How to Make DIY Seed Starting Mix: The Best Recipe

Ready to grow some big healthy seedlings? Come learn how to make DIY seed starting mix, perfect for sprouting all your favorite vegetable, herb and flower seeds. Making your own seed starting mix is really easy to do, and gives you the utmost control over the ingredients. It’s also a great way to save money if you grow a lot from seed like we do!

Truth be told, most seed starting mix recipes are pretty simple and similar, but our tutorial is “the best” (IMHO) because our adorable furry garden assistants are going to teach you how to make it! Plus I have plenty of tips for success to get the best results possible, like how to rehydrate coco coir or screen the mix to reduce unwanted lumps.

RELATED: Don’t miss our complete Seed Starting Guide with tips on how to grow the most healthy, robust seedlings possible. Also learn when to start seeds indoors with planting calendars for every zone!

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What is Seed Starting Mix?

• Seed starting mix is a soilless mixture made specifically for germinating or starting seeds. It’s usually composed of peat moss (or coco coir) and perlite, and occasionally with a small amount of compost, wetting agents, or other ingredients added.
  
  
  
• Seed starting mix has an extra fine and fluffy texture, perfect for tiny sprouts and roots to grow in with ease. On the other hand, something like potting soil is too chunky, heavy, and thick, which can smother or stunt new fragile roots.
  
  
• It’s relatively low in nutrients since seeds and young seedlings don’t need rich soil to grow. Rather, they’re very sensitive to fertilizer burn.
  
  
• Seed starting mix should have good moisture balance, offering both excellent drainage as well as long-lasting water retention so that the mix doesn’t yo-yo between soggy and dry.
  
  
• Most often, it’s used in seed starting trays or small pots to start seeds indoors (or in a greenhouse). However, you can also use this seed starting mix recipe for soil blocks.

Benefits of Making Your Own Seed Starting Mix

• As a gardener, it’s a great skill to know how to make your own seed starting mix – especially if you have a hard time finding a decent pre-made bagged mix locally. Our local availability fluccuates alot
  
  
• If you start a lot of seeds each season, you can potentially save a lot of money by buying ingredients in bulk and mixing your own big batch! For instance, we can only find 8-quart bags of pre-made seed starting mix (fairly small) at our local nursery.
  
  
• Making your own mix gives you the utmost control over the ingredients and quality. For instance, some gardeners really don’t like peat moss – so you can simply use coco coir in this recipe instead. Otherwise, it’s virtually impossible to avoid peat in bagged mixes.
  

Sometimes we buy pre-made organic seed starting mix (and add a little compost), and sometimes we make our own!

Ingredients and Recipe

• 60% (or 4 parts) peat moss or coco coir, pre-moistened (damp but not wet) and free of large chunks. See pros and cons of both options below. Coco coir usually comes in condensed dry bricks. Each 1.4 pound brick will expand to about 2 gallons of damp coco coir. Rehydrating tips to follow.
  
  
• 30% (or 2 parts) fine perlite or vermiculite. For the best results, use the smallest perlite size possible – sometimes referred to as grade 1 perlite, which is about 1/8-inch or under 3 mm granules.
  
  
• 10% (or 1 part) worm castings or other well-aged finished compost. Worm castings are the best compost choice for homemade seed starting mix because: a) they offer really gentle nutrients that won’t accidentally “burn” your plants, and b) they naturally have the perfect small particle size. We usually use homemade worm castings from our worm compost bin, but you can also buy organic worm castings online. You want the mix to be as fine and free of chunks as possible, so if you use another type of compost, be sure to break it up and screen it well first. (See screening tips at the end of this post.)
  

To measure by “parts” simply swap parts for the measurement or volume you wish to use, such as: 4 quarts peat moss (or coco coir), 2 quarts perlite, and 1 quart worm castings or compost. Or, 4 gallons peat moss, 2 gallons perlite, and 1 gallon worm castings.

Peat Moss vs Coco Coir: Pros and Cons

You can use either peat moss or coco coir in this DIY seed starting mix. Both are fine, fluffy, and offer great moisture balance – ideal for seeds and seedlings.

However, peat moss is considered far less sustainable than coco coir. Though there have been increasing efforts for sustainable peatland management and restoration in recent years, it is essentially a non-renewable natural resource. You can learn more about the issues with the peat moss industry here.

On the other hand, comparison studies consistently show that peat moss is a superior seed starting medium and results in larger, healthier, faster plant growth when compared to coco coir (hence peat’s popularity). Peat is also slightly easier to work with, as coco coir often comes in condensed bricks that must be rehydrated and broken up before use.

Instructions

To make your own seed starting mix, all you have to do is combine the called-for ingredients in this recipe together and mix well. However, follow our tips below for the best results possible, including how to rehydrate coco coir. We like to make our seedling soil in a plastic tote, which makes it easy to mix and then neatly store to use later.

Pre-Moistening

Before combining all the ingredients, it’s best to break up and dampen the peat moss or coco coir slightly. Both peat moss and coco coir are hydrophobic when they’re dry, meaning they naturally repel water. So it’s always a good idea to pre-moisten your seedling mix before use – including before filling seed starting trays or sowing seeds. Otherwise you’ll have a really hard time watering your seeds.

The moisture also helps to reduce dust from the peat, coco and perlite – none of which you want to inhale, and also makes a mess!

Slowly add just enough water to slightly dampen the medium, NOT make it soggy. If it gets too wet, peat and coco tend to get clumpy, and it will be more heavy and difficult to mix evenly with the other ingredients.

How to Rehydrate Coco Coir

To rehydrate coco coir, add the condensed brick to a large tub or bucket and add a small amount of water at first. (Expect to use about 3 to 4 quarts of water for a 1.5-pound brick of coco coir.) Let it sit for 15 to 20 minutes, see how much absorbs, and then slowly add more water until it easily breaks apart and fluffs up. Remember, you can always add more – but it’s hard to take away excess moisture if you overdo it!

Screening to Reduce Clumps

At the risk of sounding like a broken record, the best seed starting mix is free of large clumps. Just one clump in the wrong place could smother a seed or prevent its roots from growing with ease! Peat moss and homemade compost are especially prone to having a lot of lumps and chunks.

If needed, you can improve the texture of your DIY seed starting mix either by screening the ingredients prior to mixing, or screen the finished mix afterwards.

We use a DIY compost screen that we made to harvest and sift worm castings from our worm bin. It’s simply 1/2″ hardware cloth attached to a wood frame with a catchment tote placed below. Put the contents on top, move it around to sift and work it through the screen, and then remove or break up any large clumps left on top.

That’s all folks!

Pretty simple, eh? Now it’s time to grab your favorite seed starting trays (or soil blocks) and go start some seeds!

I hope you enjoyed this lesson and have wonderful success with your new DIY seed starting mix. If you found this helpful, please leave a review below! Also feel free to ask any questions you have. Best wishes and happy growing!

Don’t miss these related posts:

• How to Start Seeds Indoors: The Ultimate Guide + Seedling Care Tips
• 7 Useful Seed Starting Supplies for Success Indoors
• How Long Do Seeds Last? (Seed Viability Chart)
• When to Start Seeds Indoors: Planting Calendars for Every Zone
• 13 Best Places to Buy Vegetable Seeds: Organic, Heirloom, Non-GMO