Are Windbreaks Multifunctional?

In January, 2012, the Nottawasaga Valley Conservation Authority hosted a two day conference on multifunctional field windbreaks. It was very well attended by both industry and growers. There were excellent speakers haling from as far west as Regina, Sask., and as far east as Kamouraska, QC.

There was a lot of information about the value of planting windbreaks (aka hedgerows). In fact, there was too much to do justice here in one short article, but here are the headliners in bullet form. Windbreaks can:

  • Reduce odours by absorption and dilution
  • Reduce spray drift by intercepting spray & deflecting wind
  • Reduce noise by baffling sound
  • Control snow drift (on the down-wind side)
  • Reduce dust, which also prevents sand-blasting crops
  • Create privacy
  • Increase crop yield for certain crops at a given distance from the break
  • Enhance biodiversity
  • Create the possibility of secondary production (e.g. timber, biomass, fruit)
  • Enhance aesthetics
  • …and, of course, manage damaging wind

There is always the bad with the good, however. Windbreaks have also been known to:

  • Reduce the plantable area in an operation by encroachment or shading
  • Create a habitat for pests (e.g. birds, insects, deer, etc.)
  • Increase the labelled buffer zone by creating a natural habitat
  • Create cooler temperatures, further reducing plantable area
  • Create conditions that might favour disease incubation
  • Create a lot of additional work through windbreak management
  • Create roots that compete with adjacent crops or destroy drainage tile

One interesting outcome of the conference was the message that not all windbreaks are created equal. Considerable thought and effort should go into the planning and preparation for a windbreak. Speakers emphasized the following considerations:

  • Ground preparation is essential to creating a strong and healthy windbreak, just as it is in any perennial planting.
  • Windbreak maintenance includes pruning, weed control, irrigation, fertilization and many other activities to ensure the windbreak forms as desired.
  • Choice of plant is very important. Trees can grow quickly and require lots of maintenance (e.g. poplar) or slowly and mostly take care of themselves (e.g. cedar). Some plants are more tolerant of drought than others. Some fill in completely, while others are sparse. Some grow tall, some don't. Some have roots that grow laterally, some grow down.
  • Plant spacing is very important. Wide spacing means the windbreak is less effective early on, but requires less maintenance (e.g. removing every second tree) as it grows and fills.
  • Plan for annual management - windbreaks can become infested with invasive buckthorn shrubs and wild grapevines which can damage the health of a windbreak, where not cut out on a regular basis.

What was really interesting was that the nature of the windbreak should reflect the job for which it is intended. For example, a windbreak to stop snow drift is not the same as one that effectively filters pesticide drift. To understand this, you have to know more about how windbreaks work.

Figure 1 shows how wind speed is faster the farther away from the ground. This is because friction slows wind.

Illustration of figure shows how wind speed is faster the farther away from the ground. This is because friction slows wind.

Figure 1. Wind is slower nearer to the ground

Imagine putting a narrow row of trees in the wind. If they are somewhat sparse you should be able to see through them. This can be considered "50% porosity" and appears in Figure 2. Note that some air ramps over the tree (B) and some passes through (C). The grey area (E) denotes turbulent air mixing and (D) is all but still air. By the time the air passes by, over or through the tree, it returns to normal (F).

Imagine putting a narrow row of trees in the wind. If they are somewhat sparse you should be able to see through them. This can be considered "50% porosity" and appears in Figure 2. Note that some air ramps over the tree (B) and some passes through (C). The grey area (E) denotes turbulent air mixing and (D) is all but still air. By the time the air passes by, over or through the tree, it returns to normal (F).

Figure 2. A porous windbreak filters air

Now imagine the windbreak is so dense as to form a wall (Figure 3). Almost all the air compresses and ramps over the obstacle (B) and the dead air zone extends farther away from the windbreak (D).

Imagine showing the windbreak is so dense as to form a wall (Figure 3). Almost all the air compresses and ramps over the obstacle (B) and the dead air zone extends farther away from the windbreak (D).

Figure 3. A dense windbreak deflects air

Which situation is best for removing pesticides or odours? The porous barrier is best because it filters particulates out of the air. In fact, trees with needles outperform those with leaves. Figure 4 shows how suspended particulates are scrubbed from the upwind air, creating a clean zone downwind. Eventually, the particulates re-equilibrate, which means the size of the scrubbed area depends on the height of the windbreak. If the windbreak were intended to stop noise snow, a denser break would be preferred.

Figure 4 shows how suspended particulates are scrubbed from the upwind air, creating a clean zone downwind. Eventually, the particulates re-equilibrate, which means the size of the scrubbed area depends on the height of the windbreak. If the windbreak were intended to stop noise snow, a denser break would be preferred.

Figure 4. A porous windbreak filters suspended particulates

The shear amount, and occasionally conflicting, information got a little overwhelming, but fortunately there are specialists that can help. Contact your local Conservation Authority and they can assign someone to come to your operation and help you determine the windbreak that is best for you. Contact Conservation Ontario at 905-895-0716 or visit their website at www.conservationontario.ca.

To summarize, windbreaks offer many advantages both economic and aesthetic, but are only as effective as the planning that go into them.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca