Cold frames are simple structures—bottomless boxes with a transparent lid—that trap solar heat and protect plants from frost. They can extend your cutting season by several weeks on both ends.
Set transplants of snapdragons, stock, and sweet peas into cold frames 4–6 weeks before your last frost date. The frame protects them from hard freezes while they establish. This gives you blooms weeks earlier than unprotected plants. Cold frames also work for hardening off seedlings.
As frost approaches, place cold frames over late-season crops like zinnias, cosmos, and dahlias to protect them through light frosts and extend harvest by 2–4 weeks.
In zones 7+, cold frames can protect fall-planted ranunculus, anemones, and snapdragons through winter for very early spring blooms. Vent the frame on warm days to prevent overheating.
The simplest version: stack straw bales into a rectangle and lay an old window or sheet of clear polycarbonate on top. More permanent frames can be built from lumber and hinged polycarbonate panels. South-facing placement captures maximum sun.