ask-tom-pictureAsk Tom –

My greenhouse collapsed under the recent snow and the Quonset shape now looks more like a capital “M”. Any suggestions on repairs?

-Perplexed Grower

Dear Perplexed Grower –

I’ve seen poly houses collapse or blow away on several occasions. Each time they have gone back up stronger than before. We lost two houses in the recent snow but others survived. My first suggestion is to look for the points of failure so that the repair can eliminate that particular failure next time. With our changing climate I suspect more large snow events in the future.

Here are some common causes of polyhouse failure and possible preventive strategies to consider:

  • Wind – Greenhouses look remarkably like the leading edge of an aircraft wing. As high wind moves across the hoops there is pressure down (to collapse the structure) but also lift created (to pull up ground stakes and make it airborne). It pays to either secure ground posts with concrete or put tabs on the posts underground to resist an upward pull. Mobile home anchors might be a good idea, particularly for a retrofit.
  • Rain – Loose poly can create bags where hoops intersect the purlins. In a heavy rain hundreds of pounds of added load can take down a greenhouse. Keep the poly tight and also check the wiggle wire channels from time to time and make sure the screws attaching them to the frame are still solid.
  • Snow – The traditional approach that we used for snow protection is 2X4 posts under the center of the house every couple of hoops. In this exceptional snow storm that system failed in two ways. In some cases the supports were driven into the soil and allowed the structure to deform enough to fail. In other cases the 2X4 just splintered under the load. This failure was, by the way, with both of us unloading houses for all we were worth as snow was falling. For heated houses the strategy normally is to turn off the inflation blower and keep the house warm enough to melt snow as it falls. Another approach that we will consider next time if we get behind in removing snow is to pull the wiggle wire and get the poly off the frame.

So — our lessons learned were use bigger boards than 2X4. In another house 2X6s deformed but did not fail. I plan to make better use of the poplars that are common on our farm and cut really beefy posts instead of relying on dimensional lumber. I also plan to put those posts on some sort of footing like a trex platform. (Pressure treated wood is not allowed in organic houses.) Trex is one brand of composite wood made from sawdust and recycled plastic. Composites won’t leach toxics or rot.

Repair suggestion – I recommend cutting out the kinks and breaks in the old frame with a Sawzall or similar reciprocating saw. A circular saw with an abrasive metal blade also works but it’s a little slower. Cut enough of the bend out so that the cross section of the pipe is round again. Large pipe wrenches or a section of larger diameter pipe can often be used to fix minor bends. The key to strong repairs is to find pipe that is slightly larger or smaller than your hoops. Cut enough for about a foot overlap on either side of the repair and slip the other pipe over (or inside) the cut ends. Tek (self-tapping metal) screws can hold it in place. I have used this method before and never seen that same spot fail again. The double metal probably makes the repair stronger than the rest of the frame, even with the screw holes (which generally are not a good idea for hoops.)

I anticipate about 40 hours to reconstruct each of our houses. The cost of that time compares favorably to the cost of hauling off the old frame and buying a new one.

One other thought to consider as you reconstruct is to see if you can change the shape from Quonset to gothic. Gothic designs with a steeper roof shed snow better and I am hopeful they will reduce drips on the crops if the condensation will slide off the interior instead of dripping on our tomatoes (a leading cause of tomato leaf mold.)

Happy farming.

— Tom

Tom Elmore

Tom Elmore

Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.