Seed Starting Mix

I started my seeds about a month ago because, well, I can. Living in Arizona means I can kinda get started whenever I want as long as we don’t have an unusual late-season cold snap. Even then, it’s not like we’ll have below zero temps down here in the Valley of the Sun. Yeah, it’s awesome!

Because of the early start, I got to research and experiment with a few things well before Tim would be ready. So when Tim started ordering seeds and thinking about getting some things starting, he asked what kind of mix I used for starting my seeds in. I had originally used a commercial pre-mixed bag of stuff, but I wasn’t happy with it and looked for more DIY info. I pulled the materials together and made my own mix which I liked a lot better and outperformed the commercial stuff. I also learned a lot in the process.

Shortly after his order of seed packs arrived he let me know he was ready to dig in. It’s pretty straight forward stuff, but for those of you getting ready to start seeds for the first time, like Tim, there are a few key things you need to know about soilless seed mixes. Below you’ll get to know some of the key components to a good seed starting mix and a couple of recipes so you can make your own at home.

Seed Starting Mix Basics

Some Things You’ll Need

Perlite – Seeds need a loose, open media to grow in. If the material is to dense or compacted, the roots will struggle to develop and the sprout will struggle and possibly fail. A loose media means the roots can develop easily and spread quickly resulting in a happier, healthier sprout. A less dense media also allows better aeration, giving the roots much needed oxygen for better plant metabolism.

Vermiculite – One of the other key things we are looking for in a seed starting media is moisture control. Perlite does some of this by keeping the mix loose and providing aeration, but the absorbent qualities of Vermiculite increases moisture retention and provides nutrients for your seedlings as well. For seedlings that might be sensitive to high moisture medias and prone to damping off, vermiculite could be left out of the mix.

Sphagnum Peat Moss – Peat moss or Coconut Coir will play as the primary media in your seed mix. It’s a light, organic material that resists compaction and holds moisture well. Peat has been the go-to material for seed mix and garden soil conditioner for a long time. The problem with peat is its sustainability as a resource and the environmental damage that peat harvesting can cause. For those reasons, a lot of people are turning to a more sustainable product, Coir. Peat can be acidic so it’s usually recommended to add a little lime to it to balance the Ph. The bag of sphagnum peat moss that I found had lime pre-mixed into it and was ready for use.

Coconut Coir – Coir is a light, absorbent fibrous material harvested from spent coconut husks. It is generally dried and compacted after processing so it can be shipped easily and cheaply. The fact that it is processed as a byproduct of harvesting coconut meat and liquid makes it a nice, sustainable substitute for peat moss. From my own shopping around I have found that Coir is also less expensive than peat. Store the bricks, dry, until you’re ready to use them and once wetted and expanded, keep moist in a sealed bucket for future use. I had trouble finding coir locally at any home goods or nursery so I ordered mine online.

Other Items – You’ll also need some sort of scoop or measuring cup, a bucket for your final mix (with lid if you pre-mix a large amount), a separate bucket to wet the Coir (if you use it), a marker to label the bucket, seed trays, water as well as possibly lime and good compost.

Bringing it Together

Everyone seems to have their own recipe and preference for what medium to start their seeds in. It can depend on what you’re growing, some plants are sensitive to growing environments that are too rich or too moist. Sphagnum peat moss and perlite tend to lighten the mix and allow it to drain more quickly. Compost, vermiculite, and coir increase moisture retention. You can tailor your recipe as needed for your plants.

April Johnson, landscape and greenhouse coordinator at the Rodale Institute says, “Regulating the moisture is key. It’s easy for the soil to stay too wet, and that can lead to damping-off.” Damping-off is a common fungal disease that attacks the base of the stem and causes newly germinated seedlings to topple over and die. It is brought on by contaminated old seed-starting mix, cool, moist conditions and/or poor air circulation.

The important thing for me was being able to control the mix content myself, save some money and (as I found out) create a better product than I could buy commercially.

Seed Starting Mix Recipe #1

Basic Seed Starting Media for filling your standard seed trays:

• 2 parts coir
• 1 part vermiculite
• 1 part perlite

Seed Starting Mix Recipe #2

Basic Seed Starting Media for moisture sensitive plants:

• 3 parts sphagnum peat moss (Sphagnum moss tends to lighten the mix and allow it to drain more quickly)
• 1 part vermiculite
• 1 part perlite

Seed Starting Mix Recipe #3

For starting seeds in larger containers, or for a mix after transplant where the maturing plant will need more nutrients:

• 3 parts screened compost or vermicompost (worm castings)
• 1 part perlite
• 1 part vermiculite
• 2 parts coir or peat

If you do decide to use compost or soil in your mix Eileen Powell, author of From Seed to Bloom, recommends sterilizing the soil by baking it in a shallow pan at 275°F for at least 30 minutes. This will insure that you’re not introducing any bacteria or fungi that could hurt your seedlings. She warns though, the baking process will smell bad so be prepared for that. Derek Fell, author of Vertical Gardening, suggests baking soil at 450°F for 40 minutes to sterilize it before mixing it with equal amounts of spent coffee grounds to make his seed starting mix.

You don’t have to be super accurate with your measurements, this isn’t baking. I have been all over the internet reading about seed mix and just about everyone has a different recipe. Our friend Linda over at Garden Betty likes to keep it simple and uses an equal ratio of each: peat, vermiculite and perlite. Just get the ratios close and you should be able to achieve what you need. Mix everything together in a clean bowl or bucket, pre-moisten the mix to get a more uniform moisture content and make it easier to work with. Then you’re ready to go!

Store your unused portion in a container that allows the mixture to breathe, such as a plastic bin with a few holes to provide ventilation. Seed starting mix is best stored in a dry, cool location.