Following Storm Hannah’s destruction of my new greenhouse, I’ve been thinking about what to do next.
I really don’t think there is any point in replacing it in the same position. It’s rare for the wind to gust from the direction it did directly towards the long face of the greenhouse. If it had been at an angle to the long face then it wouldn’t have exerted the same force.
The Force is Against Us
It’s interesting to look at the force wind can exert. The formula to calculate it is pretty straightforward. According to Mr Google..
Wind pressure is given by the equation P = 0.00256 x V2, where V is the speed of the wind in miles per hour (mph). The unit for wind pressure is pounds per square foot (psf).
A normal wind for here is 25 mph so the force of the wind would be 1.6 pounds per square foot. The flat area of the greenhouse is 50 sq ft so a total force of 80 pounds. Upping that windspeed to a storm speed of 40 mph increases the total force to 205 pounds. Just a 15 mph increase more than doubles the force.
The real problem is that the wind gusts. A 60 mph gust punches 460 pounds of force on the greenhouse and 80 mph increases that to 819! We have had gusts of 100 mph which would hit 1,280 pounds.
Putting a solid fence up would absorb that force but the wind going over the top will fill the void very quickly which is why trees are so effective as windbreaks. They don’t stop the wind, they slow it down and this reduces the eddying that the solid fence causes. A 30% reduction in wind velocity makes a big difference to the force of a gust. The force of that 80 mph gust would be more than halved to 401 pounds.
Since we moved here, it seems that severe storms have become more frequent so wind fencing is something to consider whatever we do. If the predictions of climate change are true, as I believe, things are only going to get worse for us all.
One area that is no stranger to wind is the Shetland Islands. Hurricane winds in excess of 70 mph are not unusual and those, of course, gust higher. 101 mph has been recorded at ground level. Growing in their conditions is not easy.
The ideal is to grow tender crops under cover but even a good polytunnel is vulnerable in those conditions. So I was really interested to see this innovative solution to building robust growing shelters. See video at the end of this post.
Called a Polycrub it’s basically a super-tough, solid polytunnel utilising recycled salmon feed pipes from the salmon farming industry to form the structural support hoops. They’re not cheap but they’re permanent even in hurricanes.
According to their website, it’s called a Polycrub because:
Historically, Shetlanders needed sheltered space to grow-on young kale (cabbage) plants. Stone was their building material of choice and they built round, stone-walled shelters to nurture plants through the toughest conditions. Locally, this structure was known as a ‘planticrub’. Our building material of choice is polycarbonate, so we merged the name of our chosen 21st century material with a name from past growing history in Shetland. The ‘Polycrub’ was born.
Interestingly, in Lanzerote we noticed a similar walling system being used to shelter plants from the continually blowing trade winds. Very different in terms of temperature and rainfall but the same answer.
The winds aren’t as strong in Lanzerote but they are constant and dry out the plants. So sheltering from the wind also helps with the water supply – fresh water is a precious commodity on Lanzerote.