After years of tinkering with existing low tunnel designs and making various modifications, we've developed a low tunnel custom designed for use with both recycled auto tire and corrugated sheet metal beds. The low tunnel will provide the gardener with a versatile, user-friendly means of protecting and enhancing the growth of crops growing in permanent raised beds.
The basic components of this low tunnel are hoops made from PVC pipe, a polyethylene greenhouse film cover and hold-down straps made from 1/4-inch rope. The tunnel cover is held in place using rope straps located between each hoop. The straps enable the user to raise and lower the cover without unfastening or fastening the cover every time the tunnel is opened or closed.
Begin tunnel construction by fabricating hoop receptacles from 1-inch schedule 40 PVC pipe. When attached to the side of beds, the receptacles hold the hoops in an upright position.
If you intend to cover an auto tire bed, cut the pipe into 6-inch pieces. You will need to prepare 22 pieces to construct a 30-foot-long tunnel. Drill a 7/16-inch-diameter hole completely through the side of each piece of pipe 2 inches from one end. Attach the receptacles to the bed sides using 2-inch wood screws. Locate one screw in the middle of the pipe and the other 1 inch above the base of the pipe. The use of two screws will keep the pipe from rotating. When attached, the top of the pipe should be no higher than the side of the bed, and the drilled holes should run parallel to the side of the bed. Space the hoop receptacles 3 feet apart starting at the end of the bed.
If you are covering a corrugated sheet metal bed, cut the 1-inch PVC pipe into 4-inch pieces. You will need to prepare 22 pieces to construct a 30-foot-long tunnel. Attach the pipe to the 2-inch by 4-inch wood frame using 2-inch wood screws. Locate one screw in the center of the pipe and locate the other 1 inch from the base of the pipe. When attached, the pipe should be level with the top of the frame. Space the hoop receptacles 3 feet apart starting at the end of the bed.
Fabricate hoops for the tunnel out of 1/2-inch schedule 40 PVC pipe. We recommend cutting a 20-foot joint of pipe into three equal sections measuring 80 inches. An 80-inch hoop will create a tunnel about 2 1/2 feet high when used in conjunction with a 40-inch-wide bed. You will need to prepare 11 hoops for use with a 30-foot-long bed.
We recommend using 6-mil greenhouse film for the tunnel cover. Thinner film will work, but isn't as durable and will need to be replaced more often. Construction grade plastic film may be used, but will need to be replaced annually due to sunlight degradation. Porous floating row covers may be used, but only the heavier weight covers will provide any long-term serviceability due to the wear and tear on the fabric that occurs with repeated opening and closing of the tunnel.
The tunnel cover will need to be wide enough and long enough to hang over the sides and the ends of the tunnel. When using 80-inch-long hoops on a 30-foot-long bed, you will need to prepare a piece of film 8 feet wide and 38 feet long.
Proceed with the tunnel assembly by installing the hoops. Insert one end of the hoop into a receptacle and the other end of the hoop into the receptacle directly opposite on the other side of the bed. When installed correctly, the hoop ends should rest against the screws used to secure the receptacles to the sides of the beds.
To prevent excess bending of the end hoops when the cover is applied, use a piece of rope to connect the top of the hoops to the ends of the beds. If you're covering a rubber lumber bed, fasten the rope to an eye screw inserted into the rubber at each end of the bed. If you're covering a corrugated sheet metal bed, tie the rope to the sucker rod stakes at each end of the bed.
Next, install the tunnel cover. Do this on a calm day to make the job easier. Make sure the cover is centered over the hoops. Gather the cover at one end of the tunnel and twist it into a tight spiral. If you're covering a corrugated sheet metal bed, tie off the twisted end of the cover with a piece of rope and secure it to the middle two sucker rod stakes at the end of the bed using the rope. Repeat the process on the other end of the tunnel, pulling the cover towards you prior to securing to the stakes. Stretching the cover removes excess slack in the material.
With one exception, the process is the same if you're installing a tunnel on a rubber lumber bed. We use a 1-foot-long U-shaped stake made from a 3/8-inch-diameter piece of rebar to secure the tied off ends to the soil at both ends of the bed.
Next, make the hold-down straps. Select 1/4-inch-diameter rope to use for the straps. To reduce the amount of abrasion that occurs when the cover is raised and lowered, choose a rope that is soft to the touch. We've had good results using nylon braided rope. Two hundred feet of rope is more than enough to make the hold-down straps for a 30-foot-long tunnel, reinforce the end hoops and tie off both ends of the cover. Prepare the straps by cutting the rope into 16-foot pieces. You will need one strap for each hoop.
If you're covering a corrugated metal bed, start at one end of the bed and thread a piece of rope half-way through the opening between the sucker rod stake and the bed wall directly beneath a hoop receptacle. Throw both free ends over the cover to the other side in the direction of the opposite receptacle. One half of the rope should be positioned on one side of the hoop and the other half on the other side of the hoop.
Tie a loop in one end of the rope and thread the other end behind the sucker rod stake beneath the receptacle and through the loop. Pull down on the free end to cinch up the hold-down strap. Tie off the free end using a slippery half hitch knot. Repeat the process when installing remaining straps.
A restraining strap may or may not be needed for each end hoop, depending on the amount of tension desired. If you install a hold-down strap on the end hoops, an 8-foot piece of rope will suffice. Position the strap on the side of the hoop towards the center of the bed.
If you're covering a rubber lumber bed, the process of installing the hold-down straps is similar, with one exception. The rope should be threaded through the hole drilled in each receptacle.
Cinching up the straps forces the cover against the hoop. Adjust the tension so the cover can slide up and down on the hoops with some resistance. The cover should be drawn down several inches below the hoops. Adjust the strap too tight and you won't be able to raise or lower the cover; too loose and the cover will flop in the wind. The hold-down straps will have a tendency to loosen over time. If you need to cinch up a strap, release the knot, adjust the tension and retie the knot.
During the spring and fall, it is often necessary to ventilate tunnels on sunny days and close them when cold or rainy weather threatens. Because of the tension exerted by the hold-down straps, the cover will tend to remain in any desired position. Over time, however, the cover will slide down the hoops. One method of ensuring the tunnel remains fully ventilated is to tie the cover together with rope or with a bungee cord when the cover is bunched together at the top of the tunnel.
A more flexible method of ventilating the tunnel employs the use of pins inserted into holes drilled into the hoops at desired locations. The pins prevent the cover from sliding down the hoops. This technique enables the gardener to regulate the amount of ventilation on both sides of the tunnel independently of one another.
When the tunnel is no longer needed, the cover can be nested to one side of the bed. Depending on the crops grown, the hoops and straps can remain in place or be removed from the bed. Leaving the tunnel components on site year round is less work, but there is a downside – both the polyethylene cover and PVC pipe are susceptible to sunlight degradation, which will shorten their service life.