Growing Medium
What's best for starting seeds?

Growing Medium

You have your seed trays all set. Now, what growing medium will you use? I just take the easy way out and use a commercial potting soil. Some books say not to use this but it works for me. It's sterile and provides the four basic needs for proper germination. Which are;

  1. Proper aeration to let the gases exchange between the roots and surrounding soil.
  2. Enough seed-soil contact to let water get to the seed and root system.
  3. A surface that doesn't crust over to inhibit seedling emergence.
  4. Low density so the roots can grow deep.

Seeds can germinate in medium completely devoid of all nutrients. They have their own food supply at first. You can simply feed them with nutrient rich water until it's time to transplant. But if you're going to keep the seedlings in the flats for an extended period it's best to have some organic matter in the soil.

Materials Used For Growing Medium

Growing Medium

The most common materials used in making seed starting growing medium are as follows.

Potting soil or garden soil is used in combination with the others materials in this list to make a nutrient rich mixture. It needs to be rich in organic materials for the best nutrients. It also needs to be sterile or pasteurized to kill weeds, bugs and diseases harmful to the seedlings. If you want to pasteurize your own soil just heat it to 180 degrees Fahrenheit for about a half hour. Then you should be good to go.

Vermiculite is a heat-treated mineral that is porous and flakey. It retains water very well and provides good drainage at the same time. You need to use horticultural vermiculite not construction grade that may contain dangerous chemicals for plants. It can be used by itself for germination.

Peat moss is harvested from bogs, dried, milled and packaged for sale. It's nothing more that plant material from poorly drained swampy areas. Sphagnum moss is the best for seed starting growing medium. It's loose texture holds water well and tiny seeds can get a good roothold.

Perlite is volcanic ash. It looks like small plastic beads, but it's all natural. It is a lot like vermiculite but doesn't hold moisture as well and it's a little courser. It's used mostly to loosen up the growing medium a bit, sometimes in combination with vermiculite. It's not used by itself for germination.

Sand is also used to help loosen things up. But don't use sea sand because it's too salty. Lake sand is usually too fine. Course construction sand works just fine.

You can get any of these materials at your local home and garden store.

Mix Your Own

Commercial seed starting medium is available at your local garden supply store. But if you like to experiment you can mix your own medium.

Growing medium - adding vermiculite The easiest way is to just fill a flat or container to within an inch of the top with potting soil. Next put on a half-inch layer of vermiculite for the seeds to germinate in. This gives the seedlings a nice loose, sterile layer to germinate in and a nutrient rich layer for the roots once they develop. This is my way, the lazy man's way. It's hard to see in the picture at left, but there in potting soil in the bottom of that flat. The mix should be one half-inch down from the top of your container for easy watering.

If you want to make your own nutrient-free mix here are a couple of recipes for you. Cornell University developed one that is equal amounts of peat moss and vermiculite or perlite. The University of California uses a mixture of half peat, half sand. There aren't a lot of nutrients in these mixtures so you'll have to fertilize the seedlings once they develop true leaves.

If you want nutrients in a mixture, use pasteurized soil, sand, and peat in equal parts. Then you'll only need to use water on the plants until they're set out in the garden.

Have fun experimenting with different combinations of mixtures for your growing medium. I know I do. That's half the fun of gardening, trying something new. Especially in early spring when I can't get out in my cold, wet garden yet.

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