Seed Starting Mix: DIY Options And More

As you prepare for the upcoming growing season, you gather together all you need to sprout seeds at home. You’ll go for your seed catalog, your garden draft, your starting station, and most importantly, your seed starting mix.

Chances are if you’ve done this a couple of times you have a set of seed starting mixes you’ve tried. Maybe there’s one you know will work. But the seed starting mix you choose can be adjusted when you decide to grow new and different plants. 

So, what goes into a good seed starting mix recipe? And what do you need to consider when starting seeds and amending your own mix to suit it to unfamiliar plants? Can’t you just use a potting mix? Keep reading, and you’ll learn to make your own DIY seed starting mix, or you’ll learn how to modify yours to grow your own seed.

What Does a Good Mix Need?

A really good seed starting mix is not the same as potting soil or potting mix. Potting soil is not as fine as seed starting mix. A good conventional or organic seed starting mix has finer particles than potting soil because it’s designed to allow easy root growth through the medium and to keep germinating plants moist. Potting soil often also contains fertilizers, and seed starting mix may not as each seed contains all the nutrition it needs to sprout. But that doesn’t mean they can’t have fertilizer too, particularly if your own seed starter is going to keep the plant going until it’s time for it to get into the ground!

While there are plenty of non-soil-related reasons as to why your seeds aren’t germinating, sometimes the seed starting mix is the culprit as it might be too muddy, or too chunky, etcetera. Depending on when and what you want to germinate, you may need to adjust your mix slightly. So before you start your seedlings, let’s discuss a couple of basic starting mix options that anyone with access to local garden centers can make. 

Basic Starter Mixes

There are many different recipes, all claiming to be the best seed starting mix. However, you can use what materials you have on hand, so there’s no reason to feel as though you can’t have a good starter to get started with!

Let’s say you have a bag of premade starter mix on hand already, but you’re starting something that will require some time before they’re transplanted out in the garden. You can simply amend your mix to include a little more nutritive material. For instance, an 8-quart bag of seed starting mix blended with about a half quart of worm castings and a sprinkling of azomite should be nutrient-dense enough to keep the plants going for a while.

Most high-quality potting mix contains a lot of chunkier material. However, you can certainly run some of your preferred potting blends through a screen and remove the bigger bits, leaving yourself with a perfectly workable seed starter, too!

But what if you want to make your own homemade seed starting mix? There are two different recipes for a good, basic starter we’ll share with you.

Basic Peat-Free DIY Seed Starting Mix (the Epic Starter): 1 part coconut coir + 1 part pumice or perlite + 1 part compost (this part can contain up to half its volume in worm castings) + a sprinkling of azomite.

Basic Peat DIY Seed Starting Mix: 1 part peat moss + 1 part perlite or pumice + 1 part compost (again, this part can contain up to half its volume in worm castings) + a sprinkling of azomite.

Not only are these starter mixes great for sprouting seeds, but they are inexpensive. All the elements are easy to blend, and each part has a specific role to play. 

In your blend, the ideal goal is to have about ⅓ moisture-retention ingredients (such as coir or peat moss), ⅓ drainage ingredients (such as pumice or perlite), and ⅓ fillers like compost or fertility-based ingredients like worm castings. Either of the starting mix recipe types we’ve listed above are versions of a soilless seed starting mix recipe. Seed starting in these mixes will produce healthy plants, planting seeds in them is easy, and they can easily be up-potted later to larger containers.

What if you want an organic seed starting mix? That’s just a bit trickier, as it means you need to source OMRI-rated ingredients to work into your blends. But it’s certainly not impossible to do, if that’s your preference. You will get healthy plants either way!

Seed Starting Mix Additions

Certain seedlings and modes of seed germination may require adjustments to your DIY seed starting mix. Germinating seeds that won’t be planted into garden soil, and instead will be planted in potting soil, requires nutrients that will make the potting mix an appropriate home for the plant as you’re not getting the best soil minerals and nutrients from soilless blends. Let’s discuss some of the potential additions to a DIY seed starting mix that will assist plants that end up in a potting mix rather than garden soil.  

