Nurseries are finalising orders for 2021 over the next few weeks, so if you plan to plant in 2021, you need to start talking to your nursery about the species and numbers to order.
However, before you make that call, the first step is to be clear about the reason for planting: whether it is to prevent wind erosion; provide livestock shade in summer and shelter during lambing; provide additional browse to supply vitamin E in summer; manage waterlogging and salinity; reduce dam water evaporation; increase biodiversity or add pollinators and pest predators.
Your local nursery can often recommend general designs and species for each reason, but the detailed design and species list depends on the site’s soil type, rainfall and the capacity to fence. However, you can apply some general principles.
Shelterbelts and windbreaks generally consist of at least four lines of plants, as Parnells Nursery manager Bronwyn Copestake explains.
“Anything less than four lines and you are losing the effectiveness in terms of windspeed reduction. So, four lines spaced three metres apart and plants 1.5–2 metres apart within each line would be a very generalised guideline.”
The purpose of the planting will also affect the proportion of trees and shrubs in the belt.
“If you want a windbreak I would go with a higher percentage of taller trees, if you want it for biodiversity or a shelterbelt for stock I would go with a denser, shrubby configuration.”
Windbreaks can affect windspeed for up to 20 times the tree height (20H), but David Bicknell from the Department of Primary Industries and Regional Development says it is within four to 15H that you see most of the effect. However, he also says that windbreaks can cause erosion if not designed properly.
“You don’t need much of a gap underneath to cause a problem (with wind jetting through a gap). So you don’t graze under wind breaks and shelterbelts, which means they should be fenced.”
Ms Copestake agrees, suggesting that oil mallees planted in a crop year to allow them to establish is really the only option if areas aren’t being fenced [Note: Eucalyptus polybractea (blue leaved mallee) has a tree form and is slightly more susceptible to grazing compared to E. loxophleba subsp. lissophloia (‘lox liss’), which has a mallee form.]
Mr Bicknell says that having at least two lines of shrubs in the windbreak is important to stop wind jetting beneath trees, while the risk of turbulence on the downwind side is mitigated by an open overstorey at the tree-line.
“The standard design is four to five rows with trees in the middle and shrubs on the outside. The tree canopy area can be 50% open to filter wind. If trees are dense at height, windflow will be blocked which will cause turbulence and wind erosion within four tree heights downwind. An alternative to shrubs could be trees with low foliage such as an Allocasuarina.”
Placing trees four to 10 heights away from a specified area to be protected such as a yard or dam will maximise the reduction in windspeed without causing the problem of tree roots and falling branches on fences.
If the purpose is shelter for lambing or off-shears sheep, Mr Bicknell has other alternatives that might not need fencing.
“For lambing, it’s much better to have distributed low shelter everywhere such as scattered shrubs with dense lower shelter such as oldman saltbush. It is a really good system for giving even shelter, but you need specialised paddocks for it. Sometimes having the windbreaks close to the shearing shed or around shearing sheds and yards is a good idea to stop the dust in these areas.”
After you’ve worked out a density and design for your site and needs with your local nursery, success relies on good site preparation.
“Good weed control, ripping the seedling lines, and planting early once there’s been a decent rainfall. That’s your best chance of success,” says Ms Copestake.
Another key tip is to source good nursery stock, which can be improved by getting orders in early, with most nurseries finalising orders in November. So, now’s the time to clarify your aims, identify your soil type and make the call.
For more information, see the Department of Primary Industries and Regional Development’s publications “Tree windbreaks in the wheatbelt” or “Establishing effective windbreaks on the Swan Coastal Plain.”