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Chemical control is one of the tools available to control weeds within an integrated pest management framework. To introduce a chemical undervine weed control program successfully, careful consideration must be given to the initial land preparation and timing of operations at least six months before the chemical is applied.
Many problems are caused by poor initial preparation and failure to read the label of the product and follow instructions.
Firstly, it is necessary to understand the three basic types of herbicides available and how they work. They are:
These chemicals are applied to soil and remain active for varying periods. They kill the germinating weeds before they emerge from the soil but have little or no effect on existing weeds.
Residuals act in the top 50-80mm of soil. They must generally be fixed at the correct soil depth by irrigation or by mechanical incorporation. In furrow irrigated vineyards they should be applied when the soil is moist, or immediately before rain. In sprinkler irrigated vineyards, if the soil is not moist when the material is applied, a light irrigation of about 12mm should be given after the chemical is applied.
Excessive rainfall or irrigation after application may wash these chemicals into the root zone of the vines where they may cause serious damage. Alternatively, the chemical may be washed away to nearby areas and damage other plants.
Residual herbicides should be applied to bare, moist soil, or soil with a few standing weeds. Just as residuals are formulated to “cling” to soil particles, they will also “cling” to any organic matter on the surface and therefore will not be able to be incorporated into the soil. Unless incorporated, most residuals will be degraded by sunlight and air and become inactive.
Some chemicals are better suited for use in sandy soils than others. Check the label for suitability and correct application rate.
Residual herbicides will control weeds for between two and twelve months, depending on the type of chemical, application rate and soil type.
Knock-down herbicides will kill the above ground parts of weeds, acting by contact of the green tissue of the plant. Knock-down herbicides work most effectively on young weeds. They are most useful for control of annual weeds. They may not be sufficient in controlling perennial weeds which can propagate from underground parts (e.g. Couch grass) as they only affect tissue they contact.
They may however, be useful to stimulate growth of dormant buds on underground parts. This can be useful to make the weed more susceptible to the action of a systemic herbicide, once dormant buds begin to grow.
Death of sprayed areas of weeds is rapid with a knock-down herbicide.
Generally, knock-down herbicides are nonselective (which means that they damage or kill any type of plant). However, some knock-downs may be used to kill specific weeds without damaging the crop by changing application rates and timing spraying to when weeds are most susceptible. Check the label to ensure the correct herbicide and rate is chosen for this use.
Systemic herbicides usually move into the plant and may be translocated within the plant away from the point of contact toward the roots or growing tips. This is different from the action of knock-down herbicides which only affect the plant tissue they contact.
Due to the translocating mode of action, systemic herbicides may take several days or weeks to actually kill weeds. The rate of yellowing or browning usually depends on the growing conditions at the time of spraying. Systemic herbicides take effect most quickly in good growing conditions.
Because systemic herbicides are able to move through the plant, they are especially useful for controlling perennial weeds (such as couch grass) which have deep tap roots or underground rhizomes or tubers.
Applying a spray too strong or too weak. Excessive application can result in loss of crop or loss of vines. A weak spray can result in poor weed control. Use only the rates detailed on the product label.
Sprayer output must be measured and the correct amounts of chemical used.
Over-leaching of soil sterilants into the soil. To avoid this, follow recommendations of the product label and do not over-irrigate.
Failure to select the correct rate and chemical for a particular soil type. Follow recommendations on the product label. Generally, herbicides registered for use on young vines are safest to use on sandy soils.
Failure to read the label. The label contains information on weeds controlled, rates, methods of application, any precautions necessary, compatibility etc., and must be read before the chemical is used.
Damage to shallow rooted grape varieties such as Gordo, especially in sandy soils. Carefully select the herbicides to be used.
Prepare the vineyard bank well before the first planned application of chemical. The first application is usually applied before budburst so preparation should begin in early winter.
The undervine bank may need to be built up to prevent water flooding across the treated area. Allow plenty of time for the soil to settle and compact.
The bank should be kept free of weeds. This means that it will either have to be knifed several times during winter or sprayed.
A 1m wide undervine strip is usually treated; that is, 500mm either side of the vine.
Correct timing of application is crucial to the success of knock-down and systemic herbicides. If these are sprayed at the correct stage of the weeds growth, minimum rates will achieve a kill or prevent development of viable seed.
Correct soil preparation is the most important factor for obtaining good results when using a residual herbicide. Even coverage and even incorporation is only achievable into a well prepared soil surface.
Most residual herbicides initially give from four to six months of weed control, so in the first season two applications may be needed for year-round control. Refer to the container label for manufacturer’s recommendations and follow them closely.
If two sprays are used, apply the first before budburst and the second before the vine canopy falls.
If no weeds have germinated when the second spray is due, then the lowest recommended concentration may be used.
If weeds are present when the second or subsequent sprays are due then add a knock-down or a systemic herbicide.
In most cases only one application is needed in the second season. This will depend on the breakdown of the chemical and the number of weeds present. Use the normal recommended rate. Do not disturb the undervine bank as the affect of any residual chemicals will be lost and fresh weed seeds will be brought to the surface.
Under some conditions perennial weeds such as couch grass, paspalum and Johnson grass may become a problem. Spot spraying with a systemic herbicide will control them.
Always check the label to see if a wetting agent should be added and use only the one recommended. The addition of a wetting agent to diuron provides a knock-down affect on very young seedling weeds.
Whenever agricultural chemicals are to be used always:
In Victoria, certain chemicals are classified as restricted use chemicals. These chemicals are S7 poisons: ester herbicides of 2-4D, 2-4DB; MCPA and triclopyr; formulations of atrazine; metham sodium; and pindone concentrate (>2.5% pindone). Use of these chemicals is restricted to authorised persons. Check the Agriculture Victoria Chemical use section or contact the Chemical Information Service on (03) 9210 9379 for more information.
The advice provided here is intended as a source of information only. Always read the label before using any of the products mentioned. The VVBC does not guarantee that the content is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.