By Gordon Clark
Extension Master Gardener
University of Vermont
Low tunnels are the contemporary version of the “cold boxes,” made out of windows and wood frames, which older generation gardeners used to start plants early in the season. They are also every bit as good for extending your season well into the fall and early winter – even in Vermont.
These tunnels provide other forms of protection for your garden throughout the year. Think of them as the home and community gardener equivalent of the high tunnels we see on many farms.
The simplest low tunnel can be created with wire hoops that can be purchased at any garden supply store. Place the wire hoops to cover a section of your garden and then create the tunnel, using one of several types of coverings, all available at garden stores or online.
The lightest grade of floating row cover can be used to provide early-season protection from insects, such as protecting young eggplants from flea beetles, broccoli from cabbage loopers or melon plants from cucumber beetles. Don’t forget to remove it, however, when the eggplants and melons flower to allow for pollination.
Shade cloths provide protection for tender plants like lettuce against the ravages of our now much hotter summers. Heavier grades of floating row cover or plastic can be used for protection when frosts start showing up in the fall.
Always make sure to secure the sides and ends of the tunnel material. You can use rocks, bricks, boards or garden pins.
In fall and winter with storms and stronger frosts, a sturdier frame and thicker covering could be in order. A very sturdy 10-foot low tunnel frame can be made from a dozen pieces of two-foot rebar and six lengths of either half- inch PVC pipe or half-inch metal electrical conduit. You can purchase all this in any hardware or home supply store.
The metal conduit is stronger and more durable as well as less environmentally problematic, but it requires a special pipe bender to make the hoops. PVC will last several years with care and can be easily bent by hand, even more easily when you fill it first with heated sand.
Simply hammer the pieces of rebar one foot into the ground, on either side of the row at two- foot intervals, with one foot above ground and leaning into the garden at approximately a 30 degree angle. Then shimmy the PVC or metal conduit over opposing pieces of rebar, alternating back and forth, so it touches the ground on both sides.
To make sure the PVC or pipe will fit over the rebar, always test before purchasing. Repeat with the other pieces until you have six large, well-secured hoops.
For the covering you can use either a heavy, frost-grade row cover, or 6 mil clear plastic, which is sold in convenient 10- x 25-foot sheets. Again, be sure to secure at the sides and ends. The row cover fabric breathes, while plastic does not.
As a result, plastic can protect against a lower temperature, but it also heats up much faster in full sun. So if you are using this, you need to be ready to open or vent your low tunnel as needed.
If you’d like to learn how to build a low tunnel, the University of Vermont Extension Master Gardener Program is offering free hands-on workshops on Aug. 14 at the UVM Horticultural Research Farm in South Burlington (http://go.uvm.edu/emg-lowtunnel-aug14) and on Aug. 16 at the Yellow Barn Community Garden in Arlington (http://go.uvm.edu/emg-lowtunnel-aug16).