Starting seeds indoors but not sure what to fill the pots with? With hundreds of products on the market and some, unfortunately, inconsistent labeling, choosing the right material for sowing seeds can be confusing. For the best results, seed starting mix is the way to go, but what exactly is it? We’ll answer that, as well as an easy recipe to make your own.

How is seed starting mix different?

True seed-starting mix differs from potting mix or potting soil primarily in texture. While a bag of potting mix may commonly contain larger bits of bark or woody debris, seed-starting mix should be finer-textured to ensure better contact with the seeds, thereby helping transfer moisture to them to begin the germination process. 

Seed starting mixes need to be loose, almost fluffy, to allow plenty of air channels around the new baby roots while still providing access to moisture. Dense materials pack too tightly together, slowing drainage and air exchange.

While seed-starting mix doesn’t need to contain any fertilizer, many do, recognizing that most gardeners don’t wish to immediately transplant their seedlings and will grow them in the same material for several weeks. Unlike potting mix, seed-starting media should be low in fertility to avoid burning the fragile new roots of germinating seeds.

Off-the-shelf seed-starting mix is commonly described as sterile, but we don’t mean that in the medical sense of the term. In this context, we mean that the product has a much lower likelihood of containing harmful bacteria, fungi, and molds that can cause issues for seedlings after germination. They are also typically free of most weed seeds and insects.

What’s in seed starting mix?

Commonly, you’ll see that the main ingredient in seed-starting mix is either coco coir or peat, and sometimes both. Some gardeners view peat as being environmentally unsustainable. Coco coir, although made from a waste product, has its own environmental and social issues, so each gardener must decide for themselves. 

Peat and coir both serve to store water and provide the bulk of the structure. Most off-the-shelf seed starting mixes also contain vermiculite or perlite. Vermiculite also stores water, but due to its other properties, it helps aerate the medium and allows excess water to drain easily. Perlite (it’s an expanded volcanic rock, not Styrofoam) helps to keep the mixture light and airy. 

Seed starting mixes are usually sold in small bags, 8 or 12 quarts at a time, because you don’t need much to fill up a few 72-cell trays. It is more expensive than potting soil, and both organic and standard products are typically available. 

Fertilizers are not necessary for seed starting, but if you purchase a seed-starting mix without fertilizer, you will need to start fertilizing your seedlings a week or two after germination. While the seed contains all the energy needed to push out the new root and those first leaves, the young plants must gather what they need after that. Neither coir nor peat are a good source of plant nutrients. 

Why do we use seed starting mix instead of garden soil?

Seeds germinate in regular soil out in our garden beds, so why do we need to use something else when starting seeds indoors?

It is true that seeds will germinate when we direct sow them outdoors. And they’ll also do the same if you use garden soil when planting them indoors. It’s what might happen shortly thereafter that creates the problem, and the need for something other than garden soil.

Garden soil is much heavier, or denser, than potting mix or seed starting mix. Take an old yogurt container and fill it with potting mix, then fill another with soil from outside, and you’ll see the difference. That density translates to fewer air channels, slower drainage, and greater resistance for a seed to push through as it tries to break the surface. Outdoors, the channels and pores in soil are maintained by roots, soil microbial life (and worms!), and weathering processes such as the freeze-and-thaw cycle. Those factors are not present indoors. When we water frequently to keep the small soil blocks or cell trays moist for germination, the density can become even greater. It’s the same reason we don’t fill our planters with garden soil: it gets compacted and doesn’t support healthy plant growth. 

Soil from our gardens also harbors microbial life, some of which can be harmful to seedlings. Fungi and water molds that cause damping off, like Fusarium, Pythium, and Phytophthora, may also be present in garden soil, but are less common in a commercially prepared seed starting mix. Outside, the soil ecosystem and fluctuating conditions mitigate these problems, but indoor seed-starting conditions such as high humidity, low airflow, and dense sowing can increase the likelihood of disease. Better to start out with a lower chance of disease from the beginning.

Make your own seed starting mix

Full disclosure: I’ve started thousands of seeds in regular potting mix because it’s what I had on hand at the time, and I’m not always organized enough to purchase seed-starting mix ahead of time. It’s not ideal, mostly due to the larger or more coarse texture, but it works. When I make my own seed starting mix, I use this recipe.

1. You’ll need one part peat or coir, one part vermiculite, and one part perlite by volume, not weight. For example, you might use 4 quarts of peat, 4 quarts of vermiculite, and 4 quarts of perlite to make a 12-quart batch. 

2. Add all three ingredients to a bucket and stir until everything is evenly distributed. Be wary of dust at this stage. Perlite, especially, can be dusty when pouring it, so do this step outside or wear a mask to avoid inhaling it.

3. When you are ready to use it, water your mix until it is damp but not wet. If you took a kitchen sponge, wetted it, then thoroughly wrung it out, that’s the moisture level you want for your seed-starting mix. Fill your plug trays or small containers, and you are ready to sow some seeds.

FAQs about seed starting mixes 

Can I start seeds in pure compost?

Yes, and in some parts of the world this is common. Remember, seeds don’t need soil to germinate. If you wish to use compost, use a high-quality, commercially prepared product. If using your own compost, consider sterilizing it first. While good compost is quite active with microbial life, not all of it is helpful to young seedlings. Commercial compost mixes commonly achieve the higher temperatures needed to kill off the bad microbes, but in home compost piles that can be a struggle. 

Is seed starting mix soilless?

Yes, seed starting mix is soilless. Potting mix, potting soil, and seed-starting mix typically do not contain any actual soil. Not only is soil heavy, but shipping soil across state lines is heavily restricted since it can transport pests and diseases. Look on the back of the bag for an ingredient list, just like you would find on a food label.

Can I use seed starting mix for soil blocks?

Seed starting mix works quite well for soil blocks. Its finer texture and lack of chunks make it less likely to cause problems. Some potting mixes containing bark or wood pieces cause issues with properly filling the blocks in the press or with firmly pressing them before ejecting. Like any medium, you’ll need to wet it quite a bit more to get it to form good blocks.

Does the DIY recipe above work for all veggies, herbs, and flowers?

Yes, the basic recipe of ⅓ peat or coir, ⅓ vermiculite, and ⅓ perlite works well for all indoor seed-starting needs. Remember to follow seed depth guidelines, provide a warm spot, and keep the mix slightly moist, and your seeds should pop up just fine.