Hardy geraniums (which don't look anything like the pelargoniums which are also known as geraniums) start flowering now and have a good long season. G endressii is particularly good in this respect. Attractive lobed leaves and masses of shiny pink flowers.

Earlier in the year (so think now for next year) wallflowers are easy from seed, and are fragrant. Although they're usually grown as annuals, they will often keep going from year to year if you chop them back a bit to stop them being too straggly.

Potentilla is another thing that flowers for ages. Typically a small shrub with yellow flowers.

For late summer, calendula (pot marigold) with daisy-like flowers in shades of cream, brown, orange, yellow is another easy from seed. Crocosmia - tall spikes of small lily-like flowers in yellow through to red is easy but can be too much of a good thing. Nasturtiums can crawl over other plants and are good for poor soil - they'll keep flowering until killed by frost.