If you’re going to be starting seeds indoors this springunder lights, on a windowsill or in a greenhouseyou don’t want to be using regular potting soil. It’s too heavy and dense for the delicate, hair-like roots of a newly-germinated seed.
The best soil mix for seed starting is not really soil at all. It’s a growing medium comprised of sphagnum peat moss and small amounts of vermiculite and/or perlite. This blend helps ensure a consistently moist environment to encourage good germination. It is also light enough to provide lots of wiggle room for tiny roots. Just as importantly, it has been sterilized to eliminate bacteria and fungus which can cause problems such as “damping off”.
Professional Germinating Mix is my #1 choice for starting seedsespecially very tiny flower seeds like those of petunias, snapdragons and flowering tobacco. It has an ultra-fine texture because the sphagnum peat moss has been milled to remove all clumps and lumps. This mix is also a must for the APS seedstarting system because it doesn’t get waterlogged when it’s used with capillary matting.
Transplant Mix contains essentially the same ingredients as the Germinating Mix, but it has a more coarse texture. It is the right mix for starting larger seeds such as cucumbers, zinnias and marigolds. It’s also the right choice when it comes time to transplant seedlings into larger pots.
(Hint #1: I sometimes fill the bottom half of a seed flat with Transplant Mix and the top half with Professional Germinating Mix. Seedlings get the benefit of the fine texture when they’re very young, and as they mature, they send their roots down into the Transplant Mix. It saves a little money and the seedlings don’t seem to mind.)
(Hint #2: When I’m transplanting seedlings into larger pots, I’ll often mix in some worm castings or compost. This provides valuable nutrients and also helps prepare the plant for garden soil. Seedlings that are 6 or 8 weeks old have had time to develop a tolerance to the naturally-occurring bacteria and fungi in compost.