What Exactly Is the Difference Between a Cold Frame and a Hotbed?
Think of a cold frame as a sun-powered blanket: a bottomless box with a transparent lid that traps solar heat and shelters plants from wind, snow, and hard frosts. A hotbed is the same box, except it has an internal heat source—fresh manure in traditional versions, soil warming cables in modern ones—so temperatures stay 10-20 °F higher. Both are miniature greenhouses you can tuck along a south-facing wall, on a patio, or even atop an unused raised bed.
Why Every Home Gardener Needs One
The USDA Plant Hardiness Zones tell only part of the story. Even zone-6 gardeners can pick spinach through Christmas, germinate tomatoes before spring, and overwinter hardy herbs when they harness a simple frame or hotbed. The payoffs are:
- 3–6 weeks of extra harvest in fall and the same head start in spring
- Lower grocery bills and continuous salads during short, expensive market days
- Seed-starting space that does not crowd windowsills
- No heating bills—you rely on the sun, a few boards, and, for hotbeds, a single, low-watt cable
Choosing the Perfect Spot
Orientation and Slope
Face the lid due south if possible. In tight yards, southeast works, but skip north-facing walls—they never catch enough winter light. Pitch the site 5–10° so cold air drains OUT, not pools around your seedlings.
Shelter from Wind
A brick wall, fence, or thick evergreen hedge on the north and west sides blocks the icy air that steals heat at night.
Ease of Access
Site the frame within a 20-second walk from your kitchen door. During winter storms you will not trek across snow for a quick harvest.
Blueprint: Build a Cedar Cold Frame in One Afternoon
Materials (all available at big-box stores)
- Four cedar 1x8 boards (actual ¾ x 7¼ in), each 8 ft long
- One cedar 1x6 board, 6 ft long (for the 6-in-tall front)
- Two heavy-duty hinges with screws
- One clear twin-wall polycarbonate sheet, 24 x 48 in, or an old storm window
- Outdoor wood glue and 2-in galvanized deck screws
- Optional: scrap brick or pavers for leveling
Cuts
From the 1x8s cut three pieces 48 in long (back wall) and two pieces 24 in long (both side walls). Rip one 48-in board at a 15-degree slope so the back sits higher than the front, giving the lid a forward tilt.
Assembly
- Lay three back-wall boards vertically to make a 22-in–tall rear panel. Screw through the sides, glue, and square.
- Attach the 6-in front panel. The frame now naturally slopes for rain runoff and maximum light capture.
- Hinge the polycarbonate sheet to the back edge so it opens like a trunk lid, not sideways.
- Bury the lower 2 in of the frame into soil or rest on bricks to anchor against winter gales.
Total cost: 45–65 USD if you purchase new cedar; close to zero if you repurpose an old window and scrap pallet wood.
Starter Hotbed: Add Gentle Heat With an Electric Cable
Frame
Use the same cedar box above. Before filling, nestle a 50-foot, 120-watt soil warming cable in looping S-shapes directly on the ground. Cover with 1 in of damp sand and a ¾-in mesh welded wire to protect the cable.
Power
Plug the cable into a digital thermostat set 5 °F above the lowest nightly air reading. Energy draw is comparable to one old-style light bulb, so the cost per month ranges from 2–4 USD even in northern states.
Optional Hotbed Heaters
- Fresh horse manure—pack 12 in deep, cap with 2 in of rich compost, then plant directly—acts like a warm electric blanket for 4–6 weeks
- 12–15 gal black water jugs filled on sunny days, placed around root zones, release heat at night
How to Manage Temperature Anywhere Below 45 °F
Overheating kills seedlings faster than frost. A digital soil probe thermometer is worth its 10-second nightly check. Open the lid an inch when air inside the frame tops 75 °F and close it before temps fall below 50 °F. Adjust the angle of the lid slightly by wedging a stick along the back wall to create a one-inch crack—perfect ventilation on sunny winter afternoons.
Best Veggies for Cold Frames and Hotbeds
Fall & Winter (plant 8–12 weeks before first hard frost)
- Winter Density romaine lettuce
- Bloomsdale Long Standing spinach
- Russian kale (any variety)
- Claytonia and winter purslane for salad variety
- Corn mache (lamb's lettuce)
- Hardy round radishes, like Cherriette
Late Winter to Early Spring Seed Starts (5–8 weeks before last spring frost)
- German Lunchbox or Sungold tomatoes
- Jalafuego jalapenos, King of the North bell peppers
- Touchon carrots (sown thickly in mini-bands for sweetest roots)
- Broccoli, any bolt-resistant variety such as Belstar
- Cilantro—slow-bolting Santo for repeated harvests
Seeding Secrets Beneath the Polycarbonate Lid
Direct sow spinach every Thursday in September for rolling harvests. Pack soil loosely so it holds water but warms quickly. Sprinkle seeds, tamp gently, mist, then lay lightweight row cover directly on soil untacked; the fabric keeps seeds from drying out while still admitting light.
