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Winter garden protection: Greenhouses and row covers

Row covers inside a greenhouseThe heart of Eliot Coleman's winter gardening is a two-tiered method of protecting the plants from winter cold.  He grows his vegetables under floating row covers inside simple greenhouses (hoop houses), and the combination of the two is the equivalent of moving his garden 1,000 miles south to where it is three climatic zones warmer.  The majority of his growing area --- and the part I will be discussing in this lunchtime series --- is completely unheated, but he does note that for some people who have a cheap source of heat, it might be worthwhile to warm the greenhouses until they stay just above freezing.

I've always been leery of hoop houses and greenhouses because of disease and pest buildup, and Eliot Coleman notes that permanent greenhouses also have the side effect of building up soil nutrients to poisonous levels.  By cutting your growing space off from the outside world, you're promising to keep the ecosystem in balance, which is a tough job when we haven't even identified all of the microorganisms at work in our soil.  Coleman has a solution --- let nature do that hard work for you.  His greenhouses are light enough that his tractor can pull them to a new patch of ground a couple of times a year, allowing nature to heal the wacky greenhouse ecosystem left behind.  His movable greenhouses also have the side benefit of allowing him to grow heat-loving summer crops in the greenhouse until late October, then move the greenhouse onto two-month-old fall crops just as the killing frosts commence --- in essence, he gets fourteen months of greenhouse time per year for each structure.

The row covers Coleman uses inside his greenhouse are the same spun-bonded fabric you can purchase in many seed catalogs and which you'll see in my garden.  (He uses Agribon brand row covers from Johnny's.)  Since the greenhouse blocks wind and heavy weather, you can simply drape your row covers over twelve inch tall wickets made of #9 wire and spaced every four feet along the bed.  Snap a few clothespins on to hold the fabric in place, and you're done with your second tier of protection.  Unlike glass-topped cold frames or cloches, row cover fabric is self-venting and only needs to be removed when warm spring days in March begin to overheat your crops.

Movable greenhouseI was glad to see that Coleman did some research to determine the best kind of row cover since I've been confused by all the choices in the past.  Your first decision is whether to choose polyvinyl alcohol fabric or the simple spun-bonded fabric which is more readily available.  The former has marked advantages of lasting about four times as long, allowing 90% of light to pass through (versus 30 to 85%), and keeping the air underneath about one degree warmer, but with a price tag five times higher, polyvinyl alcohol probably isn't worth it.

Next choice is --- light fabric or heavy fabric?  In the seed catalogs, you'll see the heavy fabrics listed as protecting your plants several degrees colder, but in Coleman's experience, these heavy fabrics block so much of the sun's light that the ground actually ends up colder in the long run.  Instead, he recommends the thinnest row covers, which are rated as protecting your plants only down to 28 degrees Fahrenheit, but which Coleman said provide increasing amounts of protection as the weather gets colder.

I won't go into the specifics of Coleman's greenhouse construction since his scale of greenhouse --- 48 feet by 22 feet --- is beyond the scope of our homestead garden.  If you want to know more, check his book out of the local library, or visit his website (from which I snagged these photos.)  Or stay tuned for tomorrow's post, in which I'll consider ways to apply Coleman's method to the backyard.



Read more about sunrooms in this 99 cent ebook!This post is part of our Winter Harvest Handbook lunchtime series.  Read all of the entries:





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Hello Anna, thank you so much for your info on Coleman's use of the "house within a house" - I am looking forward to your thoughts on how to apply on a smaller scale. Jackie

Comment by Jackie Coalson Sexton Tue Dec 14 12:18:56 2010
PVA
Interesting. I didn't know PVA film was used to protect plants. We use it at work as a "top mould" for vacuum infusion of fiber reinforced plastics. It is also widely used in the production of LCD displays.
Comment by Roland_Smith Thu Dec 16 16:10:51 2010

Jackie --- I hope my later posts gave you some good ideas for your garden. We're looking forward to trying it out.

Roland --- I'd actually never heard of the stuff before reading the book. I'm used to the lower end fabric, which falls apart way too fast. Maybe PVA will become more available soon and fix that pesky problem.

Comment by anna Sat Dec 18 20:21:16 2010