The reasons for growing your own seedlings are many, and making your own DIY seed starting mix is a good way to reap even more benefits. Starting your own seeds not only allows you to grow a greater diversity of varieties, it also saves money, gives you more control, and is downright fun. If you’re still buying commercial seed starting mixes, now is a great time to try making your own. This article explains the ingredients you’ll need, how to mix up a batch, and even offers a few of my favorite seed starting soil recipes.
How is seed starting mix different from potting soil?
There are many different types of growing mediums available to gardeners. General potting soils (also called potting mixes) don’t typically contain any actual topsoil or garden soil. They are soil-less blends of other ingredients, and they often include fertilizers. Which growing medium to choose depends on what you plan to grow. Mixes with larger particles are best for potted perennials, tropical plants, and shrubs. Medium textured mixes are best for annual flowers and vegetables. Cacti and succulents prefer to grow in mixes with a coarse and sandy texture. (You’ll find recipes for making your own homemade versions of all of these soils in this article.)
For seed starting, however, we are looking for different traits. Primarily, we want a growing mix with a finer texture/small particle size, and good drainage and aeration. The fine texture of seed starting mixes is important to ensure germination is easy, even for the smallest of seeds. Potting blends with larger chunks can restrict seed germination.
In addition, growing mediums made for seed starting should contain no fertilizers. No nutrients means no risk of fertilizer burn on tender new roots or leaves. All seeds consist of the embryonic plant, the seed coat, and the endosperm which, like an egg yolk, contains all the nutrients the newly germinated plant needs until it is a few weeks old. Seed starting mixes don’t need to contain fertilizers because every seed already contains what it needs.
Why make your own DIY seed starting mix
There are many commercial brands of seed starting mix available at garden centers, so why would you want to go to the effort of making a DIY seed starting mix when you could just grab one off a store shelf? Well, for starters, the expense of commercial mixes can be prohibitive if you plan to start your own seeds. An 8 quart bag of good-quality mix costs between $15 and $20 USD. Cheaper blends may not be sterile and may even contain weed seeds. Sterility is important since young seedlings are more susceptible to fungal diseases and other pathogens.
Mixing your own seed-starting mix also gives you control over every ingredient, the texture of the final product, its aeration, and drainage. You can make about 40 to 50 quarts for $35 to $45 USD, depending on where you source the ingredients and the quantity you buy of each (larger bags are typically less expensive per quart). Individual ingredients will last for many years, if kept in a bag or bin in a dry location. Below you’ll find a few different DIY seed starting mix recipes, each with a different balance of these traits, but all suitable for growing your own seeds indoors. Which one you choose is up to you.
Ingredients for homemade seed starting mix
Let’s first discuss the ingredients used in the DIY seed starting mix recipes below. Each has unique traits and brings different benefits to the blend.
Sphagnum peat moss
Peat moss is mined from a unique ecosystem known as a peat bog. Here in North America, peat is harvested from bogs in Canada and certain regions of the northern U.S. Some consider peat to be a natural renewable resource, but many others don’t. It takes generations for bogs that are restored after the peat is harvested to regenerate. For this reason, other gardeners shun peat moss and opt to use alternative ingredients. Peat moss has a very fine texture, especially after it’s been sifted. Once wet, it retains moisture and slowly releases it to plants, but once it has dried out, it’s difficult to wet again. Many peat moss products contain a wetting agent to keep the moisture levels more consistent.
Coco coir (a.k.a. coconut coir, coir fiber, coconut peat, or coconut fiber)
Gardeners who opt not to use sphagnum peat moss in their DIY seed starting mix have another choice. Coco coir is the fiber of coconut husks. Here again, many consider this a natural renewable resource. However, coir is not without its downsides, too. A lot of clean water is used to process it and it needs to be moved great distances to travel from its point of creation to gardens in North America. Plus, it’s more expensive than peat moss. Coir has a light, peat-like texture and most often comes in compressed bricks that expand when soaked in water.
This volcanic rock deposit is extracted from mines and heated until it expands. After heating to super-high temperatures, the flakes expand into little accordion-like structures. Vermiculite is great at moisture retention, holding on to nutrients, and its particles are fine and lightweight, making it a great choice for DIY seed-starting mix recipes.
Fine grade perlite
Perlite is another horticultural product that’s created when small pieces of volcanic glass are heated to temperatures as high as 1800°F. At that point, they expand and pop, creating little Styrofoam-like particles. Perlite comes in several grades from fine to super coarse. For seed starting, the only one you should use is fine grade perlite so the larger particles of the coarser grades don’t inhibit germination. Perlite improves a mix’s drainage ability and aeration. Here’s more on the differences between vermiculite and perlite.