Coconut Coir

Coconut coir is everything in between the shell and the outer coating of the coconut husk that has been processed and prepared for growing mixes. Coconut coir pods are used to grow seedlings, especially in hydroponics. In this case, there is no DIY seed starting mix – just coco coir pods. It can also be added to starter mixes to improve moisture content. It’s a generally renewable resource, although it undergoes many processing stages that may have ecological impacts. Coco coir is also insect neutral due to its intensive processing, whereas traditional seed starting soil or mixes may not be. 

While they’re great for hydroponic growing, coconut coir pods are neutral and don’t have any added nutrients for long-term growth. While coco coir promotes retention of moisture, it does drain quickly and needs moisture replenishment often. It can also be expensive. So if you plan to add coir to your seed starting soil, know that it will simply help retain moisture. 

Sphagnum Peat Moss

So, we’ve discussed peat moss as a base ingredient for your own DIY seed formula. But did you know that additions of peat moss work similarly to coco coir? The material comes from peat bogs, where mosses break down in the absence of air. The result is a highly absorbent material that does not compact over time. It gives potting mixes and seedling mix a little extra acidity, making it great for blueberries or camellias – any plants that enjoy acidic soil for growth. It’s sterile and doesn’t contain pathogens or weed seeds that other media might contain. 

But good peat is hard to find. Like coir, it doesn’t contain any additional nutrients that will assist in long-term plant growth. It requires alkaline additions if you’re making a seedling mix for plants that don’t enjoy acidic soils. And finally it’s very slow to regenerate in the bog, and therefore, it isn’t the best choice for growers who want to use environmentally friendly materials. 

Worm Castings

Worm castings are worm poop, plain and simple. They are sold in bags, or they’re incorporated into a potting mix recipe from a vermicompost bin. Sometimes worm castings are pure worm poop, and sometimes they are accompanied by the materials present in a vermicompost bin. In the latter case, they have composted worm bedding and leftover food bits. For a seed starter mix, pure worm castings are better because they’re more neutral than currently decomposing material. Because the worms have consumed matter and excreted it, it won’t burn plants or delicate seedlings. They contain a wealth of beneficial microbes useful in longer-term growing situations. They increase plant yield and protect soil from diseases. They improve the soil over time and hold a ton of moisture. 

You will require other nutrient content in a DIY seed mix because they don’t contain a ton of nutrients. They also don’t stimulate root growth, which might be what you want in a homemade seed blend. Finally, you won’t want to use more than ⅕ castings in your seedling or potting mix because it can bog down the soil and make it too moisture-retentive, because castings can hold up to ten times their own weight in water. 

Blood Meal

Blood meal is an inert powder made from dried animal blood. It’s an excellent amendment for potting soil that contains high counts of nitrogen. Nitrogen supports healthy root and leaf growth. Blood meals also improve soil structure over time, making it a great addition to regular potting soil or leftover mix. It visibly changes established gardens, making them lusher, and it can do the same in a seed tray too. It’s a very cheap amendment that can be found in your local garden center. 

Because blood meal is high in nitrogen, too much will prevent the development of fruits and flowers. It also attracts insects and mammals that may smell the blood and come to check out your seedlings if they are left unattended. 

Bone Meal

Bone meal is a fine-grained powder made from bones from slaughterhouse byproducts. It supplies phosphorus to potted plants or seedling mixes. It is used to promote fruit and flowering, and the developing root system needed when transplanting seedlings. It also provides pest resistance to plants and promotes healthy growth.

This meal can’t be the only addition to your growing medium as it only supplies ⅓ of the NPK (namely, the P). Growing plants with added bone powder is great, but it takes time for the meal to break down. Like blood meal, it can attract pests, and its effectiveness is reduced in alkaline soils.  

