Our balcony was taken over in late summer by straw-like vines, wrapped around every other plant, pot and railing. Sweet peas are not known for their consideration of other plants when blooming in summer, and my experience was no different. Left on their own for a few weeks, we returned from holiday to find all the green had turned to straw, and this sweet pea straw had taken over the best part of the balcony.
With some severe hacking, we brought some form of order back to the balcony, but whilst doing this, we kept hold of the sweet pea seeds for sowing next year. Sweet peas are annuals, so the plant grown from seed is only with us for one season, never to return, hence the importance of collecting your seed! Even on a balcony you can harvest seed... and this inexpensive and interesting process will provide seed and flowers for the following year.
Sweet pea pods are easily recognisable to all, as are any pea pods! Knowing when the seed is ready is indicated by a colour change of the pods from green to brown. Be careful not to leave it too late before the pods open and disperse their contents. Remember the plants want to release their seed as soon as possible. Once the pods have lost their green colouring and turned to brown, with the sun behind them, you will be able to see the black seeds within (see images below). If you shake the pods, you should also be able to hear a rattling.
Shake off any debris from the seeds and store in a cool, dry place. Remember to label the seeds, and save for next year! Sweet peas could be sown indoors from January to April, or if you prefer to sow directly into the ground or in pots, do so in late March or April, although in my experience the results are generally less satisfactory than bringing the seedlings on indoors before planting out!