You don't have to plant summer bedders in orderly brigades to get their full benefit - in fact, unless you live in a stately home with plenty of land, that sort of specialised planting is best left in the public park. But, because they have an obligingly long flowering season, half-hardy annuals are perfect for containerised growing. Just a weekly feed and regular deadheading will keep them performing for months.
Sow these before the middle of the month, with a bit of heat (preferably in the greenhouse): ageratum, nicotiana, bidens, godetia and French marigolds (not African ones which go to mush after heavy rain).