The Right Growing Mix for Seedlings

If you’re going to be starting seeds indoors this spring—under lights, on a windowsill or in a greenhouse—you 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 seeds—especially 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.

To learn more, check out our how-to articles: Seedstarting Made Easy and Seedstarting Tips.

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8 Responses to The Right Growing Mix for Seedlings

  1. Jen says:

    Why hasn’t Gardener’s started using a more earth friendly resource such as coco peat to replace peat moss, which is a non-renewable resource? I’m always pretty conflicted when I’m faced with the choice to buy something with peat moss in it. You do carry coco peat mulch after all.

  2. Hi Jen. I know our merchants have been considering the pros and cons of peat moss for awhile now. It’s one of the reasons we’ve begun offering the recycled coco fiber. I’ll get back to you as I get a better sense as to our plans going forward. – Kathy

  3. Anonymous says:

    Umm, sphagnum peat moss *is* renewable, by any reasonable definition of renewability. For every lb of peat moss harvested/yr, 100 lbs/yr are being added by natural processes. Residential and commercial developments destroy far more bogs than gardeners ever will. The Canadian Sphagnum Peat Moss Association has published materials that make a compelling case that sphagnum peat moss can be an environmentally responsible choice []. As a customer of Gardener’s Supply, I wouldn’t want well-meaning but misinformed feedback to preclude my ability to get peat-based products. Sounds like there is a certification program for peat products that ensure proper environmental practices are in place, and would like to see that information shared with customers. Disclaimer: I am neither Canadian nor named Pete, but in the right light my beard makes my face look a little mossy. Have no idea what a sphagnum is…sounds like something you cough up.

  4. Here’s an update from our merchants. All of the peat we use in our growing mixes comes from Canada. Though peat is a indeed a diminishing resource in Europe, the peat supply here in North America is quite abundant. Here is the URL for an article published by the Canadian Sphagnum Peat Moss Association: provides some background information about the issue.Though this article does come from inside the industry, our own research is in agreement with what’s stated.For seedlings, we haven’t found a better growing medium than sphagnum peat moss. We support the responsible use of this valuable natural resource.

  5. Chris Herren says:

    I have an indoor seedling problem. Two years ago we purchased several hundred dollars worth of indoor fluorescent grow lights, trays, lids, and such to save money on store-bought seedlings. We sowed seeds, fed them with 12 hours of light a day, hardened the seedlings for 2 weeks, transplanted them into the garden on the appropriate planting days, and they did absolutely nothing! They just sat there in the garden barely growing all season long and most did not produce veggies. We subsequently sowed the same seeds directly in the garden and they outgrew the indoor seedlings and produced normal veggies. What happened? The indoor seedlings were leggy & tall when we transplanted them into the garden, but they were also very green, leafy, and mold-free. I read through your info on lights & seedlings, but can't decide if it's a temperature, light, fertilizer, or “all of the above” issue.

  6. Hi Chris,

    Your seedlings should not be leggy if they've been grown under lights. Did you set the lights so they're close to the seedlings? The bulbs should be 3-4″ from the tops of your seedlings.

    Check out this article, which gives the details on growing healthy, stocky seedlings under lights.,default,pg.html

    -David Grist, Gardener's Supply

  7. Right on point … good article …

  8. Anonymous says:

    Chris…. make sure room temps aren't too high, get those lights close, and run 'em 24/7 if you want.

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