Choosing the Right Organic Seed-Starting Mix


If you’re a first-times seed starter, finding the right seed-starting mix can be a challenge. You need a seed-starting medium that will hold water well and be loose enough that roots can get through it, but have enough structure that the roots can grip it. And it needs excellent drainage; too much water and your plants may suffer from fungal disease.

While long-time gardeners often mix their own seed-starting medium, we strongly suggest you go with a commercial version for your first couple of seed-starting seasons.

Seed-Starting Mix Components

Choose something labeled a “seed-starting mix.” Seed-starting mixes are sterile, so your seedlings won’t pick up funky diseases from the planting medium. Most are made from a light-weight mixture of peat or coir, finely ground compost, and vermiculite, which allows the mix to hold more moisture and oxygen.

Potting soil is not the same thing as seed starting mix. Potting soil is coarser, for one thing. And potting soil may not be sterile, which makes your seedlings more vulnerable to disease.

Seeds and coir seed start mix⎜ Spotts Garden Service

We start seeds using coconut coir, often available in a dehydrated block like this one.

Fertilizer vs. No-Fertilizer

Seeds use their own energy to germinate, but once they’ve used up their store, you’ll need to feed them.

Because we use straight coir to start seeds (see below), our seed-starting medium doesn’t contain nutrients. We use organic liquid fertilizers like fish emulsion or kelp meal to feed them once a week. Feed at half the concentration suggested on the container.

Seed-starting mixes that contain compost (like this one from Gardener’s Supply Company) will supply most of the food your seedlings need for about three weeks. After that, you’ll need to fertilize them.

Some seed-starting mixes have fertilizer in the mix. If you’re planning to buy one with fertilizer, be sure to look for one certified organic (look for the OMRI label). We prefer not to use mixes with fertilizer, since we like to control how much food our seedlings get.

Peat vs. Coir

Mixes have at their base either peat (mostly sphagnum peat moss in the U.S.) or coconut coir.

Peat-Based Mixes

Many seed-starting mixes have peat as their base. But peat bogs are fragile ecosystems that sink a lot of carbon, filter water, and provide huge biodiversity. Conventional harvesting of these peatlands is done in a non-sustainable manner.

However, managing peat bogs as a renewable resource is gaining ground. If you’re going to use a peat-moss-based product, look for one that comes from a Veriflora Certified Peatland Product grower, such as Sun Gro.

Coir-Based Mixes

We prefer to use coconut coir for our seed-starting medium. That’s right; the same stuff used to make door mats can be used for seed starting.

We particularly like Gardener’s Supply Coir Seed Starting Soil Mix. One dehydrated block is equal to 10 quarts, so only rehydrate what you’ll actually need; just break off a hunk of the block and drop it into a bucket of water.

Start Those Seeds

Now that you’ve chosen a seed-starting medium, check out our guide to starting your own seeds.

Amy graduated from DePauw University with a degree in physics, a lifelong love of theatre, and a problem-solving style that combines the approaches from both those fields. A Master Gardener and long-time communications professional, Amy conducts gardening seminars and blogs about gardening in addition to her work with Spotts.

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