Sustainable Seed Starting with Soil Blocks

When I first started growing on our farm in 2016, I did a lot of research into seed starting techniques. I came across soil blocking in Eliot Coleman’s book The New Organic Grower and was excited to learn about this process that both reduced the use of plastic and encouraged strong seedlings that were less susceptible to transplant shock. I have been soil blocking ever since!

There are many resources available to learn about soil blocking so I won’t cover all of the details here, but I’ll go through the basics of my process. I use Eliot Coleman’s soil mix recipe for now, though I am interested in finding a peat / coir free substitute (for a great discussion of soil mixes and alternatives check out this great new podcast series out of Alberta - The Sustainable Flower Podcast episode 4).

I use a mini (3/4”) soil blocker to start most of my seeds and then bump them up into 2” blocks once they germinate. This allows me to save lots of space and soil mix in the initial germinating phase, reducing waste related to germination failure. Also, for those seeds that germinate best with some heat, it means I have room for many more germinating seeds on my heat mat that can later be bumped up into large blocks and off the heat. I can get over 1000 seedlings on one 17” x 48” heat mat this way.

Seeds are placed one at a time into each tiny block with a wet sharp pencil or bamboo skewer. I use non-draining trays for the blocks so I can water the blocks from the bottom as needed.

For the mini blocks I made a number of trays from corrugated plastic sheeting that I had leftover from another project and some damaged sheets that I was able to source from a local supplier. When I run of these I also re-use old plastic clam shells and styrofoam trays (or whatever I can find!) that I collect from friends and family.

For the larger blocks I use old plant trays that I have collected over the years, lined with reused heavy plastic from soil mix or dog food bags, etc., and sometimes supported underneath with cardboard for added strength. This is not the most efficient process for sure but it does align with my goal of reducing and reusing plastics.

As the seedlings grow they tend to maintain their roots within the large soil blocks. Blocks can be easily removed and handled when it comes time to transplant into the garden. My experience has been that the transplants are healthy and easily make the transition to the garden beds.

The soil blocking process is definitely time consuming up front as the block making takes more time than filling flats or cell trays. There is greater efficiency at planting time, however. There are no pots from which to extract seedlings, no pots to blow around the garden, and no pots to stack and wash. That makes the huge job of transplanting much simpler and saves time in May when time is really at a premium!

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