Every year when I start getting ready to grow seedlings I mix up a batch of seedling flat growing medium. By starting with new growing medium every year I ensure there are no insects, plant disease or weed seeds. This helps to ensure healthier plants are going into my garden and I am not introducing weed seeds to the garden.
How to mix custom soil
Time to mix up the sawdust, sand and perlite that will be used as our growing medium in our seedling flats. We mixed 2 parts sawdust, 1 parts sand and 1/2 part perlite for our seedling flat, as seen in the pictures below. The perlite is optional and can be omitted to keep your costs down.
We mix everything in a wheel barrow with a flat nosed shovel before transferring everything into our seedling trays.
The Mittleider Gardening Course book recommends 3 parts sawdust to 1 part sand. It also gives options to sand and sawdust, including mixing ratios. Get your own MGC book here if you don’t already have one.
Seedling flats are shown in the Mittleider Gardening Course book on page 181. The dimensions of the seedling flats are 18″ x 18″ x 2 1/2″. Once filled with the mixed growing medium we added 1 1/2 ounces of our pre plant fertilizer mix. (Page 51 in the MGM, revision 4)
Do you prefer to use your own soil?
Not everyone wants to mess with the extra work of acquiring everything to make a custom growing medium. The correct sawdust can be difficult to find and even expensive when you do find it. You can sterilize your own soil and grow healthy seedlings. See my blog post on Seedling Success Through Sterilized Soil.
We needed sawdust to fill our grow boxes and made a quick trip to Jamesport, MO to a sawmill. This lumber mill makes pallets and will allow us to take the sawdust for FREE. We just needed to bring a shovel and fill the truck ourselves.
They do not make pallets from walnut so this sawdust is safe to use. They have huge piles of excess sawdust and were happy to have us take some away. We will make a few trips here in the near future.
What sawdust is safe for grow boxes?
Any sawdust made from trees of any variety in the United States except walnut is safe for use in your garden. Walnut sawdust contains tannic which can retard the growth or even kill it if the concentration is high enough. If the sawdust has any walnut in it you shouldn’t use it.
Can you use sawdust from plywood?
There are two reasons you should not consider using sawdust from cabinet shops. The glues used in the process of making plywood can be toxic to plants and humans.
The second reason you shouldn’t use that sawdust from cabinet shows is that the particles likely either too large or too small for use in your garden. If they come from a planer then they will be too large to permit proper drainage and will sour in your garden.
Sawdust made on table saws, radial arm saws or with handheld saws is too fine. Sawdust that is too fine doesn’t drain easily and can result in compacting. If your growing medium is compacted the plants can’t get water or oxygen to their roots and will die.
How to pick the right sawdust for your Mittleider garden
If you would like to see how to determine what sawdust will work in your Mittleider garden be sure to read this blog post. There is even a video showing you the correct size and sawdust that will cause compaction.
But to put it plainly, sawdust from any source that isn’t a sawmill running a large circular saw isn’t going to work.
Can’t find a sawmill?
If you can’t find a sawmill locally that has the correct sawdust you can use wood pellets. Pellets that used for heating work as do equine pellets. For more information check out the blog entry on equine pellet sawdust.
Our quest to be self reliant and grow a healthy garden