All About Seed Starting Mix + Recipes

While you can easily buy prepackaged seed starting mix anywhere that gardening supplies are sold it can be very beneficial (and cost-effective) to make your own seed starting medium. There are dozens of recipes to be found online, but I’ve narrowed them down in this post as well as providing an in-depth explanation of each component. Knowledge is power, right?

What Is Seed Starting Mix?

Seed starting mix, or seedling mix, isn’t to be confused with potting soil. It is a specialized product which can contain little or no fertilizer and is formulated to encourage good germination and healthy root systems. Generally it’s made up of various parts peat moss, coco coir, vermiculite, perlite, and sometimes amendments like worm castings or compost, but not always. Many department store varieties will contain no nutrients at all, which serves a purpose for certain uses; as a home gardener you need to be aware which mixes have nutrients for your seedlings and which don’t, and what this means for seedling care as the season progresses.

For instance, the ever-popular and readily available Jiffy seed starting mix consists of nothing but fine peat moss. This means that later in the season, when you pot-up your seedlings or as they put on growth, you’ll need to either add liquid fertilizer or introduce some nutrient-bolstered soil to your containers. I prefer to start with good soil and not have to introduce liquid fertilizers, but it’s really personal preference.

So what are all these components, and what do they do?

Peat moss is the most common and plentiful ingredient in seedling soil. It consists of decomposed spaghnum moss which occurs naturally in peat bogs and is harvested in an advanced state of decomposition. There have been many sources recently who have claimed that the harvesting of peat moss from bogs in non-renewable, and its use is actually outlawed in some European countries; however there are differing opinions on this matter and Canada, the largest exporter of peat moss, claims that through proper management it is perfectly ethical to use as a gardening product. It provides a sponge-like, highly aerated medium for plants to grow healthy roots.

Coconut coir is used in the same way as peat moss, but was developed more recently as a direct alternative to spaghnum products. It is a byproduct of coconut processing and is made from the ground-up husks of the coconut shell. It functions well as a soil-less medium but it can be more expensive and more difficult to find than peat moss. Some readily-available consumer products will also have increased levels of salinity which may be toxic to seedlings, so will require proper washing to desalinate the medium. When purchasing coir do your research on the processing requirements and be careful ordering online.

Perlite is made of tiny bits of volcanic glass that are super-heated to expand like popcorn and become porous and light. They retain water but also provide extra drainage for your plants so it’s a wonderful addition to any potting soil mixture. The only thing I’ve learned about perlite over the years is to check the size of the product you’re using – for instance, the MiracleGro perlite, which is readily available in most hardware stores, has particles of perlite that are much too large to be used for micro soil blocks.

Vermiculite is a naturally-occurring mineral that is similar to mica or shale. It is porous and retains water, and also helps to retain nutrients that may otherwise be washed out by constant watering. While it contains no available nutrients in itself it is a useful addition to seedling mix, and it can even be used independently as a soil-less medium for rooting cuttings or starting seeds which require very specific growing conditions.

Do I Really Need It?

Technically seed starting mix isn’t required to start your own garden seeds, but it will be well worth the (small) investment when you see the difference it makes to your young plants. The main reason why you would want to use a bagged or homemade seedling mix (rather than potting or raised bed soil) is the presence of large debris – or lack thereof. Inclusions like sticks, bark, or even small rocks can inhibit germination and root growth no matter where these inclusions are in the decomposition process. They can stunt the fragile roots of your brand new plants, cover seeds and prevent germination, or inhibit the uptake of nutrients by your growing plants. And in the case of soil blocking, it can make using your equipment nearly impossible unless you have a fine mixture of medium. That being said, many gardeners use generic raised bed soil and still get good results. You will have to experiment and figure out what works best for your needs.

Another factor to consider is soil contamination. While this may not matter to your adult houseplants or vegetables it can literally mean life or death for your seedlings – especially if you’re using soil from your yard or garden patch. Because you’re often cultivating the most perfect growing environment for young seedlings, any pests or diseases that may be lying dormant in that soil in the dead of winter are just waiting to thrive in your germination station. On the advice of many seasoned gardeners not only do I purchase my seed starting medium new in a bag I also sterilize it with boiling water before using it. You want to do everything you can to avoid preventable afflictions like fungus gnats, damping off and other things.

My favorite line of products to use for seed starting has always been Black Gold Soils. I am not sponsored or endorsed by them in any way, they just have the best products in my opinion and are pretty widely available in my area. For my own recipe of seedling mix that I whipped up specifically for use with soil blockers, I start with their seedling mix and add a few other simple ingredients. This mix does contain some nutrients so it will get your seedlings past that four-week mark before you will need to fertilize or add additional soil when potting up.

Now there are many reasons why you would want to use a soil-less, sterile seedling mix. Whether it’s for root growth, for fine seeds which require exceptionally light soil, or for specific propagating methods, many people will use a medium that contains no nutrients at all. Most commercial, bagged seed starting mixes fall under this category, however it’s much more cost effective if you buy the components yourself. Here’s simple recipe you can mix up at home for a much cheaper alternative to store-bought formulas.

When it comes to soil blocking in particular, Eliot Coleman is a firm believer in the recipe for success. His article over on Johnny’s is a great read, and contains his famous recipe for soil block propagation. This is my preferred method of seed starting and if you’re interested in more details about the revolutionary technique you can read my post all about it over here. Since Eliot shared his recipe over on Johnny’s I’ve shared a snippet below.

And finally, this year I have started experimenting with adding rabbit manure directly to our seedling mix and have been really happy with the results. Unlike other types of manure, such as chicken, horse and cow manure, rabbit pellets are not “hot” and are therefore completely safe to add directly on top of your plants in the garden. So in the interest of better soil blocking I figured the addition of rabbit manure would act as the perfect fertilizer and an additional binding agent within the formula itself, and still be completely organic and all-natural. Starting this season I will be eliminating cups for my production seedlings and will be utilizing this recipe for all my starts, whether I keep, sell or share them.

Do you have a favorite seed starting recipe? Share it with us below!