Extending the growing season is one of the most effective ways to increase reliability and productivity in a home vegetable garden. Three simple structures—low tunnels, cold frames, and caterpillar tunnels—can protect cool-season crops from frost, freezing temperatures, wind, and heavy rain. Each structure has its own strengths, ideal climate range, and best crop uses.

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Below is a detailed guide to understanding when and why to use each one.

Low Tunnels

A low tunnel is a series of hoops (PVC, metal, or fiberglass) covered with row cover fabric or greenhouse plastic, usually about 18–30 inches tall.

Best Uses

Short-term frost protection
Fall and early spring season extension
Winter protection in mild climates (Zones 7–9) or in colder climates with double covering

Best Temperatures

Fabric row cover: protects 4–8°F below outdoor temps
Clear plastic: protects 8–12°F below outdoor temps
Plastic + inner row cover: protects 12–20°F below outdoor temps (used in harsh climates like Zone 5)

Best Crops

Lettuce
Spinach
Arugula
Asian greens (tatsoi, mizuna, bok choy)
Radishes
Carrots (as winter storage under mulch)
Beets
Kale (in Zones 6–8)

Best Climates

Zones 6–8: Ideal for overwintering hardy greens with a single layer of protection
Zone 5: Very effective only with a second inner layer; used mostly for holding crops through winter

Strengths

Inexpensive
Quick to install
Great for early planting
Easy to vent on sunny days

Limitations

Snow load can collapse hoops
Small space for working
Temps can swing widely on sunny days

Cold Frames

A cold frame is a bottomless wooden or metal box with a transparent, hinged top (polycarbonate lid, old window, or greenhouse panel). It traps solar heat and protects plants from wind.

Best Uses

Starting early spring crops before outdoor soil warms
Hardening off seedlings
Winter protection for small, hardy crops
Overwintering transplants such as lettuce, spinach, scallions, and herbs

Best Temperatures

Maintains temps 10–20°F warmer than outside on sunny days
Night protection varies, but cold frames stay significantly warmer than the open garden
Can overheat above 50°F on sunny winter days—requires venting

Best Crops

Spinach
Lettuce
Mache
Radishes
Scallions
Parsley
Cilantro
Overwintered seedlings (kale, broccoli, cabbage)

Best Climates

Useful in all zones, especially Zones 5–7
In milder climates (Zones 8–9), cold frames function like mini greenhouses for year-round greens

Strengths

Reliable winter germination for hardy crops
Early start for spring planting
Excellent heat retention
Durable and long-lasting

Limitations

More expensive and labor-intensive
Requires frequent venting
Limited space

Caterpillar Tunnels (Mini High Tunnels)

A caterpillar tunnel is a scaled-down greenhouse—tall enough to walk into, usually 4–8 feet high—with hoops covered by greenhouse plastic and tied or clipped down. Ends may be open or closed.

Best Uses

Full winter vegetable production
Large plantings of greens, roots, and herbs
Long-term protection in cold climates, including Zones 4–5
Extending warm-season crops late into fall

Best Temperatures

Protects 15–25°F below outdoor temps
Holds heat far better than low tunnels
Best for climates with deep freezes or heavy winds

Best Crops

Spinach
Kale
Collards
Mustard greens
Asian greens
Lettuce
Carrots
Beets
Turnips
Herbs (parsley, cilantro)
Winter scallions
Even hardy brassicas like early cabbage and overwintering broccoli in Zones 6–8

Best Climates

Zones 4–6: Ideal for reliable winter harvests
Zones 7–8: Enables nearly year-round production
Windy or exposed sites—tunnels stay in place better than low tunnels

Strengths

Large interior space = stable temperatures
Easy to work inside during winter
Excellent for both overwintering and harvesting
Handles snow load better than low tunnels

Limitations

Higher cost
Requires anchoring and tensioning
Needs regular venting in sunny winter weather

Which Should You Choose?

Choose a Low Tunnel if:

You want an inexpensive, quick solution
You only need protection for greens and roots
You garden in Zones 6–9 (or Zone 5 with double covering)

Choose a Cold Frame if:

You want to germinate early spring crops outdoors
You need a place to harden off seedlings
You prefer a fixed, durable structure

Choose a Caterpillar Tunnel if:

You grow lots of winter vegetables
You live in a cold or windy climate
You want winter harvests—not just winter survival

Low Tunnels vs. Cold Frames vs. Caterpillar Tunnels: Comparison Chart

Feature	Low Tunnel	Cold Frame	Caterpillar Tunnel
Definition	Series of hoops covered with row cover or plastic; 18–30″ tall	Bottomless box with hinged transparent lid; solar-heated	Large, walk-in mini greenhouse with hoop frame and plastic cover
Best Use	Frost protection, fall/spring extension, light winter growing	Germinating early crops, overwintering seedlings, winter greens	Full winter production, overwintering, larger plantings
Temperature Protection	4–12°F (single cover) / 12–20°F (double cover)	10–20°F warmer on sunny days	15–25°F and very stable
Best Climates	Zones 6–9 (Zone 5 with double cover)	Zones 5–7 (works in all zones)	Zones 4–8; windy or exposed sites
Best Crops	Lettuce, spinach, arugula, radishes, carrots, beets, Asian greens	Lettuce, spinach, mache, radish, scallions, parsley	Kale, spinach, collards, lettuce, carrots, beets, turnips, overwintered brassicas
Cost	Low	Medium	High
Portability	High — easy to set up and move	Medium — semi-permanent	Medium to low — requires anchoring
Ease of Venting	Easy, but overheats fast on sunny days	Must be vented daily in sun	Roll-up sides allow controlled venting
Snow Load	Poor (collapses easily)	Good (stout with hinged lid)	Very good (arched structure sheds snow)
Wind Resistance	Fair	Good	Excellent (properly anchored)
Space Inside	Small; limited working room	Small but usable	Large; walk-in access
Best For Beginners?	Yes	Yes	Intermediate to advanced
Longevity	1–3 years (hoops + covers may need replacing)	5–20 years depending on materials	5–10 years with periodic re-covering
Season Extension Value	★★★☆☆	★★★★☆	★★★★★