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 help you determine the best distances and depth to plant your vegetables.
Seeds come in a variety of sizes and each size needs to be planted at a 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.
Planting depths for some of the more common vegetable seeds.
Recommended seed planting distances will differ slightly between various planting guides, so it is useful to consult several guides to find one that best suits your garden conditions, or better still to compile your own seed planting guide. Click this link to download the personal planting guide that I have put together. Planting Distances And Harvesting Times.
It is 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 reached the seedling stage. I often 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 survivors to cover any that are lost.
It is possible to reduce the size of mature vegetable plants by placing them closer together than the recommended planting distances. This technique works well with vegetables such as cauliflowers and cabbages, by placing the plants closer together you will get smaller and more manageable heads.
Though note that this technique does not work well with all vegetable varieties. Vegetables susceptible to fungal diseases such as Powdery Mildew (such as tomatoes) need good airflow between the plants to keep the leaves dry. Powdery mildew thrives on moist surfaces. It is best to stick to recommended planting distances for vegetable varieties that are susceptible to Powdery Mildew
