Planting vegetation as a drift barrier

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Vegetative barriers can be effective in mitigating the downwind movement
of spray particles released during normal pesticide-application operations

Line chart showing reduction in downwind deposits of spray particles

Figure 1:  Reduction in downward deposits. This is an example of a barrier reducing drift by more than 90 per cent. Predicted deposition of spray drift without a barrier (blue line) and deposition when a 3-metre-high barrier with a porosity of 0.1 (10 per cent) is positioned 10m from the edge of a field.

SOURCE: University of Queensland Centre for Pesticide Application Safety

Vegetative barriers are areas of trees or shrubs deliberately planted to act as ‘filters’ to minimise spray drift risk and to potentially reduce odour and noise.

However, remnant vegetation should not be used as a barrier: it should be protected from spray drift.

Establishing a vegetative barrier

When establishing a vegetative barrier, ‘thicker’ is not better. Increasing the number of rows of trees that are planted (to more than two alternating) may reduce the effectiveness of the barrier if the porosity becomes substantially reduced.

Thicker vegetation reduces the airflow through the barrier, which reduces filtering and the capture of spray droplets. Before planting, consider how large trees will grow, then space larger trees so that their canopies will only just touch each other when mature. Vegetation needs to be maintained right down to ground level. Establish an understorey of shrubs either side of the main row of trees to prevent possible funnelling of pesticides through any gaps in the vegetation.

Locally indigenous plants should always be given preference. They are most likely to perform well as they are already adapted to local climatic and environmental conditions.

Drought-tolerant plants are particularly useful, especially if they have thick, waxy cuticles or rough or hairy leaf surfaces, as they are more likely to resist the uptake of pesticides. Casuarina species are a useful species for including in a vegetative buffer due to their open canopy and long, thin, cylindrical leaves with thick cuticles.

Vegetative barriers can be permanent, such as planted strips of trees and shrubs, or temporary, such as strips of forage sorghum adjacent to paddocks that are sprayed regularly.

A permanent vegetative barrier is typically a narrow strip of trees and shrubs (about 20 metres wide) that has been deliberately planted close to the edge of sprayed fields to trap airborne spray droplets that may be carried away from the field by wind. Vegetative barriers do not need to be particularly tall or wide to be effective.

The height of the vegetative barrier depends on the release height of the spray.

At 50 per cent porosity (ratio of light to dark when viewed side on), the height of the vegetative barrier should be at least one-and-a-half times the release height of the spray. If the porosity of the barrier is 40 per cent, it is recommended that its height be increased to about twice that of the release height of the spray.

Immediately behind a vegetative barrier will be a reduction in drift, which is directly related to the porosity of the barrier. As distance downwind of the barrier increases, the reduction in drift decreases. However, a long distance (beyond 100 metres downwind) from the buffer, the drift will be at the level of no barrier (Figure 1).

More information:

Dr Andrew Hewitt,
University of Queensland,
a.hewitt@uq.edu.au

Useful resources:

Queensland Department of Natural Resources and Mines, Planning Guidelines:
Separating Agricultural and Residential Land Uses

PSIC (SCARM) Report 82, 2002, Spray Drift Management: Principles,
Strategies and Supporting Information

Victoria DPI Agriculture Notes, AG0860, 2002, Using Buffer Zones and
Vegetative Barriers to Reduce Spray Drift

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GRDC Project Code UQ00072, UQ00047

Region North