Garden Hoops: Low Tunnels for Frost and Insect Protection
How to Protect Plants from Freezing Without Investing in a Greenhouse
When you’re itching to get your hands dirty, garden hoops can extend the season enough to avoid disaster.
Midway between full-sized greenhouses and the gamble of throwing seeds into cold soil and hoping the last frost has already passed sits an affordable option. It’s easy to do, takes advantage of existing setups, and provides natural pest control. Materials can be used year after year and food production pays for supplies many times over.
Low tunnels incorporate the same concepts of high tunnels except they’re … well … lower. A high tunnel is a long hoop house with enough space for walking and digging. Some high tunnels can accommodate tilling equipment and small tractors. Low tunnels protect smaller raised beds and strips of ground within gardens. Hoops arch over short crops like lettuce, and either plastic or fabric lay on the framework. Also called grow tunnels, they provide greenhouse-type benefits but in forms that can be disassembled and stored later in the season.
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Wendy Baroli is co-founder and farmer at Girlfarm, a sustainable agricultural establishment within the Sierra Nevada high plains desert. Recently she founded the nonprofit Polygrarian Institute to help train a new generation of farmers to take over when the aging population retires. She uses high, low, and caterpillar tunnels for food production and teaches young farmers to do it as well.
“Here at the Polygrarian Institute at the Flying GF Campus,” says Wendy, “we know that high tunnels, low tunnels, and caterpillar tunnels are the way not only to extend seasons but prevent damage from harsh desiccating winds, early insect invasion, and a huge reduction in water use.”
Four 30ft by 70ft high tunnels assist in the majority of her crops. At 5,400 ft elevation, she would never see a ripened tomato without them. The low tunnels shelter newly planted squash, melons, and cantaloupe; sun scald, diurnal temperature fluctuations, rodents and rabbits are huge problems in the Sierras. Caterpillar tunnels, which are low tunnels within high tunnels, are employed during the coldest part of the season. They add another layer of protection for new seedlings when the large hoop house just isn’t enough.
Gardeners who have no space for high tunnels can easily incorporate the lower garden hoops to achieve a successful harvest.
Why Use Garden Hoops?
Like cold frame gardening, low tunnels extend temperatures to allow planting earlier in the spring. Some gardeners even cultivate through the winter. But because cold-tolerant plants are not cold-resistant, they will get damaged or die if struck by a hard frost (temperatures below 28 degrees.) Frost blankets, depending on quality and materials used, can keep plants four to ten degrees warmer. It also keeps the frost itself from touching the plants, which is more damaging than just the temperatures. One of the most valuable gardening tips to follow is to either strictly adhere to your area’s final frost date or use covers on your crops.
Plastic is a popular gardening tool. Tented over garden beds, it raises the temperature beneath by as much as 50 degrees on a sunny day. It also insulates at night. But plastic should never just be thrown over plants, since it can trap moisture against leaves, causing serious frost damage. Plastic lying across foliage can be worse than no cover at all. Tenting plastic over both soil and foliage provides a warmer pocket of air and keeps that moisture up high.
In the summer, it can get hot. “Full sun” doesn’t mean, “blazing, scorching, blistering July daylight.” It means, “at least six hours of sun in an area that frequently sees cloud cover.” Vegetables cultivated in the desert can wilt and sunburn, or at least have growth stunted, all of which impacts harvests. Plastic is a bad idea during the summer because it turns garden hoops into ovens but the same row cover which acts as frost blanket can cool summer greens while still allowing 80 percent sunlight to shine through.
Organic pest control is a headache in a vegetable garden. Hoops with row cover protect in an incredibly simple way: they keep bugs out. Even smaller mammals like rabbits are deterred by chicken wire bent over curved rods and pinned to the ground. Squash bugs, killed by carbaryl but few other pesticides used legally by home gardeners, fly in from a neighbor’s yard to consume pumpkins…but they won’t get past a secure row cover. By covering the plants and holding material against the soil, gardeners can avoid aphids, blister beetles, hornworm, and other destructive pests.
Farmer Wendy Baroli proudly displays the food she grows in an area where the last seasonal frost is in June. A picture taken in early February displays mizzen, chard, radish, butterhead lettuce, broccoli, and many other crops flourishing within her tunnels…during a month when nighttime temperatures drop into the teens. As nights warm to 28 in late March, she can barely keep up with growth. She assures us, “There is no doubt regardless the climate—the insect control, water conservation, and overall management of the intensive use of space—has been a boon to the modern small farmer.”
How to Make a Low Tunnel
Whether creating tunnels over the ground or raised planters, construction requires only one or two people.
First, obtain material which can arch over the garden bed. PVC rods are inexpensive but may not withstand heavy snowfall. Metal rods require special tools to bend them but afterward can last for decades. Nurseries sell ready-to-use garden hoops and cover. Avoid rods or PVC which bend at a right angle, since corners can wear against cloth or plastic, poking holes after a few wind storms.
