This is an easy garden project for a Saturday afternoon.
You will need:
Mulch cloth (optional)
Topsoil, compost, peat moss or some combination of the above
A few bricks or rocks
Heavy-duty bubble wrap (the kind with the big bubbles)
If using mulch cloth, lay it down over the area where you plan to put your raised bed. I have mixed feelings about mulch cloth. It helps with weed control for a while, but it gets in the way when I’m trying to plant seedlings. I skipped it this time.
1. Arrange your cinderblocks to form a bed. I’d recommend designing a rectangular bed, as it will be easier to work with when you create the bubble-wrap blanket for the top. Mine is circular because I planted last year’s garden in a pre-existing fire pit in the backyard, and I didn’t feel like rearranging the blocks. The smaller the bed, the more thermal mass you’ll have for keeping plants warm, so keep that in mind.
2. Fill the bed with a couple of inches of planting medium (topsoil, compost, peat moss, whatever).
3. Plant whatever cool-weather seeds you feel like growing and water them. I used a lettuce mix I found at the Co-op, but spinach, kale, radishes and some herbs are also good choices.
4. Lay strips of bubble wrap, bubble side down, across the top of the bed. Duct-tape the strips together as neatly as possible. For a rectangular bed, you can measure your dimensions and do this step inside on a flat surface, which I’d recommend. Leave enough excess around the edges to be sure all the soil is covered and to have room to weigh down the bubble wrap when you’re done.
Cold frames work like little greenhouses: The transparent plastic lets in light and traps heat, warming the soil. Cinderblock walls add thermal mass, and bubble wrap provides extra insulation, allowing you to start planting even earlier.