Seeds & Potting Compost

Compost, Potting Soil and Unwanted Seeds

• Experts at the University of Minnesota Extension recommend using one-quarter to one-third compost by volume in potting soil mixes. Too little compost might not provide adequate nutrients for the plant, while too much can water-log it or prevent proper aeration to the roots. The process of composting destroys most seeds in its mix, but occasionally, undesirable seeds remain viable. Roughly comb through your compost before adding it to potting mixes and pick out any seeds if you see them. If numerous seeds are present, you may need to take steps to kill them.

To Heat or Not to Heat

• Pasteurizing compost will kill any seeds, and this is easily accomplished in a conventional oven. Heat the material to 160 degrees Fahrenheit and maintain that temperature for at least 30 minutes. While effective, this method also kills beneficial microorganisms that help fight off soil-borne diseases. Pasteurization is probably only worth it if there are numerous seeds in the mix or if there is a question as to the compost's maturity.

The Radish Test

• Immature composts often have phytotoxins that kill seeds and embryos and may contain other nuisances, such as pathogens and viable unwanted seeds. Therefore, immature compost should not be used in potting soil mixes. For beginners, in particular, however, determining when compost is fully mature can be difficult. Generally, backyard composts take approximately six months to fully decompose. To test if your compost is mature, place compost in a few small pots and plant a radish seed in each of them. If three-quarters of the seeds sprout and grow over the next several days, the compost is mature. If not, the compost needs to decompose more before you can use it in your garden or potting soil.

Starting Seeds In Compost

• Experts at the University of New Hampshire Cooperative Extension recommend not starting seeds in compost, garden soil or potting soil. Seeds require certain conditions to germinate properly, and these materials are often too heavy or too wet, or they harbor diseases seeds can't fight off. Instead of compost, use equal parts vermiculite, milled sphagnum moss and perlite.