Build a Cold Frame This Weekend: Extend Your Season by Months

A cold frame is the most cost-effective season extension tool you can build. For $50-100 in materials and a few hours of work, you get 2-3 extra months of growing season—harvesting lettuce in December, starting seeds in February, hardening off transplants in March.

Commercial cold frames cost $200-500 and are no better than what you can build yourself from reclaimed materials. Old windows, scrap lumber, and basic hardware are all you need.

What Is a Cold Frame?

A cold frame is essentially a miniature greenhouse—a bottomless box with a transparent lid that captures solar heat during the day and insulates plants at night. No electricity, no complex systems, just passive solar gain and basic structure.

Temperature gain: Well-designed cold frames maintain 10-20°F warmer than outside air, protecting plants from light frost and extending the growing season on both ends.

Cold Frame Uses

Fall/winter:

• Grow cold-hardy greens (lettuce, spinach, kale, mâche)
• Protect root crops for extended harvest
• Overwinter tender perennials
• Shelter herbs for fresh winter harvests

Spring:

• Start seeds 4-6 weeks earlier than outdoors
• Harden off greenhouse/indoor transplants
• Protect early crops from late frost

Summer:

• Propagate cuttings
• Protect shade-loving plants from intense sun (use shade cloth on lid)

Location and Sizing

Site selection:

• Full sun (minimum 6 hours direct sun)
• South-facing exposure (maximizes winter sun)
• Sheltered from prevailing winds
• Near water source
• Level ground with good drainage
• Convenient access from house (you’ll use it more)

Size considerations:

Small (2x4 feet):

• Good for beginners
• Easy to build and move
• Sufficient for salad greens for 1-2 people
• Limited spring seed-starting capacity

Medium (3x6 feet):

• Sweet spot for most home gardeners
• Produces significant greens through winter
• Adequate seed-starting space
• Still manageable to build and maintain

Large (4x8 feet or bigger):

• Serious production potential
• Requires substantial materials
• Consider multiple medium frames instead

Height:

• Back (north side): 12-18 inches
• Front (south side): 8-12 inches
• Slope allows rain/snow runoff and maximizes sun exposure

Materials and Tools

Frame materials:

Budget option:

• Scrap 2x lumber or pallets (free-$20)
• Hinges from hardware store ($10-15)
• Handle/lift prop ($5-10)
• Total: $15-45 plus glazing

Standard build:

• New lumber (2x6 or 2x8 for sides, 2x4 for bracing) ($30-50)
• Deck screws ($10)
• Hinges, handle, prop ($20)
• Total: $60-80 plus glazing

Glazing options:

Old windows (best value):

• Free from remodeling projects, salvage yards
• Pre-framed, ready to mount
• Tempered glass is best (won’t shatter)
• Size your frame to match window dimensions

Polycarbonate panels:

• $30-60 depending on size
• Lightweight, nearly unbreakable
• Good insulation value (twin-wall)
• 10+ year lifespan

Greenhouse plastic:

• $10-20
• Shortest lifespan (1-2 years)
• Least expensive option
• Adequate for beginners

Glass panes:

• $40-80
• Best light transmission
• Heavy, requires sturdy frame
• Breakable (safety concern)

Tools needed:

• Circular saw or hand saw
• Drill/driver
• Measuring tape
• Square
• Level
• Pencil

Basic Construction Plans

Simple 3x6 Cold Frame

Materials:

• Back: 2x12 x 6 feet (or two 2x6s)
• Front: 2x6 x 6 feet
• Sides: 2x12 x 3 feet (cut to trapezoid shape)
• Corner braces: 2x2 x 18 inches (4 pieces)
• Lid frame: 2x2 to match window dimensions
• Old window or polycarbonate glazing
• 2 hinges, 1 handle, 1 lid prop
• Deck screws (3 inch)

Steps:

1. Cut side pieces to trapezoid:

   • 12 inches high at back
   • 6 inches high at front
   • 3 feet long
   • Cut slope from back to front

2. Assemble frame:

   • Attach front, back, and sides with deck screws
   • Pre-drill to prevent splitting
   • Use corner braces at each corner for strength
   • Check for square and level

3. Build lid frame:

   • Construct frame to fit window/glazing
   • Should overlap cold frame opening by 1-2 inches
   • Secure glazing to frame

4. Attach lid:

   • Mount hinges to back of cold frame
   • Attach lid to hinges
   • Install prop stick for ventilation
   • Add handle to front of lid for easy opening

5. Finish:

   • Apply exterior wood stain or paint (optional)
   • Caulk gaps for weatherproofing
   • Line interior with landscape fabric to suppress weeds (optional)

No-Build Quick Frame

For immediate use with minimal tools:

1. Stack concrete blocks or bricks to form walls (12 inches high in back, 6 inches in front)
2. Lay old storm window or polycarbonate panel on top
3. Weight or clip down to prevent wind damage
4. Prop open one side for ventilation

Not beautiful, but functional and takes 15 minutes.

