[IBC] Poly Tunnel Covering
Hello again,
I hope everyone is doing good. I was wondering if anyone might have some tips on building a poly tunnel. What thickness of poly should I use? I need a place for recent repots, a place for some Ficus benjamina that can't quite take the full on sun all day, and a place for everything else that just needs a little protection from the elements. Will these different applications call for different thicknesses of poly or any other differences in construction? Sorry to complicate something that's probably very simple. Thanks for any help! Audgen (zone 7). ************************************************** ****************************** ++++Sponsored, in part, by Evergreen Gardenworks++++ ************************************************** ****************************** -- The IBC HOME PAGE & FAQ: http://www.internetbonsaiclub.org/ -- +++++ Questions? Help? e-mail +++++ |
[IBC] Poly Tunnel Covering
At 12:24 AM 4/13/03 -0700, audgen wrote:
Hello again, I hope everyone is doing good. I was wondering if anyone might have some tips on building a poly tunnel. What thickness of poly should I use? I need a place for recent repots, a place for some Ficus benjamina that can't quite take the full on sun all day, and a place for everything else that just needs a little protection from the elements. Will these different applications call for different thicknesses of poly or any other differences in construction? Sorry to complicate something that's probably very simple. Thanks for any help! Audgen You don't say where you are, so that complicates things somewhat. A poly tunnel in a mild climate is definitely an easier proposition than one in a hot climate. The actual construction is pretty much a breeze, and there are many ways of doing it. First of all, the size- This is very important because the larger you make it, the more complicated it becomes, mostly because of ventilation needs. 15 to 20 feet long is about as big as you can make one that is film covered without positive ventilation (exhaust fans, or roof vents). It gets very hot in these tunnels even when covered with translucent poly or clear film covered with shadecloth. It needs to be tall enough so you can walk into it without bending over. After several aborted designs for poly or shade tunnels, I have come up with a strong inexpensive method that is easy to construct. It is made of hoops of PVC pipe positioned by wires and covered with either shadecloth, film, or both. 1 1/4 inch is the smallest pipe you can use and still have the strength you need. There is also a quantum leap in price in sizes larger than 1 1/4 inch. Be sure to get Schedule 40 PVC water pipe in 20 feet lengths. Don't buy a lesser grade or you will have problems. If you take a 20 foot piece of 1 1/4 inch PVC and bend it over, it will span about 13 1/2 to 14 feet in an arc that is a little over six foot high. My latest generation of hoop houses are 13 1/2 feet wide with the ends of the PVC slipped over 1 inch EMT conduit (also very inexpensive). The conduit is cut into lengths and pounded into the ground. A baseboard of treated 2x4 lumber is attached to the conduit (on the outside) by using pipe straps. The conduit should stick up above the 2x4 about 6 to 8 inches. This will give you a nice flattened arc with plenty of standup space. The ends of the PVC rest on top of the 2x4 edge, no other attachment needed. You really have to work to get the PVC over the conduit so make sure the conduit is really firmly anchored into the ground or the PVC will push the conduit and the 2x4 outward and mess up the arc. Space the PVC hoops every 4 feet. So 5 hoops would give you a 16 foot house, about as big as you can make with film, but without positive ventilation. There are any number of ways to connect the hoops so they can't move horizontally. Most commercial methods involve using a lateral strip or pipe called a pylon. My houses are 96 feet long, so I have developed a more economical and practical method, but it is not as practical for small houses. I use 12 gauge high tensile vineyard wire (available at farm supplies). This is incredible stuff and really cheap considering that you get about 2 miles of it in a roll, but you can only buy it by the roll and it's about $50. It is a huge pain in the butt to unspool without an unwinding contraption, which I made for the nursery (I use this stuff everywhere). Sometimes you can rent one. Anyhow, I stretch the wire in two lengths along the hoops of the house and fasten the ends at steel T posts, then run it down to the ground at a 45 degree angle and attach it to steel earth anchors (another story). I use these fabulous connectors called gripplers that allow you to stretch the wire with a tool (yet another story). You can get about 600 lbs of force on the wire with this system, so it is very strong. The wire is attached to the hoops with screws which will easily go into the PVC. Now that I read what I just wrote, go with pipe pylons or wood, it will be much simpler. You will still need some sort of a diagonal brace. Wooden end frames are probably the easiest, although not strictly necessary depending on whether you want to enclose the ends in winter or if you are only going to use shadecloth instead of film. Attaching the film is by far the hardest job. Attaching it along the length is a breeze. You slide it over the house and allow a two foot overlap along each side. Cover the film with earth. That's it, no staples, battens, any of that stuff. The earth is so heavy that no wind will ever pull it loose or tear it. The ends are as hard as the sides are easy. The best way is to pull the film over the end and attach it to a 2x4 frame contructed inside the arc. Nearly all commercial structures are built this way. A few companies sell these plastic thingies that slip over the PVC and the plastic film to hold it in place. These work great until the wind blows. The wind flaps the plastic continually until the edges of the retainers begin to cut through the plastic and in short order the ends of the film are loose and you are screwed because now the plastic is too short. End frames allow you to pull the plastic over the PVC and gather it and staple it. Then you can batten over the staples. This is a must or the wind will pull the staples loose or tear the plastic around them. You can get fancy and make doors if you want to, depends on how much you want to do with this structure. You can make a full blown greenhouse if you want. The plastic you use should only be 6 mil greenhouse plastic with UV inhibitors. In larger sizes it doesn't cost any more than regular poly and it will last four years instead of less than one. It is generally only available at nursery and greenhouse supply outlets so you will have to hunt for this. Online greenhouse suppliers carry it, but you may have to pay premium prices and it is quite heavy for shipping (Mellingers.com, CharleysGreenhouse.com). Do NOT use the regular stuff you find at hardware stores unless you want to replace the plastic every single season. For your purposes, you may not even want to use film, I don't. My hoop houses are covered with shadecloth only. This simplifies construction a bit because you can attach the shadecloth quickly and easily with drive screws in the 2x4 base and directly into the end hoops, no other frame necessary. I even enclosed the ends with shadecloth and made shadecloth flap doors that just hang from the top of the last hoops. Shadecloth gives you wind and sun protection, keeps critters out, raises humidity slightly and still allows excellent air circulation. Perfect for seedlings, transplants and shade loving plants. It does not however give you any freeze protection, well only a degree or two, because the shadecloth does slow the rate of radiation at night. I love these shade houses, leave them up all year, and have been using them for ten years. Of course I live in zone 8, so heavier protection is not usually necessary in winter for most woody plants. I am currently building one with a heated earth floor with an active solar system and frozen covering protection, but that is another WHOLE story. Brent in Northern California Evergreen Gardenworks USDA Zone 8 Sunset Zone 14 http://www.EvergreenGardenworks.com ************************************************** ****************************** ++++Sponsored, in part, by Evergreen Gardenworks++++ ************************************************** ****************************** -- The IBC HOME PAGE & FAQ: http://www.internetbonsaiclub.org/ -- +++++ Questions? Help? e-mail +++++ |
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