From Seed to Harvest: Step by Step in the Cold Frame
Week 0–2: Germination
Keep the lid closed unless daytime temps exceed 60 °F. Mist morning and evening. Expect spinach sprouts in 5–7 days in 55 °F soil.
Week 3–5: Thin & Top-Dress
Clip weak seedlings at soil level rather than pulling to avoid disturbing roots. Side-dress with ¼ inch vermicompost around their stems—cold soils love microbial life.
Week 6 onward: Continuous Cut-and-Come-Again
Pinch outer spinach leaves when they reach 3 in. Leave the crown and center leaves for regrowth. A two-foot-by-four-foot frame feeds two salad lovers all winter.
Pest Patrol—Yes, Even in Winter
Slugs climb out of cold soil and chew seedlings on warmer nights. A saucer of cheap beer buried flush to the frame edge lures and drowns them. Check framing gaps for mouse chewing; stuff coarse steel wool into any 1/4-inch gap.
Spring Into Summer: Convert the Same Box
By late April, remove the lid entirely. Lean it against a tree for windbreak use or stack it flat as a sturdy planting bench. Plant the frame with Paris Island romaine, basil starts, and dwarf cucumbers (Spacemaster 80 works in the 12-in depth) to give the summer garden a head start.
Common Cold-Frame Failures & Instant Fixes
Rotting stems from condensation
Open the frame 15 minutes at noon daily until the glass stops sweating.
Frozen cabbage heads
You transplanted too late. Pull immature heads now for tender baby greens, and re-seed inside 8 weeks earlier next year.
Polycarbonate cracking from wind
Replace with scrap lexan or double-pane window glass for the same insulation value but higher impact resistance.
Design Upgrade: Insulated Cold Frame for Polar Zones
Swap cedar for 2-in thick styrofoam sheets sandwiched between plywood skins and paint the exterior dark. Add a reflex-reflective insulation panel (emergency survival blanket stapled to a drywall scrap) across the inside back wall to bounce sun onto the soil. Users in northern Minnesota report the interior stays 25 °F warmer than outside on a calm January night.
Troubleshooting Electric Hotbed Cables
- If the thermostat clicks but soil stays cold, check that probes are buried—not just laid on the surface—so readings are accurate.
- If a GFCI outlet trips, dry out the box, seal all joints with outdoor caulking, or upgrade to IP67 foam-encased thermostat probes.
Seven Quick Cold Frame Hacks From Old Stuff Around the House
- Old shower door → perfect 1-inch-thick tempered glass lid
- Discarded stadium seat cushions → instant kneeling pad and seed-starting surface
- Broken patio umbrella → use the aluminum ribs plus umbrella fabric as a frost blanket on extreme nights
- Coffee-can chimney starter → place over inside corner candle for extra 5 °F rise on a one-nighter—remove before you sleep, of course
- Painted coffee cans filled with water → solar mass that stores and releases heat better than bricks
- Collapsed wire dog crate → frame trellis once the lid comes off
- Plastic storage bin flips upside-down → mixes as a micro greenhouse over young pepper transplants in March
Seasonal Calendar: North American Example
(Dates shift by 2–3 weeks for every 200 miles of latitude.)
- Aug 15: Sow spinach mix directly
- Sep 15: Plant claytonia & mache in the same frame
- Nov 1: Add lightweight cover inside the closed lid for insulation; scatter mouse trap bait
- Jan 15: Start tomato and pepper seeds in 72-cell tray placed on hotbed cable
- Feb 15: Thin and harden tomato seedlings during warm days when frame hits 60 °F
- Mar 15: Transplant lettuces to open ground, replace lids for 30-day early spinach
FAQ: Do You Need a Permit?
Small detached cold frames and hotbeds under 120 sq ft generally fall under storage-shed exemption in most US municipalities. In snowy regions, be sure your lid slopes at 30° or steeper so snow slides—otherwise the city inspector may cite a “structure drift” violation.
Seasons End: Winter Shutdown Checklist
- Harvest final lettuce leaves, then use a dry bamboo skewer to mark “moved” new seedlings you intend to transplant early spring.
- Scrape out slugs and night crawlers, toss to chickens or compost.
- Brush out loose soil, spray box interior with diluted hydrogen peroxide solution (1 part 3% peroxide to 9 parts water) to kill overwintering mold.
- Flip the casement window upside-down under a tarp to preserve the hinges.
- Label and untangle the electric cable; roll in a loose circle to avoid kinks that provoke shorts.
Wrap-Up: From One Box, Four Seasons of Food
Whether you screw together a box of cedar scraps or plug in a cable beneath an old storm window, cold frames and hotbeds deliver runway-length extensions to the incredibly short conventional gardening calendar. Build one Saturday morning with a saw, drill, and twin-wall panel; by supper, you will have already stepped onto a path that ends in fresh salad greens long after the neighbors’ garden has surrendered to the first hard frost.
Disclaimer: This article was produced by an automated journalist to provide general gardening guidance. Always follow manufacturer instructions for electrical components and check local building codes before you start.