Cold frame over a raised bed

A cold frame is the most versatile and effective season extension tool for raised beds. It works as a miniature greenhouse: a transparent lid (glass, polycarbonate, or clear plastic) over the bed traps solar heat and blocks wind, creating a microclimate 5-15 degrees F warmer than outside air. Cold frames are ideal for growing cold-hardy greens (lettuce, spinach, kale, carrots, radishes) through winter in zones 5-8, and for hardening off transplants in spring. Build a simple cold frame by hinging an old glass window or polycarbonate panel to the back edge of a raised bed. Orient the transparent lid to face south for maximum winter sun exposure. Angle the lid at roughly 30-45 degrees for optimal solar gain and rain runoff. Critical maintenance: ventilate on sunny days when interior temperature exceeds 70 degrees F by propping the lid open with a stick; overheating kills plants faster than cold. Automated vent openers ($25-40, brands: Univent, Bayliss) use wax-filled cylinders that expand with heat, automatically opening the lid. Close the lid before sunset to trap daytime warmth. Commercial cold frame kits ($75-200, brands: Palram, Juwel) fit standard bed sizes, or build from repurposed materials for near-zero cost.