible.