Certified GKH Gardening Expert
When collecting seeds, they need to be dry before storing. But if you soak your seeds before planting, you can plant right away.
Unfortunately, there is no one answer to your question. Annual flowers and vegetable/soft fruit seeds can usually be planted immediately after harvest. There are some seeds that need special treatment before they will germinate. These are often seeds with hard seed coats or pits that may take two years to germinate without "scarification". Also, many perennials, except grasses, from cold weather regions have a built-in dormancy period to prevent germination in late autumn that would result in winter kill. These seeds require "stratification" in a cold, moist environment before sowing. If you sow seed before this dormancy sets in and can protect seedlings from winter weather, some may germinate. You would need to collect and sow the seed as soon as it matures. I suspect this inherent dormancy is stronger in some plants than others which leaves you with researching each type of plant and how to grow it from seed. That isn't a very satisfying answer! All seeds should be cleaned; remove as much of the pod, fluff or pulp as possible to decrease the risk of disease.