In this section:
Weed Control
Methods of weed control vary with the system of undertree management adopted, namely, permanent sod culture, cover cropping or bare earth. Cultivation, chemical, mulching and competition are the main forms of weed control. Sole use of any one of these techniques is unlikely to optimise orchard productivity. Hygiene can aid in avoiding weed problems.
Hygiene
Weeds easily invade an orchard or spread within it on machinery and the wheels of vehicles. Few properties do not contain some problem weeds within their boundaries or close by. Where weeds are allowed to grow in the non-cropped areas of a property, (for example around buildings or along tracks and headlands), greater weed problems frequently exist in the orchard itself. If weeds are controlled around the orchard as well as within it, problems can be reduced over a number of years.
Problem weeds should be controlled before they set seed. For example, one caltrop (Tribulus terrestris) plant is capable of producing over 2000 seeds. Allowing a few invading plants of this species to seed will result in a serious weed problem for several years to come. In the case of caltrop some seeds may remain dormant for up to 6 years before germinating. Where a new weed species is detected in the orchard it should be controlled immediately, before it becomes entrenched by physically removing the plant and seeds from the orchard and destroying. This is only practical where a few plants are present.
Cultivation
Many growers regard cultivation as a simple and inexpensive means of weed control. However, its effect on soil structure, organic matter levels and root growth of the tree crop must be costed.
Reducing the seed bank
Cultivation provides rapid weed control, but also promotes weed germination. Weed seeds on the soil surface and seeds deep in the soil are relocated into the germination zone by cultivation. Although this rapid re-emergence of weeds after cultivation is an annoyance, it can be used to advantage by the orchardist. Repeated cultivations that kill the emerging weeds before they seed can rapidly reduce the bank of weed seeds in the soil. This strategy is only condoned where weed seed populations are initially high. It is a useful strategy in new plantings where the use of chemical control is restricted by the susceptibility of young trees to herbicide damage. Always bear in mind that cultivation has detrimental effects on the soil.
Vegetatively spread weeds
Some weed species spread vegetatively. Examples are nut grass and johnson grass which regenerate from tubers (nuts) and rhizomes respectively. Cultivation is not effective in controlling these weeds as they quickly re-shoot from the underground nuts or rhizomes. Rather than controlling these weeds, cultivation spreads them by dragging the nuts and rhizomes along the rows. Cultivation implements should always be cleaned after working in an infested block, otherwise clean blocks will rapidly become infested. Chemical control with a systemic herbicide is the most effective means of controlling this type of weed.
Herbicides
Two types of herbicides are available for the control of weeds in orchards. Knockdown herbicides are used to kill emerged weeds. Pre-emergent herbicides are used to prevent seeds germinating and/ or emerging. Pre-emergent herbicides are also referred to as residual herbicides as they continue to be effective for a period of time after application. By using the two types correctly and in a timely manner chemical weed control can be both effective and economical.
Knockdown
Knockdown herbicides, which may be either contact or translocated, are used for the control of emerged and/ or established weeds. They may be applied to the entire orchard floor, along the tree line or as a spot spray depending on the distribution of the weed problem. Knockdown herbicides are normally most effective when applied to healthy growing weeds which are not under water or temperature stress. Specific advice relating to each herbicide is contained on its label.
Contact
Contact herbicides only affect the parts of the plant they come in contact with. For this reason they are safer for use around the base of trees than other types of herbicides. Contact herbicides are effective for the rapid control of annual weeds and the seedlings of perennial weeds. When contact herbicides are applied to established perennial weeds, the above ground portion of the weed which is sprayed is destroyed, but regrowth normally occurs from the underground parts of the plant, that is roots, rhizomes or tubers.
Most contact herbicides should be applied in overcast weather or in the evening for best results. Refer to and follow label recommendations.
Translocated
Translocated herbicides are effective for the control of most weed species, but are generally slower acting than contact herbicides. The benefit of translocated herbicides is their ability to move within the plant and kill the below ground as well as the above ground parts of a weed. With attention to correct timing and rate of application, translocated herbicides can kill difficult to control perennial weeds such as nut grass (Cyperus rotundus), couch (Cynodon dactylon) and johnson grass (Sorghum halepense).
Do not allow translocated herbicides to contact green or growing tissue of trees (including young bark), as severe damage to the tree may result.
Pre-emergent
Pre-emergent herbicides are used for longer term control of weeds along the tree line. Depending on soil and orchard conditions along with the chemical and rate of application used, emergence of weeds can be prevented for up to 6 months from a single spray. Pre-emergent herbicides do not control emerged weeds, particularly once they have past the seedling stage. Effective control of weeds with pre-emergent herbicides relies on the removal of any weeds already growing with a knockdown herbicide or cultivation prior to application. In some cases a knockdown herbicide can be tank mixed with the pre-emergent herbicide allowing single application. Where large weeds or dense cover is present, effectiveness of pre-emergent herbicides may be reduced considerably.
Some pre-emergent herbicides will kill selected weed species present as seedlings at the time of application.
Unless a permanent bare, non-cultivated orchard is desired, application of residual pre-emergent herbicides to the between row area is not recommended. This is particularly the case if a cover crop is going to be sown in the future, otherwise poor establishment may result. Exception to this is where a selective residual herbicide is applied.
Mixtures
A number of registered herbicides are available which contain more than one active constituent, extending effectiveness over a greater range of weeds.
A number knockdown herbicides can be tank mixed with certain residual herbicides. This is useful when applying herbicides for residual weed control, but where weeds have already emerged at the time of application. Before mixing any chemicals, refer to the label or appropriate chemical company. Improper mixing of herbicides can result in poor weed kill and/or crop damage.
Application equipment
Herbicide application units specifically designed for orchard spraying are available. These units, such as the Enviromist(R), virtually eliminate spray drift.
Where more conventional spray booms are used, they should be set low and fitted with nozzles that produce large sized droplets to minimise the risk of drift and consequent damage to the tree crop. The fitting of covers and shields can reduce the likelihood of spray drift causing herbicide damage. Herbicide spraying should be performed only during still air conditions as spraying in windy conditions can result in off-target damage to either the tree crop or neighbouring crops.
When spot spraying, adjust nozzles of hand guns to produce an easily directed stream of coarse droplets.
Ensure spray units are calibrated, otherwise application rates may be incorrect giving poor control or wasting valuable herbicides.
Herbicide damage
In general, the herbicides used for weed control in orchards are non-selective. If these herbicides are allowed to contact green or growing tissue of the tree, herbicide damage will occur. Exposed roots and/ or suckers present in the orchard also pose a risk. Suckers should not be sprayed, particularly with translocated herbicides, as they are still connected to the tree via its root system, thus the likelihood of crop damage is high.
Symptoms of herbicide damage on tree crops vary with the chemical concerned, but are frequently similar to those expressed by the weeds that are being controlled. Contact herbicides tend to cause necrotic (dead tissue) spots on the leaves. Translocated herbicides can cause discolouring of the leaves ranging from white to orange, often resembling a nutrient deficiency, but distinguishable by its artificially bright appearance.
Prolonged usage of some herbicides leads to a residue build up in the soil. Poor establishment of cover crops has been linked to this many times. Overuse of herbicides, especially on light textured soils has been known to affect tree performance.
Mulching
Mulching is a useful technique of controlling weeds along the tree line. The thickness required to suppress weed germination varies with the density of the mulch material. Applying a sufficient thickness for effective weed suppression is often an expensive option.
Mulch layers insulate the soil from high summer temperatures allowing tree roots to grow to the surface of the soil. Where the tree line is left bare root growth in the top 5–15 cm is often restricted by high soil temperatures.
Other advantages of the mulch layer include improved soil moisture retention, soil structure and soil fertility. Throwing the slashings of cover crops or the permanent sod into the tree line can assist with the maintenance of the mulch layer. In most cases this alone will not build up a sufficient mulch layer to suppress weeds.
Weeds that emerge through the mulch can be spot sprayed with a herbicide.
Competition (Permanent SOD)
Competition refers to using a permanent sod to suppress weed growth in the orchard. Unfortunately, weeds are strong competitors. Permanent sod species are not, as a strongly competitive permanent sod would compete with the tree crop for water and nutrients, becoming a weed itself.
For a permanent sod to be effective in suppressing weed growth it must be given the upper hand over weeds in the first place. Most weeds can not successfully establish from seed where a thick healthy permanent sod already exists. Removal of weeds prior to planting of permanent sod by cultivation is essential. A residual herbicide is an effective means of giving the sod time to establish itself without competition from weeds. Adequate irrigation and nutrition must be supplied to rapidly establish the sod species.
Sod growth must be thick and healthy all year round. This requires a mix of species in the sod, for example rye grass, fescue and clovers. If the sod weakens or becomes sparse this provides the opportunity for weeds to emerge and establish.
Regular slashing is an important means of controlling weeds in a permanent sod. Permanent sod species are either perennial and/or prostrate in nature which makes them well adapted to survival in a regularly slashed orchard. As many annual weed species only produce seed well above ground level, slashing stops them producing seed, eventually eliminating them from the sod. Slashing does not control prostrate weeds including caltrop (Tribulis terestris)and three-corner jack (Emex australis), nor innocent weed (Cenchus longispinus).
Selective herbicides can sometimes be used to control problem weeds in a permanent sod. Check label recommendations to ensure that the sod species or the tree crop will not be harmed before applying herbicides.