Feather Meal

Feather meal is another one of those amendments that provides seedlings and mature plants with nitrogen. It is ground feathers taken from poultry that have been processed for food. This fine powder is excellent for green vegetative growth, and for promoting the anaerobic heat that helps compost break down. It also improves soil quality over time. 

The only downside to using feathers to start seedlings is they could overwhelm the mix with nitrogen, and they could be sourced from a place where diseases known to hang out on poultry exist.  

Miscellaneous Amendments

If you’re growing tomatoes, or peppers, you may find there are plant-specific foods that can help you grow seedlings. Tomato food is very common and contains high amounts of slow-release phosphorous that assists in flower and fruit production. Kelp meal is a great addition for fruiting plants to improve the flavor of your yield and increase flower and fruit production. It also assists in extending the shelf life of certain vegetables. It’s renewable too! 

Azomite is a great way to add some micronutrients to your soil. This rock dust adds just a bit of a kickstart to your plants.

Diatomaceous earth is used in seed mixes to keep out soft-bodied pests as the silicate structure cuts up their bodies and causes them to dehydrate. This also helps reduce the spread of fungal diseases that come from contact with fungus gnats, as the gnats are eliminated by the diatomaceous earth. 

If you’re working hydroponically, you’re familiar with organic and synthetic fertilizers that come in liquid form to provide nutrients to plants. These can also be added to soil as well. 

Garden lime is a great addition for mixes that contain a lot of acidity, as it raises the pH of whatever media it is added to. It’s often added to native soil where the content is sandy or silty and acidic. 

Starting Mix Preparations

Now that we’ve discussed the best soilless mix for starting seeds, and we’ve explored some of the amendments you can add, we’ll talk about getting ready to plant seeds. You’ll want to gather your materials for starting seeds inside: the number of seeds you need, starting containers, larger pots with drainage holes, your seedling heat mat, and your grow lights if you’re growing indoors. 

If you’re incorporating things like compost, or even a proprietary blend of soil as a base for your mix, sift them through a ⅛ inch screen to ensure there are no chunks and the mix is light and fluffy. Large particulate will block some seedling germination and prevents the healthy growth of roots. 

Consider the types of starting pots you’d like to use. Peat containers allow you to plant the entire container in the soil, protecting the roots from transplant shock. Our Epic 4 and 6-Cell Starting Trays do the same by allowing you to pop the entire start out, soil and all, and plant it directly into the garden bed or container. Soil blocks are another option and do the same as our 6 cell trays. The only downside is they’ll need to be formed somehow and can dry out easily. 

Starting Modes

While most seedlings don’t have to become a potted plant for a while, plants like peppers and tomatoes may benefit from an up-pot before the final transplant. Tomatoes and peppers will need a good potting mix to be planted in after they develop true leaves. The best potting mix will always consider the needs of the plant first. And it will often contain the same seed blend you made for your other plants, as a base. A good blend could reuse seed starting mix, with added worm castings and sand for drainage. Added tomato or pepper food will support long-term growth as well. 

When you make starter blocks, you’ll have to add garden lime to ensure excess water isn’t compromising the form of the block. The lime also balances any fertilizers you may have added, like blood meal and bone meals. A wrung-out sponge is perfect for herbs that don’t like as much moisture as other seeds. Base whatever pot you choose for starting seeds indoors on the destination and needs of the plant, and you’ll be successful in your seed starting adventures. 

Also, note that fast-sprouting seeds that haven’t sprouted in the past few days after a couple of weeks may not germinate. Plant more seeds in this case!

Frequently Asked Questions

Q: What is the best starting mixture for seeds?

A: You can purchase commercial blends, but the DIY equation provided in this piece is just as good! And it’s cheaper.

Q: Can you start seeds in potting mix?

A: You can, but you’ll need to provide amendments. See above. 

Q: Do I need seed starting mix?

A: While you can start plants in potting mixes, a seed starter mix is best because it helps roots grow in preparation for transplanting.