Make Your Own Seed-starting Mix
Between the veggies and herbs we grow on our urban microfarm and all the plants we offer to our small town Montana neighborhood, we start about about 500 of our own plants beginning in February. If we bought a $15 bag of seed-starting mix every time we planted out another tray, we’d spend hundreds on starting mix alone. So we make a LOT of our own. I’ll share my recipe in a moment, but first just a little background on where we’re growing and the challenges we face.
The height of seedling-starting season is concentrated in late March and early April in our region. Cold-loving plants are actually planted-out in our mini hoop house from mid-April on, and a few frost-tolerant varieties are directly planted in the unprotected garden beds in early May. We’re in Zone 4, gardening at just below 5000 feet, so our planting dates for everything else (tomatoes, warm-loving herbs, etc.) fall between June 1 to 5. We’ve seen hard frosts as late as June 12. But in recent years, it can just as easily be 90 degrees in mid-June. If we wait till, say, June 10 to plant out broccoli or cabbage, those plants will be hit so hard by too much heat within days of transplanting, that it will stunt their growth for the entire season.
This is NOT an easy place to grow one’s own food. Here’s how we pull it off:
Start plants indoors, as early as possible, but not so early that they’ll be root bound before plant-out.
Plant 2 to 4 seeds in every pot. 2 weeks after they sprout, thin down to the strongest plant in each pot. (Painful task, but we’ve got to do it!)
2 weeks before their respective transplant date, plant trays go outside to get a lengthy time hardening off under frost cloth. If you do a similar system, remember to check covered plants daily for watering needs.
Transplant into the garden as early as possible, always ready with frost cloth OR shade cloth!
With this approach, we’re able to select for plants that are doing the best in OUR micro-climate, instead of purchasing imported plants from a big box store. Store-bought plants may look awesome at time of purchase, but they’re likely accustomed to a way-too-idyllic growing environment and will fail to thrive here. (Though, if you don’t have time or room to start your own plants, the “next best” option is to buy plants from a local/regional Montana grower.)
For us, growing our own veggie starts means we make lots of seed-starting mix. First, let’s look at the most common recipe you’ll find anywhere online, and then I’ll share the adjusted blend that works best for me.
The most common seed-starting mix ratio that you’ll see in books and web resources looks like this:
8 cups peat moss (or coconut coir)
1 cup perlite
1 cup vermiculite
1 tablespoon Garden Lime (aka dolomite) – Used only if peat moss is the base for your mix (peat moss is a bit acidic and the lime/dolomite brings the pH to a more neutral level)
Keep in mind, this is a light weight mix that serves one purpose: it lets seeds germinate, root, and emerge with little resistance. Its main feature is that it’s lightweight and won’t cake or crust, which can prevent germination or pinch tiny sprouts. This is not a nutritive blend (the bagged versions sold in stores are not typically nutritive either). Once a seedling has “grown through” the nutrition present in the seed itself, it will need some liquid fertilizer. (When seedlings outgrow their starting pods, we also “pot up”: take the little seedlings out of their pods and place them in four-inch pots with a nutritious potting mix. But that step needs to wait for another blog post!)
All that said, I’ve found that the 1:1 ratio of perlite and vermiculite in the above mix doesn’t work so well for me. It doesn’t drain well enough, and it holds water a little too well, sometimes resulting in root rot. Let’s quick clarify what vermiculite and perlite are, then we’ll look at the Stone’s Throw Seed-Starting Recipe.
Vermiculite: a tan-colored mineral structurally altered by heat, known for its ability to expand in the presence of water and retain (hold onto) that water content
Perlite: a mineral product derived from obsidian which, after processing, yields tiny white pellets known for their ability to increase drainage in potting substrates and soils
In a seed-starting mix, vermiculite retains water while perlite promotes drainage. Healthy seed-starting requires and fine balance between both qualities. Sprouting seeds absolutely must remain moist. But too much moisture can eventually rot a seed or baby roots.
CAUTIONS: Both vermiculite and perlite are mineral products. Perlite is often confused with Styrofoam—it’s not that! It won’t leach toxins into your plant’s growing substrate. Both of these ingredients are mineral-based and safe. HOWEVER, they are incredibly dusty when you pour them. Do NOT inhale rock dust. Be wise. Work outside, wear a mask, and stand upwind of these ingredients as you pour and stir them. Additionally, I prefer to wear rubber coated garden gloves when handling this mix, as the vermiculite and peat moss tend to severely dry out my hands.
Now for that adjusted recipe. I find this light adjustment provides a better balance of water-retention versus drainage for young roots.
Stone’s Throw DIY Seed-Starting Mix Recipe
Ingredients:
8 cups peat moss
1 tablespoon “garden lime” (dolomite)
2 cups perlite
1 cup vermiculite
Directions:
1. Pour the above ingredients into a 5-gallon bucket. Remember to use a filtration mask and stand upwind while pouring and mixing perlite and vermiculite. Do not inhale rock dust.
2. Put on garden gloves and mix thoroughly with your hands. These ingredients mix best dry.
3. HOT TIP: Once the dry mix is evenly distributed, add water a few ounces at a time, mixing and kneading again after each splash of water. Keep adding water and mixing until the blend resembles a “crumble crust” for a pie. You want it damp, but not soaked or sloppy.
Use this mix right away, while it’s still damp. If you fill your starter trays while the mix is damp, you won’t have to fuss with trying to water the seed-starting cells with dry mix in them. The latter often results in the highly buoyant peat floating up and out of the cells! Then the seeds shift location, they sometimes don’t germinate well… and it’s a big mess. Wet this mix before using it. It’s important and will save you much heartache. Stated as one who’s made a few mistakes!
Happy planting!
A Note on Peat Moss: In our region, peat moss is far more available than coconut coir. However, we’re aware of the environmental impact of peat extraction. The product at our local garden nursery is harvested out of peat bogs in Canada and extraction is very hard on that delicate biome. Not good. We use more than two cubic feet every growing season—a lot for a single household and neighborhood plant-grower. We get it. This is not sustainable. But we’re still looking for a coconut coir provider who can get the product to us at a remotely reasonable cost.
Right now, in my region, peat moss is a fraction of the cost of coconut coir. We simply can’t afford coir. Using it as a primary ingredient ends up costing more than buying name-brand seed starting mixes.
Yes, please let us know if you’re aware of a coir provider that ships in bulk (two cubic feet minimum) at a reasonable rate. We are interested and would gladly share an affiliate link in a future post!