Ventilation (Critical!)

Why ventilation matters:

• Cold frames can reach 80-100°F on sunny days, even in winter
• Excessive heat kills plants
• Ventilation prevents overheating and provides air circulation

Ventilation methods:

Manual:

• Prop lid open on warm/sunny days
• Close in evening and on cold days
• Requires daily attention

Automatic vent opener:

• $30-60 device
• Wax-cylinder expands with heat, opens vent
• Closes when temperature drops
• Set-and-forget convenience
• Best investment for busy gardeners

Ventilation schedule:

• Open when temps inside exceed 60-65°F
• Close before temperature drops below 45°F
• On very cold nights (below 20°F), cover with blankets or tarps for extra insulation

Using Your Cold Frame

Fall/Winter Greens

Plant in October:

• Lettuce (cold-hardy varieties like Winter Density, North Pole)
• Spinach
• Mâche (corn salad)
• Claytonia (miner’s lettuce)
• Arugula
• Asian greens (mizuna, tatsoi)

Planting:

• Direct seed or transplant
• Plant densely—harvest individual leaves, not whole plants
• Succession plant every 2-3 weeks through October

Harvesting:

• Begin harvesting when plants reach 4-6 inches
• Pick outer leaves, allow center to continue growing
• Harvest continues through December in most zones (longer in mild climates)

Spring Seed Starting

Timing:

• Start seeds 4-6 weeks before last frost
• Earlier than unprotected outdoor seeding
• Later than indoor seed starting

What to start:

• Cool-season vegetables (lettuce, broccoli, cabbage, peas)
• Hardy annuals (calendula, snapdragons, sweet peas)
• Perennials that need cold stratification

Technique:

• Use seed trays or pots inside cold frame
• Water carefully (overwatering is common mistake)
• Ventilate daily once seeds germinate
• Harden off gradually before transplanting to garden

Hardening Off Transplants

Process:

1. Place greenhouse/indoor-grown transplants in cold frame
2. Keep lid closed first 2-3 days
3. Begin opening lid for increasing periods
4. After 7-10 days, transplants are hardened off and ready for garden
5. Prevents transplant shock and sunscald

Maintenance

Fall setup:

• Clean interior, remove debris
• Check hinges, tighten screws
• Replace damaged glazing
• Apply fresh wood treatment if needed

Winter:

• Brush snow off lid (weight can break glass)
• Monitor temperature, ventilate on sunny days
• Water plants when soil is dry (yes, even in winter)
• Harvest regularly to prevent overcrowding

Spring:

• Transition from greens to warm-season starts
• Clean thoroughly before seed starting
• Check ventilation system

Summer:

• Use for propagating cuttings
• Add shade cloth if using for shade-loving plants
• Or leave empty, open lid

Advanced Techniques

Insulation for extreme cold:

• Line interior walls with foam board insulation
• Adds 5-10°F temperature buffer
• Permanent installation or seasonal

Heating for below-zero protection:

• Strings of outdoor Christmas lights (old incandescent type)
• Generates just enough heat to prevent freezing
• Used only on coldest nights

Succession planting:

• Don’t plant once and forget
• Re-seed every 2-3 weeks for continuous harvest
• As early plantings are harvested, replace with new sowings

Double-glazing:

• Two layers of glazing with air gap
• Significantly improves insulation
• More expensive, more complex to build

Cost-Benefit Analysis

Investment:

• DIY cold frame: $50-150
• Commercial cold frame: $200-500

Returns (estimated):

• Fresh greens October-March: $200-400 grocery store value
• Seed starting supplies saved: $50-100
• Transplant purchases avoided: $100-200
• Total annual value: $350-700

Payback period: One season for DIY build, 1-2 seasons for commercial.

Want to visualize where a cold frame will best fit in your garden layout and plan seasonal crops? Gardenly’s AI design tool  generates garden layouts showing optimal cold frame placement and seasonal planting schedules for year-round production.

October is perfect for cold frame construction—build it now, plant fall greens immediately, and by December you’ll be harvesting fresh salad while neighbors scrape frost off their windshields. One weekend project, months of extended harvest.