How to Make a Greenhouse for Seedlings

Constructing the Frame

• 1

  Cut the lumber for the front piece so it is at least 18 inches wide and as long as needed to fit the windows you're using or space constraints.

• 2

  Cut the lumber for the back piece to the same length, and at least 24 inches wide.

• 3

  Cut two pieces of lumber to the length required for the width of the frame. Measure and mark 18 inches on one end and 24 inches on the other. Use a straightedge to draw a line from one mark to the other and cut the lumber along this line. These two pieces will form the sides of the frame and will slope from the low front of the box up to the higher back.

• 4

  Cut two 2-by-2s to 18 inches in length and two 2-by-2s to 24 inches long.

• 5

  Use nails or screws to secure the sides of the frame to a 2-by-2 at each corner.

Constructing the Cover

• 6

  Attach old glass windows to the box frame with hinges on the high side of the frame. If you don't have old windows available, you can construct a clear cover with clear fiberglass.

• 7

  Cut two 2-by-2s to the length of the frame and two 2-by-2s to the width of the frame. Nail or screw these together to form a rectangular frame for the cover.

• 8

  Drill holes in the edge of the fiberglass and attach it to the frame for the cover with nails or screws.

• 9

  Attach the cover to the box frame with hinges on the high side. Since fiberglass weighs less than glass, a hook and eye at the front of the box will keep the lid from blowing open during high winds.

Preparing for Seedlings

• 10

  Dig a trench 6 inches deep and the length and width of the coldframe. If you want to heat the inside of the bed, dig a trench 18 to 30 inches deep and at least the length and width of the coldframe. You should place the coldframe in an area that receives full sun, with the slope facing toward the south.

• 11

  Place 6 inches of coarse gravel into the trench. Cover the gravel with a sheet of burlap or piece of wire screen.

• 12

  Fill the trench with fresh manure mixed with straw or another dry plant material, such as dry autumn leaves, if you intend to heat the enclosure, adding it on top of the gravel. As the manure and straw decomposes, it will produce heat. Tamp down the manure mixture with your feet or a shovel until the surface is firm.

• 13

  Place the coldframe over the trench and spread 4 to 6 inches of seed starter medium on top of the gravel or manure. Monitor the temperature, if you're using manure, and don't plant the seeds until the soil temperature drops to 85 degrees F. You can prop open the lid to the enclosure to lower temperatures, if needed.