Build a $25 Cold Frame, Harvest Greens Into December

The first hard frost does not have to end your salad season. A cold frame, which is just a bottomless box with a clear lid, is the cheapest season extender I own, and this fall I built another one for $24 using a reclaimed window sash as the lid. Iowa State University Extension and Outreach puts the gain plainly: cold frames extend the growing season by two to four weeks in both the fall and the spring. Four extra weeks of lettuce, spinach, and arugula in November, plus a four-week head start in March, from a box I built in an afternoon. That is the best return on $24 in my whole garden.

What I built, and what it cost

My frame is sized to the window, not the other way around, which is the trick that keeps the cost down. A neighbor's renovation gave me a sound 2 by 4 foot window sash with intact glass for free; thrift and salvage stores near me sell them for $5 to $10. The box is two untreated 2x10 boards for the sides, cut so the front wall sits lower than the back wall. That slope, about 2 inches of drop, sheds rain and angles the glass toward the low fall sun. My parts list: boards $14 (salvage lumber), two hinges $6, screws and a prop stick $4. Total $24. The same frame with all-new lumber and a new polycarbonate lid priced out at $58 at my hardware store, and a ready-made kit of the same size was $129. All prices are my local October 2026 prices.

One 2 x 4 ft cold frame, three ways, at my local prices in October 2026. Season extension of two to four weeks per Iowa State University Extension and Outreach.

Build it in an afternoon

Cut the side boards on the slope first: I mark the back board at 12 inches tall and the front at 10 inches, and angle the side pieces between them. Screw the four boards into a rectangle, set it directly on a prepared bed in the sunniest spot you have, and level it by digging the high side in rather than shimming the low side up. Hinge the window sash to the back board so the lid opens away from you. Add a notched prop stick for venting. That vent is not optional equipment: Iowa State Extension notes the frame heats up fast under sun because the soil and frame absorb heat by day and radiate it back at night, and on a bright 55-degree afternoon the inside can cook tender greens if the lid stays shut. I prop mine open a few inches any sunny day above about 50 degrees and close it before evening.

What to grow in it now

Sow or transplant the cold-tolerant crops, not the summer leftovers. I direct-sowed spinach and arugula in the new frame this week and tucked in lettuce transplants that were crowding another bed. Leaf lettuce, spinach, arugula, mache, and green onions are my reliable fall frame crops; they grow slowly in the cold but they grow, and the frame keeps frost off the leaves, which is what actually kills quality. Water sparingly: the lid traps moisture, and soggy soil in cold weather rots more plants than frost does. When nights head below about 25 degrees, I throw an old blanket over the glass at dusk and take it off in the morning. That one habit has carried my spinach through the first December cold snaps more than once.

The honest limits

A cold frame is not a greenhouse, and Extension is clear that it offers no active heat, just stored solar warmth and shelter from wind, rain, and ice. In a zone colder than mine, two to four weeks is the realistic fall gain, not two months. Glass sashes are also heavy and breakable; if you have small children doing garden chores with you, a polycarbonate lid at the $58 build level is the safer spend. But for the price of two bags of bagged salad a week across a month of extra harvests, my $24 box pays for itself before Thanksgiving.

FAQ

How much season extension does a cold frame really add? Two to four weeks in the fall and again in the spring, according to Iowa State University Extension and Outreach.

What size should I build? Build to your lid. Mine is 2 by 4 feet because that is the window sash I had; sizing the box to a free or cheap lid is what keeps the cost at $24 instead of $58.

Do I need to vent a cold frame in fall? Yes, on sunny days. Extension guidance notes the frame stores solar heat, and greens can overheat under a closed lid even when the air outside feels cool.

What are the best crops for a fall cold frame? Spinach, leaf lettuce, arugula, mache, and green onions in my beds. Choose cold-tolerant greens, not warm-season crops.

Glass or plastic for the lid? Glass is free if you salvage a sash and transmits light well, but it is heavy and breakable. Polycarbonate costs more and is the safer choice where kids handle the lid.