Best Cold Frame for Your Garden, Plus Row Cover Types

What Makes a Good Cold Frame

The best cold frame for a home garden is a bottomless box about 3 by 6 feet, with a clear lid that slopes toward the south, a back wall near 18 inches and a front near 12 inches, and a lid that is easy to prop open. Those proportions come up again and again in university extension guides. They matter more than the brand or the price, because a cold frame that is the wrong size or hard to vent ends up unused.

Iowa State University Extension describes a cold frame as a raised bottomless box with clear covers that acts much like a small greenhouse. It shelters plants from wind, rain, snow, ice and cool temperatures, and Iowa State puts the gain at two to four weeks of extra growing season in both fall and spring. Gardeners use one to grow cool-season crops such as lettuce, spinach and radishes, and for hardening off seedlings raised indoors. South Dakota State University Extension adds a third job: holding late-season root crops such as carrots, beets and turnips.

Cold Frame, Row Cover or Low Tunnel

A rigid frame is one of three common ways to protect crops from frost, and the other two cost less and cover more ground. Floating row cover is a spun fabric laid over the bed. A low tunnel is the same fabric held up on hoops.

University of Wisconsin Extension notes that row cover can be laid directly over low-growing, flexible crops, while taller crops may need hoops under the fabric. University of Maryland Extension describes low tunnels 15 to 30 inches high, with hoops spaced 2 to 4 feet apart, as especially easy to make and use.

Structure	Typical size	Suits	Daily attention
Cold frame	3 by 6 feet	Lettuce, spinach, radishes, hardening off seedlings	Lid must be vented on sunny days
Floating row cover	Any bed	Low, flexible crops and insect exclusion	Edges need anchoring
Low tunnel	15 to 30 inches high, any length	Taller crops across a whole bed	Hoops every 2 to 4 feet, edges anchored

For a first season, fabric is the easier start: it goes on any bed and folds away afterward. A cold frame makes sense when you want a fixed spot for seedlings each spring and salad greens each fall. For the fall side, the list of what to plant in August shows which crops are worth covering.

Size and Shape to Look For

Glass sash sets the traditional size. Purdue Extension notes that standard commercially available sash is 3 feet by 6 feet. Iowa State advises that the frame be no more than 3 feet deep from front to back, so that you can reach every plant from the low side. The NC State Extension Gardener Handbook gives 4 to 5 feet as about the widest span that is convenient to reach across, which applies when you can get at the frame from both sides.

The lid should slope. Iowa State’s ideal is about 12 inches high in front and 18 inches high in back. Purdue asks for a back that is 6 to 12 inches higher than the front, with a minimum slope of 1 inch per foot. South Dakota State describes the result as a slope of about 10 to 15 degrees. The tilt sheds rain and lets more sunlight reach the plants.

Check the inside height against what you plan to grow. A 12 inch front wall is enough for salad greens and seed flats. Taller crops fit better under a low tunnel, which can reach 30 inches.

Frame and Lid Materials

Most cold frames have wooden sides. Iowa State lists cinder blocks, bricks, poured concrete, plastic lumber and straw bales as alternatives, and the NC State handbook recommends a wood species that resists decay, such as a good grade of cedar.

Among bought frames, the Royal Horticultural Society notes that models with clear sides and lids, especially aluminium and glass, are the most popular. It adds that frames with wooden or brick sides are better where protection from cold is the goal. That is a useful rule when comparing products: all-clear frames let in the most light, and solid sides hold more warmth.

For the lid, South Dakota State recommends UV-stable plastic or glass and names polycarbonate, polyethylene film and acrylic as workable clear plastics. Old storm windows, window sashes and shower doors are common recycled covers. Whatever the material, the Royal Horticultural Society’s test for a lid is that it be robust, easy to open and able to give varying degrees of ventilation while keeping heavy rain off the plants.

Weight cuts both ways. The same RHS guide warns that lightweight frames are cheap and easy to move but vulnerable to wind damage, so they need to be pegged down.

Where to Put It and When to Vent

Face the frame south. Purdue calls for full exposure to the sun with a southern orientation preferred, on a well-drained spot that is protected from wind and near a water supply. Iowa State suggests setting the north end near a house, garage or fence. Banking soil around the outside of the frame conserves heat, and the RHS suggests lining the inside with bubble plastic for some insulation against frost.

Venting is the daily chore. Iowa State’s threshold is to raise the sash when the air inside the frame rises above 85 degrees Fahrenheit, so a thermometer inside the frame is worth having. South Dakota State advises venting or removing the lid entirely on warm, sunny days, and notes that cool-season vegetables grow best at air temperatures between 50 and 80 degrees Fahrenheit. Close the lid again before sunset to hold the day’s heat overnight. Purdue recommends watering in the morning while the sun is shining.

Row Cover Types by Weight

Row cover is sold by weight per square yard, and the weight sets both the frost protection and the amount of light that gets through. University of Maryland Extension gives these figures:

Weight class	Ounces per square yard	Light transmittance	Frost protection
Lightweight	0.45	90% to 95%	2 F
Medium	0.5 to 1.0	70% to 85%	4 to 6 F
Heavy	1.5 to 2.2	30% to 50%	Up to 8 F

Wisconsin’s guide summarizes the same range as 2 to 8 degrees of frost protection depending on weight. The NC State handbook puts both row covers and low tunnels on wire hoops at 4 to 5 F.

The tradeoff is light. Heavy fabric passes as little as 30% of sunlight, so it suits cold snaps and crops that have finished growing. Maryland describes the lightweight grade as marketed as an insect barrier, and for that use Wisconsin advises burying the edges completely. The guide to row covers for pest protection covers that job in detail. For frost, Maryland lists sod pins, boards, bricks, sandbags, rocks or soil as anchors. Heavy covers also warm the bed by day; Wisconsin notes that heavyweight fabric raises daytime temperatures 10 F or more.

Maryland puts the useful life of row cover material at 1 to 3 years, so treat it as a consumable and store it dry between seasons.

Choosing What to Buy First

Start from the job. For hardening off seedlings in spring and growing lettuce, spinach or radishes into fall, pick a cold frame near 3 by 6 feet with a sloped lid that props open at more than one height. For frost protection over a whole bed, a medium-weight row cover at 0.5 to 1.0 ounce gives 4 to 6 F and still passes 70% to 85% of light. For taller crops, add hoops and build a low tunnel 15 to 30 inches high.

Before ordering a frame, measure the sunny, south-facing spot you have in mind and confirm you can reach across the frame from the front. Then look closely at the lid: how it opens, how it stays open, and whether the frame is heavy enough or can be pegged down against wind.