DC summers are hot and wet. A raised bed needs a mix that drains after a storm and holds water through a week of 95 degree days. Bagged potting mix dries out too fast. Compost alone stays soggy and shrinks by half in a season.
The mix
We fill beds with three parts by volume:
- 40 percent screened topsoil
- 40 percent compost, half leaf compost and half aged manure compost
- 20 percent coarse material, pine bark fines or perlite
For a 4 by 8 ft bed that is 12 inches deep you need about 1.2 cubic yards. Order in bulk from a local supplier. Bags cost three to four times more per yard.
Why the coarse material matters
Pine bark fines keep air in the mix after the compost breaks down. Without them the bed settles into a dense layer by August and roots stop growing. Add two inches of fresh compost on top each spring and the bed stays loose for years.
What to skip
Skip peat. It repels water once it dries out. Skip sand. Sand plus clay topsoil makes concrete. Skip fill dirt of any kind.