Depth of Seed
It is very important to sow seeds at the right depth. Some seeds need light to germinate and will never sprout if below the soil surface. Other seeds need no light, but require a particular combination of air and moisture and must be planted deeply enough so that the young plant roots have adequate support from the soil.
Planting at the wrong depth can cause poor seed germination and poor seedling development. While all reliable seed companies give you that information on the seed packet, the general rule of thumb for sowing seeds is to cover them with soil at a depth of only three to four times a seed's thickness. Firm the soil to make sure that contact is made between the seeds and the damp soil.
Spacing and Thinning
A common mistake of beginning gardeners is planting too closely. Seeds are so small that it is hard to believe they'll ever grow into the large plants described on their packets.
Yardeners and beginning gardeners try to squeeze more plants into the garden than conditions can support. The result is undersized plants and possibly some diseases and pest problems as well.
Beginning gardeners should follow the spacing instructions on the seed packet. Only plant annuals more closely than is usually recommended if the soil is very rich and well prepared.
Closely spaced plants will generally require more fertilizer and water than normally spaced plants.
You must also be more diligent about pest control, since the close spacing makes it easier for the pests to spread from plant to plant.
There is a limit to how tightly you can arrange plants in a bed, even when soil is quite rich. If you plant annuals more closely than conditions allow, you will get too much competition between the plants, and productivity will suffer considerably. Begin conservatively, and as you gain experience and improve soil conditions, you can gradually plant more closely. A safe guideline for planting annuals in beds is to space plants no more 75 percent closer than recommended on the seed packet.
Thinning Is Tricky – There are two ways to thin newly seeded seedlings. You can very carefully pull up the unwanted seedlings with your fingers, or you can cut off the unwanted seedlings with a scissors. Our preference, if you don’t have a huge garden to thin, is to use the scissors technique. No matter how careful you might be, hand thinning is going to disturb the roots of the plants you are leaving in place. Even a little bit of disturbance is going to cause some stress and set the plant back a bit.