How to Make your own Mini-Cold Frame

We are always looking for ways to extend our growing season. There are many ways do this including using row covers, moving potted plants indoors and planting cold tolerant crops. We’ve used cold frames many times over the years to grow our own crops into the fall and winter months and protect plants early in the growing season. This mini-cold frame is an easy and fun way to extend the season.

In this article I will share simple instruction on how to build your own simple wooden cold frame as well as suggestion on what and how to grow in your frame.

You might also like the cold frame we built about 10 years ago that is slightly larger but based on the same ideas. You can read more about it here: How to build a medium sized cold frame.

Cold Frame Greenhouse for a longer growing season — Mini-Cold Frame – this is the old one we built at our last house.

Contents

Basic Supplies for Building a Mini-Cold Frame

Window: The first thing you need for your cold frame is a window. Because the rest of your dimensions for wood will be based off the size of the window, I am not sharing exact dimensions. You can often find windows at yard sales or used online – just make sure there is no lead paint as you will be growing food beneath this window.

Wood – you will need wood for the 4 sides of your box. Since the base of the wood will be sitting in the ground, it’s best to get weather resistance wood like cedar. Avoid pressure treated wood as this isn’t good to grow food near. You can choose and width board although they should be at least 4 inches and preferably at least 6 inches. Here are the rough dimensions:

Front of box – 1 board the same length as the window (minus 1 inch)
Back of Box – 1 boards the same length as the winter (mins 1 inch)
Sides of box – (times 2) 2 boards the width of the window. Cut one board on the diagonal from corner to corner
Supports (2×4 or 2×2) 2 boards the same as the width of your boards +1 inch and 2 boards double the width of your board +2 inch
Back support – add an additional 2×4 the length of the back to help support the weight of the window

Hardware: you will also need hardware including exterior screws and hinges.

Building your Mini-Cold Frame

To assemble your cold frame cut all pieces of wood and lay them out.

Connect each side using the supports as framing. You want to make sure that the box is slightly smaller than the window so the window completely covers the box. I kept me supports about 1 inch below the bottom of the box to help keep the box in place in the ground. You can also adjust and put your supports on the outside of the box so the window sits inside – this makes it a little easier with the diagonal board.

Add a support across the top of the back board and connect this to the back supports. This helps support the window.

Once you’ve built the box, you will attach the window to the box using hinges. Make sure the hingers are places so the window opens and closes all the way.

Keep a few small boards on hand to prop open the window for both planting as well as ventilation.

Placing your Mini Cold Frame

You can put your cold frame in the garden in a permanent position or since it is small you can move it around. I pressed my frame into the garden so the bottom was just below the surface of the ground and then amended the soil in the frame with some compost to raise the level slightly. Make sure your frame faces south.

Since I was using this primarily to grow some winter crops, I placed this in early October in zone 5 and planted some cold tolerant greens.

How to Use A Mini Cold Frame

Here’s how to use one effectively:

Placement: Choose a sunny spot for your mini cold frame, ideally where it receives at least 6 hours of direct sunlight daily. Place it on a flat, well-drained surface like a garden bed or a raised bed.
Ventilation: To prevent overheating, open the cold frame lid slightly during the day, especially if temperatures rise. On colder days, keep the lid closed to retain warmth.
Watering: The mini cold frame helps maintain humidity, but check the soil regularly. Water when it feels dry, ensuring the plants don’t dry out while still maintaining a cool environment.
Hardening Off: If you’re starting seedlings indoors, use the cold frame to gradually acclimate them to outdoor conditions. Place the plants inside the cold frame for a few hours each day, increasing the exposure over time.
Seasonal Use: In spring and fall, you can use the mini cold frame to grow hardy vegetables like lettuce, spinach, and herbs. It’s also great for protecting early-season crops from frost or extending the growing season into late fall for more frost-tolerant plants.

Best Crops for a Cold Frame

The best crops to grow in a cold frame are cold tolerant and don’t grow too tall. For this mini-cold frame you may need to stay away from cabbage and kale that may be too big for your small frame.

Lettuce – Cold frames help extend the growing season for various types of lettuce, such as butterhead and Romaine.
Spinach – This hardy leafy green thrives in cool weather and can be grown early in spring or late in fall.
Kale – Kale is a cold-tolerant vegetable that grows well in the protection of a cold frame, especially in early spring or fall.
Arugula – A fast-growing salad green that can be grown in a cold frame, providing an early spring harvest.
Radishes – These quick-growing root vegetables do well in cool temperatures and can be harvested early in the season.
Swiss Chard – Swiss chard is a hardy green that benefits from the gentle warmth of a cold frame during cooler weather.
Carrots – Cold frames can help extend the growing season for carrots, allowing you to grow them into late fall.
Beets – Like carrots, beets can be grown in a cold frame to protect them from frost, extending their harvest period.
Cabbage – Cabbage is another cool-weather crop that grows well in the slightly warmer environment of a cold frame.
Herbs – Many herbs like parsley, cilantro, and chives thrive in cool weather and can be grown in a cold frame for an early harvest.