Building a Cold Frame for Your Garden: Simple Construction and Season Extension

Building a Cold Frame for Your Garden: Simple Construction and Season Extension

A cold frame is the simplest and cheapest way to extend your growing season. It is a bottomless box with a clear or sloped translucent top, set directly on the ground. The sun heats the air and soil inside, and that warmth keeps your plants alive weeks or even months outside the normal season.

You can build one from a few pieces of lumber and a clear plastic panel. The total cost is often less than twenty dollars. You can finish the build in an afternoon. The payoff is a small plot of garden that stays warm enough to produce food long after the rest of your garden has gone dormant.

This guide shows you how to build a functional cold frame, where to place it, what to grow in it, and how to manage the temperature inside so your plants thrive.

How a Cold Frame Works

A cold frame is a passive solar collector. The clear or translucent top lets sunlight through. The soil and sides inside absorb that light and slowly release heat. On a sunny day even in January the air inside can be thirty or forty degrees warmer than the air outside. At night the mass of the soil releases heat back into the frame, keeping the interior a few degrees above the outdoor temperature.

This temperature difference is what gives you an extended season. A cold frame can give you a spring head start of four to six weeks before your last frost date. In fall and winter it can keep hardy greens alive well into January in Zone 7. With a heavy blanket or old quilt on top on very cold nights, you can add another six to twelve weeks of protection.

You do not need electricity, a heater, or any mechanical parts. The sun does all the work. Your job is to build the frame that captures the sun and to manage ventilation so the inside does not overheat.

Choosing a Location

Where you put the cold frame matters more than how you build it. A poorly placed frame wastes more sun than a poorly built one.

Face the transparent side toward the south or southeast. This angle catches the most sunlight during winter when the sun tracks low across the southern sky. A tilt of about forty five degrees on the top is a good starting point. It balances winter sun angle with rain runoff.

Put it against a wall or fence on the north side. A solid barrier behind the cold frame blocks cold wind and reflects a small amount of light back into the frame. A brick or stone wall is ideal because it absorbs daytime heat and releases it slowly through the night.

Make sure the ground drains well. A cold frame set in a low spot where water collects after rain will develop a swamp inside. If your garden has heavy clay soil, dig out the area and add a two inch layer of gravel or crushed stone at the bottom before placing the frame.

Give yourself room to work. The frame should be no wider than about three feet so you can reach the center from the edge without stepping inside. A standard size is six feet long by three feet wide. The back wall should be about two feet high and the front wall about one foot high to create the slope.

What You Will Need

Here is a simple lumber cut list for a six by three foot frame:

• Two pieces of two by eight, six feet long (front and back walls)
• Two pieces of two by four, three feet wide (side walls)
• Four pieces of one by four, three feet wide (corner reinforcements for the side walls)
• One clear rigid PVC panel or double wall polycarbonate sheet, six by three feet (cut to size from a hardware store)
• Exterior grade wood screws and galvanized nails
• Exterior wood sealant or linseed oil (optional)
• Two to three hinges for the top cover (piano hinges or regular butt hinges work)
• Bungee cords or lid stay hardware to hold the top open
• A roll of hardware cloth, about eighteen by twenty four inches (for the bottom to keep out rodents)

You can substitute cinder blocks or reclaimed pallet wood for the sides if lumber is expensive in your area. Cinder blocks require no cutting or screwing. You stack them, level them, and set the clear panel on top. The tradeoff is that blocks are heavier and harder to move if you want a portable frame.

The transparent panel is the most important component. Clear rigid PVC is the cheapest option at most hardware stores. It is lightweight and easy to cut with a utility knife. Double wall polycarbonate costs a bit more but insulates better and lasts longer. Do not use regular glass. It is heavy, dangerous to cut, and will shatter if a branch or hail hits it.

Building the Frame

Step One: Build the back wall. Take your two six foot two by eights and screw them side by side to make a thick solid wall. The double layer gives the back wall extra insulation and mass, which helps it hold heat through cold nights. If you prefer a thinner profile you can use a single two by eight. The height of this wall determines how tall your plants can grow. Two feet is a good starting point. Most leafy greens do well in a frame of this depth. If you want to grow taller crops like peas or determinate tomatoes in spring, make the back wall three feet high.

Step Two: Build the side walls. Cut two pieces of two by four to three feet wide. Screw two one by four reinforcements along the inside edges of each side wall for extra strength. These walls will sit between the back and front walls. The front wall should sit about one foot high. The side walls angle slightly to match the slope. Mark the cut angles on the two by fours before cutting: the outside edge meets the back wall at full height and the inside edge meets the front wall at one foot. This creates the sloped shape.

Step Three: Assemble the box. Lay the back wall on a flat surface. Place the two side walls against it, angled properly. Place the front wall against the bottom of the side walls. Screw through the side walls into the ends of the back and front walls. You should now have a sloped box with one end open.

Step Four: Add a rodent barrier. Lay a piece of hardware cloth across the bottom opening of the frame. Screw or staple it to the inside bottom edges of the side, front, and back walls. This keeps mice, voles, and other burrowing pests from getting into your soil. You can skip this step if rodents are not a problem in your garden, but it is cheap insurance.

Step Five: Add floor reinforcement. If you want extra durability, screw two or three one by twos across the bottom of the frame at the front and back. These act as feet that keep the side walls from bowing inward over time. You can also skip this step if you are building a simple seasonal frame and plan to store it for summer.

Step Six: Attach the cover. Cut your clear panel to six by three feet. Screw hinges to the top of the back wall and to the clear panel. Two hinges are enough for a panel this size. Test the opening. The cover should lift smoothly without binding.

Step Seven: Add a latch and opener. Screw a simple hook latch to the front wall so you can close the cover securely at night. For keeping the lid open during the day, run a bungee cord from the inside of the clear panel to the top of the front wall, or install a lid stay hardware kit from a hardware store. The lid needs to stay open on warm days even when you are not around to watch it.

Step Eight: Seal and finish. Brush exterior wood sealant or linseed oil on all surfaces. This slows rot from rain splashing against the sides. You do not need to paint the inside or the clear panel. Let the wood dry completely before moving the frame to its final location.

Setting It Up

Place the frame in the final position before you install the clear cover permanently. Check the sunlight. At solar noon in the middle of winter the sun should strike the clear panel as directly as possible. Adjust the position if a tree or building casts a shadow between nine and three in the afternoon.

If the ground is uneven, level the spot with a rake and a board. A frame that sits on a slope will have one side open to wind and the other pressing into the soil. Use bricks or stones under the low side to bring the bottom into level.

Fill the frame with soil. A mix of topsoil and compost at about a fifty fifty ratio works well. Pack it down lightly. The soil needs to be deep enough for roots to reach a few inches below the surface. Four to six inches is sufficient for cold frame crops.

Replace the clear cover. On a sunny winter day check the temperature inside within an hour of noon. A simple maximum minimum thermometer, which costs about ten dollars at a garden store or online, is very helpful. It records the highest and lowest temperature inside the frame each day so you can see at a glance whether conditions are safe for your plants.

What to Grow in a Cold Frame

A cold frame is best suited to hardy and semi hardy crops that tolerate cool temperatures. It is not a substitute for a greenhouse in summer. The clear top will bake most warm season crops on a sunny March or April day if you forget to open the lid.

What grows well in winter:

• Spinach. Grows slowly but steadily through frost and light snow.
• Kale. Very cold hardy. A thick snow cover on top of the frame actually insulates it.
• Lettuce. Cold frame lettuce stays tender longer than outdoor lettuce in winter.
• Chard. Hardy to about twenty degrees Fahrenheit.
• Mustard greens. Fast growing and very cold tolerant.
• Radishes. Small fast radishes like Cherry Belle can mature in a cold frame through late fall.
• Cilantro and other cool season herbs.

What grows well in early spring:

• Peas. Sow peas directly in the cold frame three to four weeks before your last frost.
• Spinach. Spring spinach grows fast under the glass.
• Radishes. Quick crops that mature before the frame gets too hot.
• Transplant starts of broccoli, cabbage, and kale. Hardening off seedlings in the frame before transplanting them outdoors saves weeks of indoor grow light time.

