|The basic 22 x 48 metal frame of my moveable high tunnel. This size allows for sufficient ventilation through end wall vents.
|The sliding side-wall entrance alleviates the necessity for an end-wall entrance, so the end walls are stronger.
By Phil Norris
Photos by the author
Here in Maine, the short growing season makes some sort of season extension desirable. Unheated high tunnels have proven themselves to be profitable, and moveable high tunnels have been around long enough for the advantages to be evident. Several companies are putting out well engineered rolling greenhouses, and the farmer now has some choices.
I wanted a moveable high tunnel so that I could get away from the insects that plague stationary greenhouses; I wanted this high tunnel to roll easily so that it could be moved several times each season; I wanted plenty of clearance so that the high tunnel could roll over established crops; and I wanted it to vent entirely through the end walls.
I had originally planned for a 60- by 22-foot house. I have to admit that in designing this moveable high tunnel, I leaned heavily on the advice of my neighbor, Eliot Coleman, who has done a great deal of pioneering in moveable greenhouses. It was he who convinced me that a 60-foot high tunnel wouldn’t vent sufficiently through the end walls – that there would be a dead spot in the middle. He suggested that a 48- by 22-foot house would vent properly. He was right.
Designing the end wall was the biggest challenge. Most manufacturers provide the basic frame, including bows and cross members, but leave the end wall up to you. I drew various end wall possibilities, but the entrance always seemed to be the structural sticking point. The doorway seemed to destroy the integrity of the wall. And I sure as heck didn’t want to have to duck under a beam to enter the house. The answer: a side wall entrance. That meant I could use a 22-foot-long piece of steel at waist height on both end walls. That would be integrity. You hardly ever see entrances on the side walls of greenhouses. Could it be that difficult?
I built the house in the fall of 2013 and got it covered with poly just before the cold weather set in. At first the side wall door was a flap of poly sealed around the edges with wiggle wire. It worked that first winter and kept snow out but it was cumbersome. It wasn’t elegant. In the spring I bent some steel tubing to the same shape as the side wall and welded a door frame, then screwed on a piece of polycarbonate. I hung this 4-foot-wide door with standard sliding barn door hardware, which consists of an 8-foot-long hollow galvanized steel track with two wheeled “trucks” that ride inside it. The door is hung from the trucks. The bottom of the door is free and is held down by gravity.
The roll-up end wall is fastened with wiggle wire to a piece of double wire lock positioned 42 inches above the soil. When the poly is rolled up, it will be well above the lower beam, giving an honest 32 inches of clearance. The rolled up poly is secured in the “up” position by slipping a rope loop over the end of the roller. It does take two people to roll up the end walls and two people to move the house.
I covered the west end wall with poly using a scissors-type closure. Simple and cheap, it has worked out fine. The east wall is covered with rigid polycarbonate; because the greenhouse is exposed to the battering east winds, I wanted a strong wall in that direction. It features removable panels for warmer seasons.
|The house moves over four plots along a 200-foot track.
|A scissors-type closure and triangular vent at peaks provide ventilation.
|An automatic vent in the fully open position
|This sprinkler system and a wireless thermometer move with the house, enabling us to irrigate any of the four plots as necessary.
The house rolls on two 2-inch galvanized pipes. My site is slightly curved, so I laid out the pipes with a gentle curve and then built the house on top of them. The whole greenhouse is uniformly curved. The house rolls more easily than any other moveable greenhouse I have seen. Two people can push it by leaning on the corners. No tractors, no winches. We have to tie it down in the summer so that a breeze won’t send it down the 200-foot track. Pre-tensioning each bow with a come-along and then locking it in at exactly 22 feet before erecting it took some extra time but has probably contributed to ease of rolling. The strong end walls with an uninterrupted beam 32 inches above the soil and diagonal braces to keep the wheels from splaying out also probably contributed.
The house is anchored by a 3-foot ground post driven into the clay just off each corner. The corner chains lead from these ground posts to eyebolts on the house. Six additional ground posts are used inside the house, three along each side, giving a total of 10 ground posts for the winter position of the house. In summer, when winds are mild, a quarter-inch piece of nylon rope at each corner has been more than adequate. If a hurricane is expected, we will roll the house to the winter position and secure it.
I wanted some sort of simple automatic vents, not necessarily capable of venting the whole house but to exhaust some heat in case a cloudy day suddenly turned sunny. The obvious place to put the vents was at the peak. I used hinged, triangular-shaped vents made of polycarbonate on a wooden frame attached to a couple of nonelectric vent openers. These vent openers are sealed hydraulic tubes that are designed to lift a heavy vent. In this horizontal application they have worked effortlessly and flawlessly. They start to open the vents at about 70 F and are wide open at 85 F. Also, in the middle of the house, I installed a wireless thermometer, which beams a signal to my office. It monitors and graphs the temperature and humidity in the high tunnel. During a Maine summer with both end walls fully vented, the temperature in there never exceeded 99 F. The peppers loved it.
Another unexpected use of the high tunnel has been as a rolling sprinkler. I installed a 32-sprinkler overhead system that attaches with a garden hose to any one of four water spigots. It takes only a minute to roll the house onto any one of the four plots, hook up the hose, and water a thirsty crop. It takes two hours to provide an inch of water, after which the house can be rolled back to its starting position.
We still have to work out the best planting dates and rotations for this four-plot system on our farm. We envision that the tunnel will be tied down and zipped up securely for a winter crop of greens. Then in the spring it will be rolled over a bed of strawberries. When the strawberries are well underway, the house will be rolled onto a summer crop, such as peppers and melons. Then in October the house will be rolled onto a fall crop of kale, Asian greens, lettuce and spinach. One plot will always be in cover crops, such as peas, vetch and oats. At the end of our first growing season with this high tunnel, we were amazed at how much produce we harvested from these four 48-foot plots. The high tunnel hadn’t paid for itself in one year, and we didn’t expect it to. But it is well on it’s way.
About the author: Phil Norris lives in East Blue Hill, Maine, and with his wife, Deborah, operates Clayfield Farm. He can be reached at www.clayfieldfarm.net. This article is adapted from one that ran in the November 2014 issue of Growing for Market, https://www.growingformarket.com.