Tuesday, May 14, 2013

Forest Garden Windbreak, Part 1

Thanks to the Cowlitz County Sheriff's Office Department of Emergency Management for this visual metaphor
Yet another one to be filed in the cluttered “we learn from our mistakes” tray. We observed, we measured, we surveyed and we drew a lovely plan with coloured circles representing the mature canopies of just the right amount of fruit, nut and nitrogen-fixing trees. We thought that the first thing to plant in a forest garden was the tree layer, especially when one considers how long it takes trees to grow and to grow to an age when they’ll start fruiting. We now realise that the first thing to plant in a forest garden is the windbreak.

A little snowbell tree, Halesia carolina, never made it through its first winter and it was only after its demise that I turned to The RHS A-Z Encyclopaedia of Garden Plants and found that, although it’s hardy, it should be sheltered from cold winds. The field being planted up is exposed to the north (cold, dry winds) and the west (winds from the south west and west, often wet).

Oft-repeated permaculture wisdom is to observe for a year, seeing your land through four seasons before making planning decisions. It seems strange but it’s only this winter that I’ve become aware of just how exposed this site is and just how windy it gets. Perhaps it’s that we sometimes only see what we want to see and are otherwise oblivious to something staring—maybe that should be blowing—us in the face. I notice it now. In fact, I automatically note what’s happening wind-wise each time I visit the field … and it’s often impressive. So, we need some sort of windbreak.

Our two reference books are Patrick Whitefield’s How to Make a Forest Garden and Martin Crawford’s Creating a Forest Garden, both are useful but the latter has more specific detail on windbreaks.

Martin talks about a “quiet zone of protection” just behind the windbreak, followed by a turbulent area. There is a calculation: the quiet zone “extends for seven or eight times the height of the windbreak.” Our forest garden is 33 metres wide (E/W) and 50m tall (N/S). That suggests our northern windbreak should be 6½m tall and the western edge 4½m tall. We’ll use that as a guide but taper the western windbreak towards the southern end so we don’t shade out the afternoon sun or affect the wonderful view too much.

Martin also suggests planting trees and shrubs that grow to the size required and therefore don’t require onward maintenance. Nice idea, but we think that particularly difficult to manage as trees will grow to different heights dependant on their soil and situation, in our case being closely planted with other shrubs and trees. We decided on a line of trees with a line of bushy shrubs in front, 1.2m between plants and 1m between the lines, the second line offset from the first by half, so that the shrubs block up the holes at the base of the trees.

We thought the list of plants suggested by both books were a bit limited, so in the next blog, I’ll explain how we chose our plants and what they are.