The best planting medium to use to germinate your seed is a mix consisting of about 1/3 peat, with perlite and/or vermiculite making up the balance. It is also helpful to add some aged, processed, fine-textured bark for drainage and tilth. If you are really gung-ho on do-it-yourself projects, you can experiment with these ingredients to create your own special blend. Following are brief descriptions of these components:

1. Peat – Decomposed aquatic plants. It can vary from acidic to almost neutral, holds water well, and contains about 1% nitrogen, necessary for plant growth. Baled peat is a poor medium for seed starting, as water does not penetrate it easily, nor does it have good drainage or aeration. But, ground, it is a great component for growing mixes. One drawback: If it dries out completely, it will repel water. If this happens, the potted medium will need to be soaked until the peat will again hold water.

2. Vermiculite – Expanded mica. It can hold large quantities of water for long periods of time. Light, sterile, neutral on pH, holds nutrients, provides good aeration and contains magnesium and potassium, two elements necessary for good root growth.

3. Perlite – Volcanic ash. Does not absorb water, but holds it on its surface, is neutral pH and carries no nutrients. Its contribution to growing mixes is good aeration. It also stays cool, and so is a good component in mixes used for seed that prefer cooler temperatures to germinate. One property of perlite that you need to be aware of: it will float to the surface of the seed bed when watered; bottom watering helps minimize this tendency.

4. Decomposed, fine-textured bark – Adds organic matter to the growing medium, aiding water retention while also encouraging good drainage and aeration for the roots. If you have a lumber processing plant in your vicinity, this is an excellent and cheap source not just for your planting mix, but also as an additive to your garden soil. Just bear in mind that it needs to be aged for a year before it is really useful. (In my garden, I use new bark on my walkways and then the following year, shovel it into the adjacent beds to work into the soil.) Otherwise, bark is available in bags in nurseries and garden centers.

Personally, I much prefer the convenience and guaranteed correct recipe that you get from buying a ready-made seed starting medium! The seeded containers need to have humidity levels kept high. You can accomplish this by covering the containers with plastic wrap, plastic bags or domes.

Ordinary garden soil works wonderfully well for outdoor direct-sow seeds such as Corn, Beans, Squash, Marigolds and Zinnias, plus a number of others that actually prefer to be sown where they are to grow. However, it is definitely not a recommended medium for successfully starting seed indoors or in a greenhouse. Garden soil is inclined to poor drainage in a small container, and often carries many diseases that will cause problems for your small seedlings. Ordinary garden soil also harbors weed seeds that will compete with your seedlings.