DIY Cold Frame: Stretch Your Arizona Garden into Winter with Scrap Wood and an Old Window
Every October I get the same itch. The garden is finally waking up again after summer, the nights are cool, and I want just a little more protection for the spinach, lettuce and other greens that hate a hard freeze. A cold frame is the cheapest way I know to stretch an Arizona garden deeper into winter without building a full greenhouse.
You do not need fancy materials. Scrap lumber, a few hinges and an old window are enough. I have built frames from fence boards, pallet wood and leftover 2x4s, and every one of them bought me warmer soil, calmer wind and earlier, longer harvests. Here is how I build a simple DIY cold frame, where to place it in Arizona sun, how to vent it so plants do not cook, and what to grow inside it through the cool season.
What a Cold Frame Actually Does
A cold frame is basically a mini greenhouse with no heat source except the sun. The clear lid lets light in, the wood sides block wind, and the closed box traps warmth so the soil and air inside stay several degrees warmer than the open garden. That small difference is huge for tender greens on a frosty night.
In Arizona it also works the other way. Our winter days can still be bright and warm. A closed frame can overheat by midmorning, so venting is just as important as insulation. Think of the lid as a thermostat you open and close by hand.
Cold frames pair beautifully with other season-extenders. If you already use DIY row cover hoops, a cold frame is the next step for a permanent little plot of greens: https://www.greenthumbhobbies.com/post/diy-row-cover-hoops
Materials: Scrap Wood and an Old Window
Here is the simple materials list I start with. Adjust sizes to match whatever window you find.
One old window or storm window for the lid. Single-pane glass or clear polycarbonate both work. Avoid cracked glass if kids or pets are around, or swap in a polycarbonate panel.
Scrap lumber for the box. 2x6, 2x8 or fence boards are common. The back wall should be taller than the front so the lid slopes toward the sun.
Four corner posts or screws and brackets to hold the box square.
Two hinges to attach the lid to the back wall.
A stick or adjustable prop to hold the lid open for venting.
Optional: exterior wood screws, wood glue, exterior paint or sealant, weatherstripping along the lid edge, and landscape fabric for the floor.
If you need hardware or a clear panel, here are Amazon searches that help: cold frame hinges and props https://www.amazon.com/s?k=cold+frame+hinges&tag=greenthumbh0a-20 and polycarbonate greenhouse panels https://www.amazon.com/s?k=polycarbonate+greenhouse+panel&tag=greenthumbh0a-20
Yard sales, Habitat ReStores and neighbors replacing windows are gold mines for free or cheap lids. Measure the window first, then build the box to fit. Fighting an odd-sized lid is not worth it.
Rough Cut List for a Window-Sized Frame
Exact numbers depend on your window. This is the pattern I use for a lid about 36 inches wide by 30 inches deep, which is a common storm-window size. Scale up or down to match yours.
Front wall: one board the width of the window, about 8 to 10 inches tall.
Back wall: one board the same width, about 14 to 18 inches tall so the lid slopes.
Two side walls: cut as trapezoids or rectangles that step from front height to back height, depth matching the window.
Optional bottom rails or floor boards if you want a movable frame instead of setting it on bare soil.
Lid stops or a cleat so the window cannot slide off when open.
Aim for a slope of roughly 30 to 45 degrees facing south. In Arizona that angle catches winter sun nicely without turning the box into an oven as fast as a flat lid would.
Build Steps
You can build this in an afternoon with a saw, drill and measuring tape.
Measure the window carefully. Note width, height and how thick the frame is where hinges will attach.
Cut the four walls. Make the back taller than the front. Dry-fit the box on a flat surface and check that the window sits evenly on top.
Assemble the box. Screw the corners together. Corner brackets or pocket screws keep things square. If you are using rough scrap, pre-drill so the wood does not split.
Set the box in place before hanging the lid, or build it where it will live. More on placement below.
Attach the hinges along the back edge so the lid opens toward the north and the sloping face points south.
Add a prop stick. A simple wooden stick with a few notches, or a commercial automatic vent opener if you want to get fancy later, keeps the lid from slamming shut in the wind.
Seal and paint if you want. Exterior paint or a clear sealer helps scrap wood last longer in our sun. Keep finishes plant-safe on any surface that touches soil or leaves.
Optional weatherstripping under the lid reduces drafts on the coldest nights.
That is the whole structure. No foundation required for a small frame. If your yard is uneven, shovel a level pad of soil or set the box on bricks.