Plan out garden rows in straight lines before planting. Arrange a soaker or weeper hose so you don’t have to pull material back every time you water. Now plant the garden. Hoops arch over, inserted several inches into the ground on either side.
For added stability during the winter, run a straight pole down the length of the hoops, attaching to the top of each arch. Keep the pole beneath the hoop so it doesn’t rub on row cover or give the wind an extra surface to grab. Also, consider adding more hoops in snowy or wind-prone areas so each section doesn’t have to hold as much weight.
Now cover your garden hoops. Hold the edge of the plastic or frost blanket against the ground on one end while someone else carefully unrolls the material, allowing it to lie atop the hoops. Be sure the material reaches the ground on all sides before cutting. Now tack it to the ground using rocks, landscape pins, or jugs of water. If your area is wind-prone or you expect snowfall, consider also clipping the material to the hoops themselves.
Protecting raised beds is similar: install hoops overtop, sinking the ends into the soil on each side. Stretch protective material over and pin it against the dirt or the bed’s border.
Weeding or harvesting is simple: Untack the row cover and pull it back in the area you intend to work. If you used plastic during the cold season and now want to incorporate shade, simply roll the plastic up and replace with row cover.
Four to 10 degrees protection isn’t much, so low tunnels aren’t recommended for growing basil unless it’s near the final frost date. Cold temperatures can damage basil well above freezing. And only so many miracles can protect tomatoes if they’re transplanted too soon. But adding a few tweaks to garden hoops can make them warmer.
Water has amazing insulating properties. Those walls surrounding tomato plants prove how just a little water can avoid a lot of damage. If the weather threatens your crops, despite protective material, set full jugs within the tunnels, beside plants. Keep crops well-watered prior to a freeze.
Old-fashioned Christmas lights can add eight to 10 degrees more warmth. Avoid LED lights, since they produce very little heat, and purchase C7 or C9 bulbs if possible. Tiny tree lights can also warm plants, though not as much, but they also don’t burn leaves. Run strands of outdoor lights along the hoops, pinning up high so they don’t touch plants or row cover. Plug the lights in when temperatures dip. Christmas lights add decorative assurance that your vegetables will survive the night.
Purchase a fully inclusive but flimsy grow tunnel, paying $15 to $20 for ten feet of protection. Or cut costs by reusing and repurposing materials.
A visit to the hardware store reveals many items which can create tunnels. Garden centers sell the arches pre-made while PVC lies within the plumbing section and gray electrical conduit sits beside lighting. Plastic electrical conduit is cheaper than PVC, and is more flexible, but the flexibility isn’t the best option when snow falls. Thin plant stakes or supports bend as necessary; if they aren’t smooth enough, wrap sharp edges with tape. Choose supports according to whether you expect snow, wind, heavy rains, or if you grow within a sheltered environment.
For very low crops, like spinach, reuse bent and damaged tomato cages. Use heavy duty wire cutters to clip through the horizontal circles. Then lay the cage out flat. Bend in the opposite direction you just worked, poking the three or four bottom wires into dirt on one side of the spinach while pinning the top edge of the cage to the ground on the other.
Or use cattle panels: new or old. A sixteen-foot length, four feet wide, can cost as low as $16. The four-foot width is perfect to accommodate a four-foot-wide roll of frost blanket. With help from one or two other people, bend the widths toward each other to make a sixteen-foot-long arch. Place this over the row and clip cloth over that. Using cattle panels ensures garden hoops can withstand more weight and can last for many years while PVC may break down in the sunlight.
Frost protection can get expensive, depending on what you buy. The best protection costs the most. But if it’s handled with care it can be reused year after year. Avoid sharp corners. Drape carefully so you don’t snag material.
If you only need a few degrees’ protection, or you want shade, special fabric isn’t necessary. Old, threadbare sheets work just fine. Use white or cream-colored sheets, because they let in more light. Purchase at yard sales or linen supply companies. Unlike row cover, old sheets can be thrown into a washing machine if they become muddy.
Thicker plastic, such as 6 mil, withstands more abuse. But most plastic breaks down in the sun unless it’s expensive UV-buffered greenhouse film. Greenhouses often need re-covering if plastic becomes punctured. Offer to purchase or trade for used greenhouse plastic. Remove plastic as soon as you no longer need it, carefully rolling it up and storing away from the light.
Hold row covers against the ground with bricks, rocks, or landscape pins. Though they’re more expensive than the rocks dug from soil, landscape pins take up less room. And though seed companies sell high-quality pins, purchasing in lots of 100 from online superstores can save a lot of money.
Clip plastic or frost blankets to hoops using metal binder clips, if the hoops are small enough. Binder clips are strong and office supply stores may offer 1,000 for $8. Clothespins also attach material in sheltered areas.
Whether garden hoops are installed large-scale, such as at the Polygrarian Institute, or to cover a small bed of winter kale, they are a cheap and easy way to avoid damage from the heat or cold, wind, or pests.
Do you use garden hoops? What have you grown beneath garden hoops?