Low tunnel hoops are invaluable assets for any vegetable gardener looking to significantly extend their harvest and mitigate damage from pests. These structures function as portable, miniature greenhouses, providing essential protection against adverse weather conditions and various garden nuisances. Constructed from readily available materials like PVC conduit or heavy-gauge wire, and covered with specialized lightweight fabrics, low tunnels are a quick and simple solution to maximize production in your garden year-round.
Understanding Low Tunnel Hoops
A low tunnel hoop structure is composed of two primary elements: hoops (the structural, arched framework) and a cover (the protective layer). Strategically deployed, these covers allow gardeners to manipulate the microclimate around their plants. Used across all four seasons, they shield crops from unexpected frost, excessive heat, high winds, and a wide array of insect and mammal pests (including deer, rabbits, cabbage worms, and flea beetles).
Key Benefits of Low Tunnel Tunnels
- Season Extension (Cool/Cold Crops): Tunnels easily add an extra 6 to 8 weeks of harvesting time for cool-season vegetables like arugula, parsley, and winter lettuces in autumn, and allow for early spring planting.
- Early Start (Warm Crops): They help pre-warm the soil and protect tender, heat-loving transplants (e.g., tomatoes, peppers, melons) from unstable late-spring temperatures during their crucial establishment phase.
- Pest Exclusion: When covered with insect netting or mesh, tunnels create a physical barrier against damaging insects and larger garden visitors. Crucially, covers for pest exclusion must be installed before pests become noticeable.
- Succession Planting: In summer, tunnels topped with shadecloth or row covers reduce soil temperature and water evaporation, helping autumn and winter crops germinate and establish during hot, dry periods.
Hoop Materials and Tunnel Sizing
Choosing Hoops
The choice of material determines the tunnel's strength and size, and often depends on the crop height and the expected weather load (e.g., snow or strong winds):
- 9 Gauge Wire: Best for narrow, short tunnels suitable for low-growing crops like salad greens or root vegetables. This wire is easy to bend by hand and cut with wire cutters.
- 1/2 Inch PVC Conduit: A good balance of strength and flexibility, creating tunnels approximately 3 feet high, suitable for taller crops like kale or broccoli. PVC is easy to bend by hand and is typically sold in 10-foot lengths.
- 1/2 Inch Metal Conduit (EMT): Ideal for heavy-duty, permanent tunnels needed for winter harvesting. Metal requires a specialized metal hoop bender to form the U-shape, offering significantly greater strength than PVC.
Tunnel Dimensions
Tunnels should match the width of your garden bed (commonly 3 to 6 feet wide). Hoops should be spaced strategically for adequate support of the cover:
- Wire Hoops: Space approximately 18 to 24 inches apart.
- PVC/Metal Hoops: Space approximately 2 1/2 to 3 feet apart.
Selecting and Securing Covers
Types of Low Tunnel Covers
The cover chosen is dependent on the goal: temperature modification, moisture control, or pest exclusion.
- Polyethylene (Plastic): Used for maximum heat retention (spring/autumn/winter). 6 mil greenhouse-grade, UV-stabilized poly is highly recommended over thin construction poly, as it offers durability and longevity (4-5 years).
- Row Covers (Garden Fleece/Agribon): Lightweight, permeable fabric offering frost protection (varying degrees depending on weight), and allowing rain and light through. Good for mild cold protection and light pest control.
- Insect Mesh/Netting: Fine mesh fabric used specifically for pest exclusion (e.g., cabbage worms, flea beetles). It provides excellent airflow and allows light and water through.
- Shadecloth: Woven material that reduces the intensity of sunlight, used in late spring/summer to delay bolting in leafy greens and aid seed germination by reducing soil temperature.
Installation Tips
Once hoops are secured, covers need to be held taut and firmly secured, especially to prevent damage from wind.
- Secure Hoops: Insert the ends of the hoops several inches deep into the soil. For PVC/Metal, if the soil is compacted, pound 1-foot rebar stakes 5-6 inches into the ground first and slip the conduit over the stakes for added stability.
- Attach Covers: For PVC/Metal conduit, use snap clamps to securely hold the cover to the hoops. For wire hoops or all cover types, the ends and sides of the cover should be weighed down with sandbags, rocks, or lumber to prevent wind lifting.
Year-Round Strategy and Care
Seasonal Use Cases
Low tunnel hoops are versatile enough to be utilized in every season to optimize garden conditions:
Spring 🌷
- Early Planting: Use polyethylene or heavy row cover 4 to 8 weeks early for frost protection of cool-season crops.
- Soil Warming: Cover beds with polyethylene for a week before transplanting heat-lovers (tomatoes, etc.).
- Shade/Bolting Delay: Switch to shadecloth in late spring to keep leafy greens from bolting (going to seed) due to rising temperatures.
Summer ☀️
- Establishment: Use shadecloth or row cover to protect new plantings (successive crops, fall vegetables) from scorching heat and reduced moisture evaporation.
- Pest Control: Use insect mesh to prevent damage from summer pests.
Autumn 🍂
- Frost Protection: As temperatures drop, use row cover or polyethylene to extend the harvest period of cool-season crops.
- Late Pest Control: Continue using insect mesh to protect late-season brassicas (cabbage, broccoli) from pests like cabbage worms.
Winter ❄️
- Deep Cold Protection: Use a combination of a layer of row cover (insulation) topped with a layer of heavy-duty polyethylene (sealing) for maximum protection of hardy winter vegetables (e.g., kale, tatsoi, leeks).
Essential Low Tunnel Care (Watering and Venting)
When using non-permeable covers (like polyethylene), active management is required:
- Watering: Covers like insect mesh and row covers allow rain through. Polyethylene, however, does not. Monitor soil moisture regularly under poly covers and water as needed. On mild days, you can temporarily remove the poly cover to allow natural rainfall.
- Venting: This is critical to regulate temperature and prevent excessive humidity, which can promote fungal diseases. On mild or sunny days, vent by lifting the ends of the tunnel cover. On very warm days, pull the entire cover to the side. Always replace the covers by late afternoon to trap heat for the night.
Frequently Asked Questions About Tunnels
What is the main difference between using PVC and Metal Conduit for hoops?
PVC Conduit is lighter, more flexible, and can be bent by hand, making it ideal for quick, temporary, or seasonal tunnels. Metal Conduit (EMT) is much stronger, requiring a special bender, but it offers superior durability and can handle heavy loads (like snow or high winds), making it the choice for serious, long-term winter growing structures.
Why is UV-stabilized polyethylene recommended over construction plastic?
Construction-grade plastic is not treated to withstand solar radiation. It degrades quickly (often within weeks or months), becoming brittle and tearing. UV-stabilized, 6 mil greenhouse-grade polyethylene is designed specifically for agricultural use; it resists degradation from sunlight, maintaining its integrity and light transmission quality for several years, reducing plastic waste.
How often must I vent my low tunnels?
Venting frequency depends heavily on the weather and the cover type. On sunny days, even if the ambient air temperature is cool, the temperature inside a polyethylene-covered tunnel can rise rapidly (the "greenhouse effect"). You should vent daily when sunny, lifting the ends to allow for airflow. Failure to vent can cook the plants or cause excessive humidity, leading to disease.