Home & Garden: Seed-Starting Mixes

Whether you’re a veteran gardener with a greenhouse and many years of seed-starting experience or a novice who’d like to try raising a few select plant varieties from seed, a key to raising robust plants is in your starter mix. You might ask, why do we use seed-starting mix at all, if plants are ultimately going into the garden; why not use garden soil or potting soil?

The answer is that out there in nature, seeds fall on top of the soil, often into leaf mold or other porous materials. Rain falls and is drained away and warmth from the sun gets the plant to send up leaves and send down roots. But for raising a seed inside, in the heated, ultra-dry winter air, garden soil is too dense and compact and can get hard and crusty after watering. It’s also chock-full of weed seeds and rife with microbes that, while not a problem out in the wide world, can cause problems in an indoor environment without natural checks and balances.

A seed actually contains everything it needs to sprout: just give it water, warmth and sunlight. But once it does sprout, its roots are delicate and vulnerable, so a sterile, fluffy, soil-less mix that retains moisture yet drains well, allowing plenty of air around the roots, gives those early seedlings their best start.

The only drawback to using a seed-starting mix is that once your seedlings are on their way, they need potting up into larger containers before it’s time to plant them out. For those new to raising their own seedlings, learning that there’s a second, messy round of transplanting to go through before your plants go out into the garden is an unhappy surprise, but having larger containers with lots of root material around the seedlings makes transplanting into the garden much easier, for you and your plants. Further, if you have to hold off on setting plants out, they have plenty of nutrients to hold them over for a while.

Because you don’t need a large amount of starting medium, buying a seed-starting mix is relatively inexpensive. But you can also mix up a batch of your own, which is more economical and allows you to know exactly what’s in it. To mix up a batch, use a large bin with a snap-on lid or a five-gallon pail, depending on the quantity you need, and start with two parts of compost. The compost will slowly release nutrients into the mix, helping to feed seedlings as they grow. You can use your own garden compost, or buy some, but your own compost will have to be pasteurized first, by baking it for a while in the oven. To pasteurize, take a large aluminum baking pan and cover the bottom with three to four inches of compost, insert a meat thermometer in the center and place in a preheated 200° oven. Once the center reads 160°, bake for 30 minutes. Cool the compost thoroughly before using, then break up clumps with your hands or sieve it to get a fine texture. This is obviously a messy undertaking, and I do recall that the first and only time I did this the odor of baking compost permeated the kitchen, so feel free to buy your compost.

Next, add two parts coir, which is fiber that is extracted from coconut husks, making it a sustainable, plentiful alternative to peat or peat moss. I love peat moss and used it for years, but it was a guilty pleasure because in my heart I knew that extracting peat damages fragile bogs that take hundreds of years to regenerate, while coir is abundant. Coir comes in a block that you rehydrate by soaking it in a bucket with water until you can easily break it apart.

Finally, add one part perlite, which will both lighten the mix and improve its air content. Perlite is is a mineral, a form of volcanic glass that is mined all over the world. It’s those tiny white specks you see in most potting mixes, and it serves to improve soil aeration, keeping it loose and well-drained. If you prefer not to use perlite, sand is a fine substitute, but it’s much heavier.

Once you’ve combined all your components, you can pop the lid on your container and store it in a cool dry place. When you’re ready to plant, moisten the mix before you use it, so it’s damp but not sodden. You can use anything that will hold the mixture — plug trays, plastic pots, seed flats, recycled containers, paper pots — the possibilities are myriad and we’ll go over them in another article. Whichever containers you choose, gently press down your seed-starting mix as you fill them and be sure to fill them at the corners. Since this mix isn’t easily compacted, don’t hesitate to firm it down so there’s enough mix for roots to expand.