What to avoid:

• Tomatoes, peppers, and eggplants. These need sustained warmth a cold frame cannot provide in early spring and will bake in late spring.
• Squash, cucumbers, beans, and corn. All warm season crops that need soil temperatures above sixty degrees and steady warmth.

Managing Temperature

Temperature management is the single most important skill for cold frame gardening. Open the lid whenever the inside temperature reaches sixty five degrees on a sunny day. Even when it is thirty degrees outside the sun can heat the interior past that quickly. The plants will wilt and cook in an hour if you leave it closed.

On cold nights close the lid securely. Add an insulating layer on top of the cover if the forecast drops below twenty degrees. An old blanket, a quilt, or a layer of straw matting adds enough insulation to protect hardy greens through most Zone 7 winters. Remove the insulation in the morning so the sun can warm the frame again.

Ventilate on any sunny day when the outside temperature is above forty five degrees. Crack the lid open an inch or two in the morning before the temperature climbs. As the day warms open it wider. Close it late in the afternoon so the frame retains heat through the night.

In late April and May a cold frame can become uncomfortably hot for even cool season crops. On warm spring days you may need to remove the clear cover entirely and use it as an open-air cold frame until your last frost date has passed. Once the risk of frost has passed the frame can be dismantled and the soil used for a regular garden bed.

You do not need to watch the frame constantly. A lid stay keeps it open while you are away. The maximum minimum thermometer tells you what happened while you were sleeping or working elsewhere. If you see the high side of the thermometer reading above seventy five on a winter day, the lid stayed open too late or closed too soon. Adjust the timing.

Pest Management

Cold frames attract pests just like open gardens, and the enclosed space can make infestations worse if you do not monitor them.

Slugs and snails thrive in the cool damp environment of a cold frame. Check the underside of leaves and the soil surface in the evening with a flashlight. Hand-pick them and drop them into a bucket of soapy water. A shallow dish of beer buried level with the soil surface will also catch them.

Mice and voles can nest in the soil inside a cold frame. The hardware cloth at the bottom keeps most burrowing rodents out. If you notice tunneling in the soil, check the barrier and repair any gaps.

Aphids can build up quickly in the enclosed space. Spray them off with a firm stream of water from a hose or picking them off by hand works well inside a cold frame where you can control the water pressure without soaking the entire bed.

Watering a Cold Frame

Watering a cold frame is simpler than watering a regular garden bed. The cover keeps rain and snow out and slows evaporation dramatically. You will water far less often than in an open bed.

Check the soil surface by pressing your finger two inches down. If the soil feels dry at that depth it is time to water. Use a watering can with a fine rose or a spray nozzle on low pressure so you do not wash the soil away. Water in the morning on sunny days so the foliage has time to dry before night.

In the deep winter when the sun is weak and temperatures stay low plants grow very slowly and may need water only once every two to three weeks. In spring when the sun is stronger and growth picks up you may need to water every week or every few days. Let the soil be your guide, not the calendar.

Maintenance and Long Term Use

A wooden cold frame left outside year round will last five to ten years with minimal maintenance. Check the sealant or oil every spring and touch up any peeling areas. Tighten any loose screws or hinges. Inspect the clear panel for cracks or cloudiness each season. A panel that has become too yellow or cracked to transmit light should be replaced.

Store the clear cover indoors during summer if you plan to keep the frame permanently in place. Hot summer sun will bake anything growing inside a cold frame and the covered structure can become uncomfortably hot for your plants. You can also dismantle the frame in spring, clear the soil for a regular garden bed, and rebuild it in late summer when you need to start fall crops.

The Payoff

A cold frame is one of the smallest gardening tools you can own and one of the most useful. It costs almost nothing to build. It requires no energy to operate. It gives you fresh food during the months when your garden is otherwise empty and gives seedlings a warm head start in spring when the rest of the ground is still frozen.

Start small. Build a frame that fits your space and your budget. Plant something hardy like spinach or radish. Watch the thermometer. Learn what your frame does on a sunny day and on a cold night. After one season you will understand it better than any guide can tell you.

The frame is a classroom. The plants are the teachers. The sun is the energy source. All you have to do is show up, watch, and adjust.

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— C. Steward 🥕