Where to Place a Cold Frame in Arizona
Placement matters as much as the build.
Face the sloping lid south. That is non-negotiable for winter light.
Choose a spot with full winter sun. Morning sun plus midday sun is ideal. Avoid deep shade from walls or evergreen trees.
Watch reflected heat. A frame against a south-facing block wall can get very warm, which is great on freezing nights and dangerous on a 75-degree afternoon. Plan to vent early in those spots.
Keep it convenient. You will open and close this lid almost every day in winter. Put it where you will actually walk past it.
Think drainage. Do not set the box in a low spot that puddles after rain or irrigation.
In the low desert I often park a cold frame at the end of a raised-bed row so I can sow spinach and lettuce there while the open beds handle radishes and carrots. In the high desert I use frames to start earlier in spring and hold greens later into fall.
Venting So You Do Not Cook Your Plants
This is the skill that separates a helpful cold frame from a plant sauna. On a sunny Arizona winter day, a closed frame can shoot past 90 degrees inside even when the air outside is in the 60s.
My rules of thumb:
Open the lid on any sunny day once morning chill burns off, even if nights are cold.
Prop it a few inches for mild days, wider on warmer afternoons.
Close it before dusk so the box traps heat for the night.
On hard freeze nights, close it fully and consider an extra blanket or frost cloth over the lid after dark. Remove covers in the morning so plants get light.
Never leave it sealed for days if you will be out of town on a sunny stretch. Crack it or ask a neighbor to vent.
If you forget once, you will see wilted greens and learn fast. I still check mine every morning with coffee.
What to Grow in a Cold Frame
Cold frames shine with crops that like cool weather and stay relatively short.
Spinach: One of my top crops under glass. Baby leaves come fast, and plants keep producing in the protected microclimate. More spinach timing and varieties: https://www.greenthumbhobbies.com/post/spinach-arizona
Lettuce and salad mixes: Butterhead, romaine and looseleaf all do well. Cut-and-come-again mixes are perfect for small frames.
Radishes: Fast and happy in cool soil. See https://www.greenthumbhobbies.com/post/radishes-arizona
Asian greens: Tatsoi, bok choy and mustard greens love the protected chill.
Beets and carrots for baby harvests: Sow densely and pull young. Full-size roots need more depth and space, but baby beets and carrots work: https://www.greenthumbhobbies.com/post/beets-arizona and https://www.greenthumbhobbies.com/post/carrots-arizona
Herbs: Cilantro and parsley appreciate the shelter on frosty nights.
Transplant starting: In late winter I use the frame to harden off seedlings before they go into open beds.
Avoid tall crops like broccoli or indeterminate tomatoes. They outgrow the box and block light. Also skip heat lovers in the coldest months. The frame is for cool-season plants, not for forcing summer crops in January in the low desert. Our winters are mild enough that tomatoes belong in spring, not under a tiny lid.
Soil, Watering and Winter Care Tips
Fill the frame with the same quality mix you use in raised beds: plenty of compost, loose texture and good drainage. If the frame sits on native soil, loosen and amend the top eight inches before the first sowing.
Water less often than open beds but do not let the soil dry out. The lid reduces evaporation, so it is easy to overwater. Check with your finger. Morning watering is best so leaves dry before night.
A few more winter tips:
Succession sow. A small frame empties fast when you harvest. Reseed every couple of weeks.
Rotate crops if you use the same frame year after year, or refresh the soil with compost each fall.
Clean the lid. Dust and hard-water spots cut light. A quick wipe makes a real difference.
Watch for pests. Aphids and slugs like the cozy box too. Hand-pick and keep the interior tidy.
Weight the lid in wind. Spring winds can catch an open window. A hook-and-eye or a brick on a closed lid prevents accidents.
Cold Frame vs Row Cover vs Greenhouse
I use all three ideas at different scales. Row cover hoops are cheap, movable and great for long beds. A cold frame is sturdier, warmer and perfect for a concentrated greens patch. A greenhouse is wonderful if you have the space and budget, but it is overkill for a few salad crops. Start with scrap wood and a window. You can always go bigger later.
Wrapping Up
A DIY cold frame is one of the highest-payoff weekend projects in an Arizona garden. Build a box from scrap wood, hinge an old window on top, face it south, vent it on sunny days and fill it with spinach, lettuce and other cool-season greens. You will harvest earlier, lose fewer plants to frost and feel like you cheated winter a little. Measure that window, make your cut list and get the saw out this weekend. Your January salads will thank you.
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