Blend Your Own Seed Starting Mix

The countdown to seed starting is on!  Each week I’m trying to get a few little things in order, so when it comes time to start those tomato seeds, I’m ready to go. Last week I mentioned a basic formula for blending your own seed starting mix as an alternative to purchasing pre-made seed starting mix, and that is exactly what I checked off my to-do list this week.

Why Blend Your Own
In general, there isn’t anything intrinsically wrong with bags of starter mix you buy at your local garden center.  I’ve used them, they work well, and if you only need a small amount, the convenience factor definitely comes into play.  With a little bit of careful label reading you can find a good mix, but there are some advantages to blending your own:

  • When you blend your own seed starting mix, you know what’s in it.  Many commercial blends have added chemical fertilizers, which is important to know if you want to grow your vegetables organically.
  • You can make as little or as much as you want at one time.  A “part” can be an entire bag of material or just a few scoops.
  • If you’re going to use a lot of seed starting mix, it’s more economical.  The bulk of the mix is compost, something that you can either get for free, or very inexpensively at a garden center if your compost is frozen solid and under a foot of snow (I paid about $1.50 for a 40 pound bag).  The bags of the other materials will go a surprisingly long way and all will store for multiple seasons if some is leftover.

You may find that there are some slight variations in different formulas for starter mix (usually a matter of personal preference), but essentially seed starting mix needs to be fine, light, and regulate water well. The seed starting mix that I like to use is a blend of the following components, all of which are easy to find at your local garden center:

  • Compost: Organic matter will not only help the soil mixture breathe, but it will provide the kind of slow and gentle feeding that seedlings need.  If your compost contains larger pieces of organic matter that is not fully decomposed, you may want to sift the compost and use only the finer material.
  • Sphagnum Peat Moss: Decayed and dried sphagnum moss is called peat or peat moss.  Peat is lightweight, retains moisture, and adds additional organic mater to the mix.  Alternatively, you can use coir, a fiber from the husks of coconuts, as a substitute for peat.
  • Perlite: Perlite is a volcanic glass that has been heated until it expands or “pops” into small, lightweight kernels (the little white specks in a lot of potting soil mixes).  Perlite improves aeration and prevents the mix from becoming too dense or compacted as a result of frequent watering.
  • Vermiculite: Vermiculite is a mineral that is also heat-treated like perlite.  The fine bronze-colored flakes are important for proper drainage.  Vermiculite allows both air and excess water to freely flow through the soil mix.

To start blending your own starter mix, combine 4 parts compost, 2 parts peat, 1 part perlite, and 1 part vermiculite in a container for mixing.  Dig right in with your hands (yay! you’ll have dirt under your fingernails for the first time this season!) and start blending the mixture together until there are no longer pockets of one material and the starter mix has a consistent appearance (a good blend of all four components).  Dampening the mixture with a little water will help keep the dust down and make filling the starter pots a little easier.

That’s all there is to it!

Leftover blended seed starting mix can be stored to be used a later time.  It’s important to keep starter mix as sterile as possible, so allowing the mix to dry out excess moisture and storing it in a container with a cover will prevent bugs, molds, and mildews from making themselves at home in the soil mix and wrecking havoc on tender seedlings.  Likewise, leftover materials can also be stored for future use.  I use a large plastic tub to store the individual bags of peat, perlite, and vermiculite together, but I like to use any leftover compost in the garden (throw a handful in the hole before you transplant the seedlings – they’ll love it!).

If you do not have the storage space or the desire to save leftover seed starting mix from season to season, just work whatever is leftover into your garden soil.

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