For one of my DIY mixes, I incorporate coarse sand (sometimes called builder’s sand or all-purpose sand; not play sand). Sand improves drainage and creates a heavier mix that’s great for starting larger seeds.
All of these products are available from your local nursery, a local garden center, or various online sources. Note: Do not substitute coarse perlite for fine perlite in the recipes below. If you can’t find fine-grade perlite, choose a recipe that does not include perlite.
DIY seed starting mix recipes
Here are some of my favorite recipes for a DIY seed starting mix. The first two are the basic blends we used at the nursery where I worked for many years (I probably sowed millions of seeds into these blends!). The third and fourth are mixes I use in my garden now, depending on what I’m growing.
Recipe 1: Basic Seed-starting Soil Mix
1 part sifted peat moss
1 part vermiculite
Recipe 2: Premium Drainage Seed Mix
1 part sifted peat moss
1 part vermiculite
1 part perlite (fine grade only)
Recipe 3: Peat-free DIY Seed Starting Mix
2 parts coco coir
1 part vermiculite
1 part perlite (fine grade only)
Recipe 4: Seed Starting Soil Recipe with High Drainage
2 parts sifted peat moss or coir fiber
2 parts vermiculite
1 part coarse sand
This is the mix I often use for larger seeds.
How to blend the ingredients
Mixing soilless seed starting blends is easy. Use a one gallon bucket as your measure if you plan to create moderate amounts. For large amounts, use a 5-gallon bucket as your “part” measure. For smaller amounts you can use a 4-cup or even a 1-cup measuring cup to represent a single “part”. Only mix as much as you plan to use within the next month or two.
I like to create the blend in a concrete mixing bin, but a clean wheelbarrow, large plastic storage bin, or another container will work, too, depending on your preference. The instructions are simple for creating a homemade seed starting mix.
- Wear a dust mask. All of these ingredients create airborne fine particulates that you don’t want to breathe in. They can irritate your lungs.
- Measure out the number of “parts” needed for each ingredient and dump them into the mixing vessel. Alternate between ingredients if you’re mixing a large batch.
- Add enough water to create the moisture level of a wrung-out sponge. Add a little water at a time, mix the ingredients, then add more water as necessary. You can use a trowel, your hands, or a small shovel to mix it.
- Continue to stir the seed starter mix until all the ingredients are very well blended.
Using your homemade seed starting soil
Now it’s time for the fun part – sowing the seeds. You can store your DIY seed starting mix in a plastic bin for a few weeks, but don’t secure the lid tightly or mold can develop on the soil. Leave it open to the air but add more water if it looks like its going to dry out completely. Use within a few months (remember, the individual, unwetted ingredients will last for years, so only mix as much as you need at any one time). When you’re ready to sow your seeds, add plastic 6-packs or other planting containers into a 1020 nursery tray and fill them up with your seed starter mix. Any of these mixes can also be used for soil blocking, too.
Then it’s time to plant your seeds. After your seedlings germinate, grow, and go on to develop their first true leaves (typically the second set of leaves they grow), it’s time to transplant them into a larger container. At this stage, they require a different potting soil blend that contains nutrients.
Remember, when your seedlings run out of the food inside their own seed coat, it’s time to provide them with an outside source of mineral nutrition. Here is the soil mix recipe I use to up-pot my transplants. For this transplanting soil mix, the nutrients come in the form of finely screened compost and granular organic fertilizer.
DIY transplanting mix recipe
2 gallons sphagnum peat moss or coir fiber
2 gallons vermiculite
1 gallon finely screened compost (adds beneficial microbes and nutrients)
3 TBSP pulverized garden lime to neutralize the pH (if you use peat moss; omit if you use coir fiber)
2 TBSP granular organic fertilizer
3 TBSP worm castings
This transplanting mix can be blended in the same manner as your DIY seed starting mix. You’ll notice that the amounts are specific volumetric measurements, not “parts”. This is because of the fertilizer. You can halve the recipe, but do not use a smaller or larger bucket or container to measure or you’ll risk over- or under-fertilizing your transplants.
It’s a DIY winner
As you can see, it’s easy to blend your own premium seed mix. I think you’ll enjoy having more control over the process and saving some extra pennies (so you can buy more seeds, of course!). This article will even fill you in on when and how you can reuse potting soil.
For more on starting your own seeds, please visit these articles: