cold frames

Flats and Cold Frames

Constructing Planting Flats and Cold Frames

Now that the active gardening season has ended and we have time before spring, use some of that freed-up time to prepare for spring planting. Two essentials for getting your garden off and running in the spring are planting flats and cold frames. Both of these make great woodworking projects for the winter months, and when spring arrives, you will be ready to get going with the fun of gardening.

Let’s look at planting flats first.planting flats and cold frames

Many gardeners start their seeds in individual pots, but good old-fashioned wood planting flats still come in handy. (My mom used these for years, starting trays of tomato plants in her south facing porch.) Good for starting seeds, they also make convenient portable trays to hold individual pots.

Plus, you can custom make them to fit a windowsill, a plant stand or a greenhouse shelf. Flats have several advantages over growing seeds in containers:

  • They hold more plants in a given space
  • They last longer than disposable containers
  • Flats are easier on the eyes than multiple sizes and shapes of individual containers.
  • They can be virtually free if you make them yourself from scrap wood.

The size of your shelf or bench will determine the dimensions of your flat, but they should be no more that 14 inches by 18 inches, and up to 4 inches deep. One inch lumber will make a sturdy flat, but half inch boards could do. Here are the steps:

  • Cut your boards to dimension for the 4 sides of the flat, and calculate how many bottom slats you will need, allowing an eighth inch space between the bottom boards for drainage. Cut the bottom slats to size.
  • Fasten a wood cleat as a handle to the two short sides of the flat.
  • Nail the 4 sides together, lining them up squarely. Use 3 nails at each corner.
  • Check the corners for squareness, and nail on the bottom slats, leaving an eighth inch between them for drainage.

Building your flats is a good project for a rainy or snowy day, and when planting season arrives, you will have a head start.

Cold Frames Construction

The second essential for extending the growing season in spring or in fall is a cold frame. Early frosts, late snow and spring rains can keep us out of the garden. With a cold frame or two, you can easily have a controlled micro-climate where you can stretch the growing season.planting flats and cold frames

A cold frame is simply a bottomless box with a hinged or removable clear cover. That old window behind the garage, or one you can pick up at the recycling depot could form the lid. Another idea is fiberglass or rigid plastic panels mounted over a wood frame. Even 6 mil plastic mounted on a frame can make an acceptable lid.

There are no standard sizes for cold frames, but one less than eight square feet is too small. The size may be determined by the dimensions of the recycled window, or you could use two windows for the lid. Make the frame slightly smaller than the lid so water drains off the edges rather that running into the frame.

The frame is often made of wood – exterior plywood, cedar or even fir or spruce boards. The wood sides should be screwed together and reinforced at the corners with 2×2’s. Construct the walls of your cold frame so you have a sloping lid to shed rain and effectively capture sunlight. A back of 18 inches and a front of 12 inches are good dimensions. Hinge the frame of the cover to the back of the cold frame, and place the whole cold frame in a sunny and level spot. You may want to drive supporting stakes into the ground around the cold frame to keep it well anchored.

By taking time over the winter and early spring months to make these two gardening projects – planting flats and cold frames – you will have a head start as the spring weather arrives.

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