Blend Your Own Seed-Starting Mix

Get better results + save money by learning how to make your own potting soil.

January 25, 2016

April Johnson, landscape and greenhouse coordinator at the Rodale Institute near Kutztown, Pennsylvania, grows literally thousands of organic vegetable, flower, and herb transplants every year. (It's a great way to get the garden going; you should try Starting Seeds On Your Kitchen Table this year.) Many of her seedlings end up in the Institute’s production and display gardens; others are sold to local gardeners at two spring fundraisers. After many years of experimenting with recipes for indoor seed-starting mixes, Johnson has settled on this general formula:

4 parts screened compost
1 part perlite (a mineral available at most garden stores)
1 part vermiculite (another mineral available at most garden stores)
2 parts coir (coconut fiber)

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Related: Earthworms Are Nature's Plow

To keep the dust down, lightly moisten the ingredients before blending them thoroughly in a dishpan or wheelbarrow.

This mix strikes a balance between moisture retention and drainage, both of which are necessary for seedlings. “Regulating the moisture is key,” Johnson says. “It’s easy for the soil to stay too wet, and that can lead to damping-off.” Damping-off is a fungal disease that causes newly germinated seedlings to topple over and die. Some flower seedlings—Johnson mentions pansies, snapdragons, Gem marigolds, and lisianthus—tend to be more sensitive to too much moisture. For those, she makes a special batch of the mix, using less compost and replacing coir with peat moss. Sphagnum peat moss and perlite tend to lighten the mix and allow it to drain more quickly. Compost, vermiculite, and coir increase moisture retention.

The compost in Johnson’s mix is made mostly from shredded leaves and other garden debris—but she avoids any organic materials that might introduce weed seeds (still not making your own? Learn The Easiest Way To Compost). Having compost in the mix means that seedlings rarely need to be fertilized until they are moved outdoors to the garden; the compost provides a constant mild feeding. Compost also counters the natural acidity of peat moss. In mixes that don’t include compost, add ¼ teaspoon of lime for every gallon of mix.

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