How to Build a Cold Frame (Extend Your Season by 4–6 Weeks)
DIY cold frame from wood and windows—grow cool-season crops before spring and after fall frost.
TL;DR
- Time: 6 hours
- Cost: $
- Yield:
- Difficulty: Intermediate
Supplies
Tools
Steps
Pro Tips
Ventilation is crucial. On sunny days above 70°F, crack the cover open. Seedlings can cook in a closed cold frame. Use a hinged cover with a prop stick for easy venting.
Wet soil in winter can cause rot. Water cold frames in early morning so excess moisture evaporates. Or water less frequently.
In spring, remove the cover on mild days (above 60°F) to harden off plants. Cold-frame crops acclimate to outdoor conditions gradually.
Double-layer plastic (air gap between layers) insulates better than single-layer. If you have plastic sheeting, build a frame and staple plastic on both sides.
Old windows are free if you ask neighbors for discarded ones. Check Facebook Marketplace or Craigslist—lots of folks have old windows.
A cold frame is passive, but on extremely cold nights (below 20°F), you might lose tender seedlings. On those nights, throw a blanket over the cover for extra insulation.
Warnings
Don't leave a closed cold frame in direct sun on a warm day. Interior temps can exceed 100°F and cook plants. Prop the cover open.
Don't use pressure-treated wood. Use untreated pine or cedar. Chemicals leach into soil where food grows.
Research & Sources
- Cold Frame Construction and Management
Dr. Dan Drost, USU Extension Horticulture (2024) - Season Extension Techniques for Utah
Dr. Heidi Kratsch, USU Extension (2023) - Cool Season Crop Production
Utah State University Extension (2024) - Sevier County Season Extension Guide
Sevier County Extension (2023)
Frequently Asked Questions
What's the difference between a cold frame and a greenhouse?
A cold frame is passive solar, no heat source. A greenhouse is larger and often has heating. Cold frames are perfect for season extension; greenhouses are better for year-round growing.
Can I use it year-round?
In spring and fall, yes. In summer, it's too hot without ventilation. In deep winter, the soil freezes hard and plants don't grow. Spring and fall are ideal.
What if I don't have an old window?
Use clear plastic sheeting (6-mil) or rigid plastic panels. Both work fine. Plastic gets cloudy after a year; replace annually. Windows are free and last forever, but plastic is simpler to install.
Do cold frames need watering?
Yes, but less than outdoor gardens. The cover traps moisture. Check soil daily. Water if the top 2 inches are dry. Overwatering is a common mistake in cold frames.
