Garden · Weekend · Intermediate
Cold Frames, Row Covers and Extra Weeks of Growing
Colorado State's 'up to 10°F' row-cover margin, the difference between a radiation frost and a wind-driven freeze, and why tunnels must be vented.
This is for educational purposes only. Not professional medical, electrical, or safety advice.
Warning
Row cover extends the season; it does not make a tender crop frost-proof. During a prolonged wind-driven freeze, expect the protection to fail and harvest or protect the crop another way.
- 01
Treat frost protection as a margin, not a guarantee
Colorado State University Extension reports that floating row covers can provide up to 10°F of protection depending on the material and its grade. An 'up to' rating is not a promised temperature increase, especially during a prolonged freeze.
- 02
Know which kind of cold is coming
A calm radiation frost is when covers can help substantially, because the ground and plants are losing heat to the sky. A windy advective freeze brings genuinely cold air, and protection may fail — so plan the crop, not just the cover.
- 03
Cover to hold soil heat and seal the edges
Row cover works by retaining heat from the soil, so secure the edges with clips, pins or boards and choose a fabric weight appropriate to the crop and the forecast.
- 04
Uncover when pollination is needed
Remove covers when insect-pollinated crops are flowering, or the covers will keep the pollinators away as effectively as they keep the cold out.
- 05
Vent cold frames and tunnels in the daytime
Cold frames and plastic tunnels need daytime ventilation because sunlight can overheat them even in cool weather. Prop or remove the lid as temperatures climb.
- 06
Put a thermometer inside and keep your own numbers
Use an internal thermometer rather than a guessed protection value, and record what each structure actually did on your site. A universal numerical protection rating for homemade cold frames is unconfirmed.