What Is a Low Tunnel? Structure, Season Extension and Ventilation
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What is a low tunnel? It is a temporary, low structure made of hoops covered with plastic sheeting or fabric, set over a single row of plants. The hoops hold the cover above the crop, and the enclosed air warms under sunlight. Low tunnels are used to start cool-season crops earlier in spring and to keep them growing later into fall. Oregon State University Extension describes them as temporary structures designed to cover a single row of plants. This page explains how a low tunnel is built, why it extends the season, what it protects against, and why the sides must be managed.
Quick answer
- A low tunnel is a temporary structure of plastic sheeting or fabric draped over hoops to cover one row of plants.
- Oregon State University Extension says low tunnels protect crops from rain and extend the growing season.
- Iowa State University Extension says many vegetables can be planted earlier in spring or grown later into fall with protection from low tunnels.
- Hoops are typically spaced about every 4 to 5 feet along the row, according to Oregon State, so the cover is supported.
What a low tunnel is
A low tunnel is a simple, low cover with two parts: a frame and a covering. The frame is made of hoops that arch over the row. The covering is a sheet of plastic or a layer of fabric draped over the hoops. Oregon State describes the structure as made of plastic sheeting draped over hoops, designed to cover a single row of plants. The structure is temporary, so it can be taken down at the end of a season or moved when a crop is finished.
The single-row design sets low tunnels apart from larger structures. Oregon State lists low tunnels alongside other season extenders, and it treats them as the smaller option. A low tunnel covers a row, not an entire bed or a building.
How a low tunnel extends the season
A low tunnel works by trapping warm air close to the plants. Sunlight passes through the plastic and warms the air and soil underneath. The cover slows the loss of that heat at night and reduces the cooling effect of wind and rain. Oregon State states that low tunnels protect crops from rain and extend the growing season. Iowa State University Extension adds that many types of vegetables can be planted earlier in the spring or grown later into the fall with this protection.
The effect is most useful for crops that tolerate cool weather. Iowa State gives an example that some crops, such as spinach or cilantro, can be reliably carried over through the entire winter with protection from a poly low tunnel. The sources do not claim that every crop will survive under a low tunnel in every climate, so a gardener should match the crop to the local frost dates and the expected cold. Both sources frame the structure as a tool for timing, so the same tunnel can serve an early spring planting or a late fall planting, depending on the weather.
Parts of a low tunnel
The frame is the first decision. Oregon State recommends placing hoops every 4 to 5 feet down the bed or row to provide adequate support for the plastic. The hoops need to be anchored deeply enough to remain in place. Oregon State also advises installing the anchoring object deeply into the ground to provide stability and prevent collapse, which matters where wind is strong.
The covering is the second decision. Oregon State describes plastic sheeting as the cover for a low tunnel. Iowa State refers to a poly low tunnel in its winter example. The sources checked for this article do not compare cover materials in detail, so a gardener should check product specifications before buying.
Why ventilation matters
A low tunnel traps heat, so it can become too warm on a sunny day. Oregon State notes that a certain design feature can reduce the need to manually ventilate the tunnel at higher temperatures, which indicates that ventilation is a routine task. Venting is the counterpart of the heat trap: opening the sides lets excess warmth out, while closing them holds warmth in during cool periods.
This is the main tradeoff. The structure that protects a crop from cold also raises its temperature when the sun is strong. A gardener who opens the sides too late may scorch young leaves. A gardener who keeps the sides closed through a warm day may cause the plants to overheat. The sources describe the need to manage the sides, but they do not give a single temperature at which every crop must be vented.
Crops that suit a low tunnel
Iowa State’s examples of cool-season crops include spinach and cilantro. The sources emphasize that not every crop is suitable. Iowa State notes that not all crops can handle being frozen without tissue damage, so the choice of crop should be made with that limit in mind.
For more on the vegetables mentioned here, see how to grow sugar snap peas and how to grow dill from seed.
How we chose
The sources were chosen from university extension websites that describe low tunnels and season extension. Two pages were opened and checked for HTTP 200 on 2026-10-27. The Oregon State University Extension publication was chosen for its definition, hoop spacing and ventilation notes. The Iowa State University Extension page was chosen for its description of seasons and crops. Retail product pages were not used for any claim in this article.
What we could not verify
The two extension pages describe low tunnels in general terms. They do not give a single temperature for venting, a fixed list of cover materials for every climate, or a measured temperature gain under a given cover.
What the sources do not say
The sources do not state how long a plastic cover will last, and they do not give a dollar figure for building a low tunnel in a home garden. They also do not provide a complete crop list for every region. Readers should check local extension guidance and product specifications before building a structure.
Frequently asked questions
Is a low tunnel the same as a high tunnel?
No. Oregon State describes low tunnels as temporary structures covering a single row. High tunnels are larger structures, and the sources checked here treat low tunnels as the smaller option.
How far apart should the hoops be?
The spacing given is about 4 to 5 feet between hoops, which is the guidance for holding the plastic up along the row.
What does a low tunnel protect against?
According to Oregon State, the cover shelters the row from rain, and the structure is used to lengthen the picking window at either end of the year.
Why should the sides be opened on warm days?
A low tunnel traps heat, so it can overheat plants in sunlight. Oregon State notes that ventilation is part of managing a tunnel at higher temperatures.
Can a low tunnel carry crops through winter?
Iowa State University Extension says some crops, such as spinach or cilantro, can be reliably carried over through the entire winter with protection from a poly low tunnel.
Sources
- Oregon State University Extension, EM 9333, “Low tunnels for season extension in Oregon: Design, construction and costs”: https://extension.oregonstate.edu/catalog/em-9333-low-tunnels-season-extension-oregon-design-construction-costs (opened 2026-10-27)
- Iowa State University Extension and Outreach, Yard and Garden, “Using Low Tunnels to Extend the Season in Your Vegetable Garden”: https://yardandgarden.extension.iastate.edu/how-to/using-low-tunnels-extend-season-your-vegetable-garden (opened 2026-10-27)
