seed planting & propagation
planting seeds
seed planting depths and distances
All seed packets come with instructions on how far apart and to what depths they should be planted   While these instructions are a good starting point there are some basic principles that can be applied to all seeds.

Seeds come in a variety of sizes and each size needs to be planted at different depth for maximum germination. If you plant seeds to a depth of one and a half to two times the width of the seed you will generally get a good strike rate, providing all other factors are right. Below is a visual depth guide for some common seeds.

Illustration of vegetable seed planting depths
Planting depths for some common vegetable seeds

seed planting distances
I believe the basic principle governing all seed planting is that it is generally better to plant more seeds then required in a given area and then thin them out to your intended spacings for mature plants when the seeds have developed into small seedlings.  I usually plant seeds at half the recommended distances on the seed packet.

The reason for doing this is that if some of the seeds fail to germinate or are eaten by pests there will usually be enough that survive to cover any that are lost. As I harvest a lot of my own seeds I can easily afford to waste some seeds, but even commercial seed packets come with far more seeds than the average urban gardener is likely to use before the use by date expires. So it is not really a waste to plant more seeds than you need.

While this principle remains true for all seed plantings there are some minor variations depending on the size and type of seeds sown.