Learn how to make an easy, affordable mini-greenhouse using row covers and low tunnels for season extension and natural pest control.
During any time of year, a visitor to my Zone 6 garden will find at least a couple of low tunnels at work. Supported by wire hoops or arches made from wire fencing, my garden tunnels are covered with row cover and/or plastic when it’s cold to create mini-greenhouses. During winter, they provide protection from wind, hail, and most critters while speeding soil warm-up for summer crops. In summer, I cover the greenhouse tunnels with lightweight row cover or tulle to exclude insect pests such as flea beetles and squash vine borers, and to provide shade for heat-sensitive crops such as lettuce. The cycle begins again when I plant fall-sown onions, such as ‘Olympic’ and ‘Top Keeper,’ or hardy greens inside my multipurpose, portable mini-greenhouses.
Any garden tunnel has three parts — the support hoops or arches, the cover, and the pins, ropes or weights to keep the edges secure. For supports, many gardeners use hoops made from stiff, 9- or 10-gauge wire, or they make their own hoops from inexpensive half- to 1-inch diameter poly pipe (the type used for underground water lines). Pipe hoops are more likely to stay erect if they are slipped over sturdy rebar stakes, or into sleeves made from rigid metal or PVC pipe. They can also be attached to the outside of framed garden beds with metal brackets (visitthe Image Gallery for examples). Tunnels made using fence-wire arches will be more secure if staked down with U-shaped metal pins.
Whether made from wire, plastic pipe or another smooth, non-snagging material, your hoops should be the right length to arch over your garden beds. For 3-foot-wide beds, hoops cut 76 to 80 inches long are best. Hoops are usually spaced 2 to 3 feet apart, so you may need a lot of them. The cheapest way to go is to buy a spool of 9- or 10-gauge wire or poly pipe and cut the hoops yourself. If that doesn’t seem doable, consider hoops available online. Expect to pay $25 for 20, or more for double-wide Super Hoops ($15 for six).
I have used a set of wire hoops for 10 years and will probably use them for 10 years more. In some instances, however, arches made from wire fencing work better than hoops. The main advantage of fencing arches in winter is their ability to withstand heavy loads of ice or snow without collapsing (as hoop-held tunnels are prone to do).
Several seasons back, I received a letter from a reader in Washington, D.C., who was harvesting 6-inch-wide spinach leaves in February under a snow- and ice-covered tunnel supported by an arch of wire fencing covered with plastic, and I’ve been using fencing arches ever since. In spring, I can plant compact varieties of peas under an arch-supported mini-greenhouse, and then take off the cover and let the peas use the arch as a trellis after the weather warms. Later, I do the same thing with pickling cucumbers — first using row cover over a fencing arch to exclude insect pests, and then removing the cover to let in pollinators and allow the cukes to ramble up through the arch that’s now doubling as a trellis.
Two types of fencing dominate my collection of arches — woven-wire fencing with big, 6-inch openings, and stiff welded wire with 2-by-4-inch openings. I use the more flexible woven wire in situations when I know I’ll want to uncover the tunnel and reach in often — to weed carrots or onions, or to harvest leafy greens, for example. I choose the arches made from the smaller fencing if I need to protect newly planted beds from animals or birds. Also, you can staple the plastic to the arch and then the unit is very easy to take on and off as needed. Another option is to construct a rigid, portable mini-greenhouse by connecting electrical conduit pipe to a rectangular foundation frame, as shown in the Image Gallery. This greenhouse tunnel is light and rigid enough to pick up and move.
Several studies have measured the temperature differences inside and outside low tunnels that are covered with various materials. During a winter in Durham, N.H., temperatures under tunnels covered with row cover and a second layer of plastic were more than 20 degrees Fahrenheit warmer than outside the tunnels. Additionally, the top 1 1/2 inches of soil inside the garden tunnels never froze, while the outside soil was completely frozen. Studies from The University of New Hampshire have shown that the combination of row cover topped with sheet plastic is the best way to overwinter onions, broccoli, kale and other hardy crops in the chilly Northeast. (Learn more in Use Low Tunnels to Grow Veggies in Winter: Quick Hoops.)
In climates with milder, sunnier winters, be careful when using sheet plastic to cover beds, with or without a layer of row cover underneath. On a sunny, 65-degree day, a plastic-covered mini-greenhouse can quickly heat up to 130 degrees or more, which can be lethal to most plants. From Zone 6 southward, plants are often safer beneath row cover than they would be tucked in with both row cover and plastic.
I like both perforated and slitted plastic row covers, which are made of thin plastic peppered with holes or long rows of vents. These covers capture daytime heat while allowing excess warmth to escape. They admit a little rain, but hail bounces off of them. At night, the covers deter rabbits and deer. Easy to handle and tolerant of substantial wind, most pieces of perforated or slitted plastic get at least three years of use. Best used during the last three weeks before and two weeks after your last spring frost, a season-specific mini-greenhouse is the best way to capture the warmth of sunny spring days for plants that need it, such as cucumbers, melons or early tomatoes. It’s also a great way to transition veggies that have become accustomed to the filtered light beneath row covers to the open garden. Expect to pay about $12 for a 6-by-20-foot perforated or slitted plastic row cover.
When it comes to using greenhouse tunnels to exclude summer insects such as flea beetles, squash vine borers and squash bugs, garden hoops are all you need to support lightweight row covers. It is usually best to hold even featherweight covers above the foliage to prevent friction with the tender new plants. To save money and material, consider reusing an old sheet or curtain. You may need stakes to keep covers from sagging. To keep the stakes from punching holes in your covers, slit tennis balls and pop them over the tops of the stakes.
Like many tunnelers, I used to struggle to keep my plastic in place during strong winds. I’ve learned that row covers and perforated plastic can be held steady with bricks, pieces of firewood, old bicycle inner tubes filled with sand or water, or sandbags. But even when I buried the edges, winds over 40 mph sometimes ruined tunnels, especially those made with unperforated plastic. Then a friend suggested using a second set of garden hoops over the first, on the outside of the row cover — a great technique that I still use to secure various covers over low tunnels.
But truth be told, lash lines work better than outer hoops when it comes to holding down plastic-covered tunnels. To create lash lines, diagonally cross soft nylon line (such as the kind sold for clothesline) over the hoops, threading the rope through hardware in your bed frame (or running board) on the long sides of the bed. In the permanent beds maintained at the Noble Foundation, a nonprofit agricultural institute in Oklahoma, ropes have been installed across low tunnels between pipe hoops, which are spaced about 3 feet apart. In addition to holding the plastic in place, the ropes make it easy to remove the weights from one edge, roll back the plastic for weeding, and then quickly pull it back into place.
My garden hoops and arches stay in the garden year-round, but I store covers that are not being used in a large storage bin. The less time the covers spend in the sun, the longer they will last, and it’s great to have them ready and waiting when I need a tunnel fast. Not counting my spades and hoes, my tunnel-building gear is among my most valuable garden equipment.
Want to see low tunnels in action: Watch our new video about season extension options, featuring Editor-in-Chief Cheryl Long in MOTHER EARTH NEWS Gardening Video: Season Extension.
Check out our best ideas for DIY cold frames, greenhouses and more with this great resource, Year-Round Gardening: Our Best Plans for Greenhouses, Hoop Houses, Cold Frames and More.