Greenhouse and Cold Frame Construction for Food Production

Cold Frames First

Build a cold frame before you build a greenhouse. They cost almost nothing, teach you how plant protection works, and deliver immediate results. Once you understand how temperature and ventilation interact in a cold frame, you'll build a better greenhouse.

A cold frame is just a box — four walls and a transparent lid. The lid admits sunlight and traps heat. The box keeps wind off. That's the entire mechanism.

Building a Cold Frame

Materials for a 4x4 foot frame:

• Four boards of 2x12 lumber (or use salvaged wood — rot resistance matters, so cedar or treated lumber is best)
• Two hinges
• One clear lid: an old storm window, double-pane glass, or 6-mil clear plastic stretched over a frame

Construction:

The classic design cuts the back boards taller than the front so the lid slopes toward the south. This angle sheds rain and maximizes sun angle. Aim for a slope of about 4 inches over 4 feet — the back wall stands 16 inches high, the front wall 12 inches.

What to grow: Cold frames excel for spinach, kale, mache, claytonia, arugula, Asian greens, and Swiss chard. These crops tolerate 25°F when protected. Hardy lettuce varieties handle 28°F. Start seeds 6 weeks before your last frost for transplants.

Ventilation: This is the rule that gets ignored and kills plants. On a sunny 40°F day, the interior of a closed cold frame can hit 90°F within two hours. Crack the lid whenever daytime temperatures exceed 45°F. On clear days above 35°F, you may need to prop it fully open.

Use a maximum-minimum thermometer inside your cold frame for one season. You'll be surprised how hot it gets and how cold it doesn't get.

Building a Hoop Greenhouse

A hoop greenhouse — also called a high tunnel or caterpillar tunnel — is the most cost-effective structure for season extension. The design is simple: bent pipes or conduit create arched ribs, plastic film covers them, and end walls close it off. A 12x20 foot tunnel can be built for $300-500 in new materials or less with salvage.

Materials for a 12x20 Foot Hoop House

The ribs:

• 10-gauge galvanized wire mesh makes cheap, durable hoops for tunnels under 6 feet tall
• For walk-in height, use 1-inch diameter EMT conduit (electrical metallic tubing) — available at any hardware store
• Rebar driven into the ground as receiving stakes holds the conduit ends

The skin:

• 6-mil clear polyethylene rated for greenhouse use (UV stabilized). Clear maximizes light; white diffuses it (better in high-UV climates)
• For colder climates, double-poly with an air gap between layers cuts heat loss by 40%

End walls:

• Plywood or extra poly film
• At minimum one end needs a functional door

Site Selection

South-facing orientation. The long axis runs east-west so both sides get sun throughout the day. Avoid the north side of any building, fence, or tree line. Wind protection matters — a windbreak 50-100 feet north cuts heating demand significantly. Level ground simplifies construction and drainage.

Foundation Options

Hoops don't need a permanent foundation. Three options:

Ground stakes: Drive rebar 18 inches into the soil at each hoop location, 4 feet apart down each side. Slip EMT conduit over each rebar pair and bend the arch. Fast, cheap, removable.

Baseboard frame: Build a rectangular frame of 4x4 lumber anchored with stakes. Attach rib ends to the frame. More stable in high wind. Allows you to seal the plastic to wood rather than soil.

Buried baseboard: The most permanent option — set 4x6 boards in shallow trenches so the top edge is at ground level. Bury the plastic's base edge in a trench along the outside of the board and backfill. Extremely weathertight.

Construction Sequence

Ventilation — the Critical Skill

A closed greenhouse on a sunny day goes from 40°F to 90°F in under an hour. Tomatoes stop setting fruit above 85°F. Most greens bolt or wilt above 75°F. If you build a greenhouse and don't ventilate it properly, you'll kill more plants than frost ever would.

Passive ventilation targets:

• Open vents or end doors when interior temperature exceeds 65°F on overcast days
• Open fully when temperatures exceed 75°F on any day
• On hot sunny days, shade cloth over the plastic cuts temperature by 10-15°F
• Roll-up sides are the simplest passive ventilation system

Temperature goals by crop: | Crop | Ideal Day Temp | Night Low Tolerance | |------|---------------|---------------------| | Lettuce/greens | 45-65°F | 25°F | | Tomatoes | 65-80°F | 50°F | | Cucumbers | 70-85°F | 55°F | | Peppers | 70-85°F | 55°F | | Basil | 65-75°F | 55°F |

Heating Options for Year-Round Production

An unheated greenhouse in zone 6 stays 10-15°F warmer than outside on most nights. That gets you through most winters with cold-hardy crops. For warm-season crops year-round, you need supplemental heat.

Low-tech heating:

• Water thermal mass: black-painted 55-gallon drums filled with water absorb daytime heat and release it slowly at night. Three drums in a 12x20 tunnel add meaningful buffering.
• Insulation: double-poly with air gap, bubble wrap on north walls, foam board on end walls
• Row covers inside the tunnel: two layers of frost protection inside a greenhouse multiplies the effect

Active heating for cold climates:

• Propane torpedo heater (60,000 BTU): heats a 12x20 tunnel in extreme cold but burns through fuel fast
• Thermostatically controlled electric ceramic heater: expensive to run but precise
• Wood stove: effective but requires constant monitoring and chimney penetration through plastic

For emergency food production, prioritize cold-tolerant crops and passive systems. Reserve fuel-burning heaters for protecting irreplaceable starts.

Managing Your Greenhouse Through the Season

Starting Seeds (6-8 Weeks Before Last Frost)

The greenhouse earns its keep in late winter when you're starting tomatoes, peppers, and eggplant under cover while the ground outside is still frozen. Maintain 65-75°F for germination. Use a seedling heat mat under trays — soil temperature matters more than air temperature for germination.

Hardening Off

Plants started inside a warm greenhouse are soft. Before transplanting outdoors, expose them to outdoor conditions gradually over 7-10 days. Start with one hour of outdoor exposure, increase daily. Skip this step and your tomatoes will sulk for two weeks after transplant.

Pest Management

Aphids, whiteflies, and spider mites thrive in the warm, protected environment. Inspect undersides of leaves weekly. Catch populations early — a heavy infestation in a greenhouse spreads faster than in the field.

• Aphids: Yellow sticky traps catch flying adults. Blast colonies off with water. Introduce ladybugs if available.
• Whiteflies: Yellow sticky traps. Insecticidal soap spray (2 tablespoons per gallon of water) on leaf undersides.
• Spider mites: Increase humidity. They hate wet conditions. Spray water on leaves daily when infested.

Winter Greens Calendar

With a simple unheated or minimally heated greenhouse in zones 5-7:

| Month | What to Grow | |-------|-------------| | September | Plant spinach, mache, claytonia, kale starts | | October | Harvest begins, plant more lettuce | | November-January | Harvest cold-hardy greens, growth slows | | February | Supplemental light or warming restarts growth | | March | Start tomato/pepper seeds; greens production accelerates | | April | Transplant warm-season crops in; greens harvest continues outside |

This calendar assumes USDA zone 5-6. Adjust 4-6 weeks earlier or later for your zone.

The Cold Frame + Greenhouse System

The most productive approach combines both structures. The cold frames handle hardy greens year-round with minimal management. The greenhouse handles warm-season crops, seedling starts, and provides a buffer for the most sensitive plants. Between them, a family of four can grow meaningful food in almost any US climate without electricity or purchased fuel — though heat-loving crops will require some supplemental warmth in northern zones.

Start with one 4x4 cold frame this year. Understand how temperature management works. Then build your first hoop tunnel. The tools and knowledge transfer directly.