I have been collecting seeds. The best time to do this is on a dry sunny day, which is certainly what we have today. It involved a courageous sally into the garden in the early afternoon heat to gather the seeds - but then I could retreat to the cool of my desk to sort and label them, ready for sowing.
You can collect seeds across a broad swathe of the year - clearly the essentials are that the seed head is ripe, but the seeds have not yet been dispersed and that the weather is fine and dry - damp seeds will rot if you keep them for any length of time. At the end of summer many perennials, trees, shrubs and annuals are ripe for collecting, but do bear in mind that a seed is the product of two sets of gametes (just like a human) and it is possible that their offspring will not fully resemble the parent who is carrying the seed head.
This is, of course valuable when it comes to evolution and plant breeding, but a bit of a pain if you collect your tomato or chilli seeds and sow them expecting exactly what you got last year. The only way this can happen is if there are no near relatives close by to add some wild card genes to your new plant's genetic mix . For example, if you only plant one variety of tomato in your garden, then your tomato seeds should be the same variety as their parents. If you have several varieties, that may not be the case. If you want a clone (i.e. genetically identical example) of a plant you love then you need to reproduce it vegetatively (for example by a cutting or bulbil), which we are not talking about here.
Any way - a hot Monday afternoon was in front of me so I went into the garden with paper bags and small china ramekins to gather seeds from three of my favourites - Acanthus mollis, Echinacea pallida (a wonderful dry or prairie garden perennial), Nigella (do I need to gather these - they seed with alarming promiscuity? Answer, yes - they are for someone else).
With Nigella I hold a paper bag over the head of the seedpod, bend the stem over and shake it. The pepper pot seed head sheds its seeds into the bag a little like a poppy does. For the Echinacea, which has a cone shaped prickly seed head I bend the head so it points into the china pot and scrape my fingernail along the seed head. As if by magic long, thin seeds drop into the pot. For the acanthus I pick off large, ripe seed capsules individually and carry them back into the house to be split open, so that the bean like seeds can removed in the relatively cool environment indoors.
An agapanthus seed head - not quite ready yet, the seed pods need to be completely dry and brown.
Echinacea pallida seeds which I collected from the adjacent seed head.
The aggressively prickly seed heads of Acanthus mollis. Gloves a good idea.
Once inside the house I tip the contents of each container individually onto a sheet of white paper. I emphasise this - it is easy to get mixed up if you have collected lots of similar looking seeds - so do them one at a time and ensure you have them labelled in some way throughout. Using a pencil or paintbrush for the smaller seeds, separate out the chaff from the seeds and discard the chaff. If you are not sure which is which then Google your seeds and some helpful soul has usually uploaded advice and photos. Label and store your seeds in paper envelopes. Do not use plastic bags as the seeds could sweat and then rot.
Sorting Echinacea seeds - if you fold the sheet of paper before you tip the seeds onto it then you can easily funnel the seeds into an envelope
if you are storing seeds for any length of time then put them in a clean dry, sealable container in the fridge. This excludes moisture, light and warmth and so minimises the risk of your seeds attempting to germinate before you are ready for them to do so. I do this with excess seeds when I have ordered them on line, saving chilli or tomato seeds I bought this January to plant early next spring. Sarah Raven has estimated that, as a rule of thumb, seeds deteriorate at an average rate of 10% a year, so even after 10 years you may still get something to germinate - you just need to sow the seeds more thickly and hope. There are exceptions to this rule, however. Parsnips, for example, are notoriously difficult to germinate if the seeds are not fresh.
There is a lot of complicated advice available about when to sow seeds, whether they should be vernalised (exposed to a period of cold), sown on top of the soil or covered in soil, given bottom heat or top heat......it can get a bit daunting. My advice is, that if you gather seeds from the garden then you should sow them when they would have naturally fallen from the parent plant (i.e. very soon after you have collected them).
When sowing into trays I use either a good proprietary seed compost or a mix I have made myself. If the seeds are big - for example the Acanthus - then I would use a pot and push them about a centimetre into the soil, putting about 4 seeds into each pot. If they are fine, like Nigella, I would scatter them thinly on the top of the soil and then cover them with a thin layer of soil, grit or vermiculite to hold them in place. Then put the seed trays outside in a protected place, such as a cold frame with a netting cover (to keep mice and birds away), and leave them alone. Ensure that when it rains the water can drain away freely.
With a bit of luck your seeds will germinate at the time most appropriate to the plant's requirements. This could easily be next spring - as is the case with Eryngium, for example. Thereafter, ensure that they do not dry out and that they remain protected from snails, mice and (in our case) our chickens. Just leave them to develop at their own pace in the little nursery you have created for them; I really do believe that nature knows best. Once the little plants have developed two true leaves pot them on into individual pots and when strong enough to face life in the garden, plant them in your chosen location.
Eryngium giganteum seedlings which germinated this spring. Miss Ellen Willmott, a Victorian horticulturalist, used to sprinkle these seeds directly onto the ground, with memorable results.
My system doesn't always work - and it is frequently augmented through finding plantlets which have self seeded naturally and moving them to my preferred place - but it is easy to do and is extremely rewarding. Give it a go, you have very little to lose.