All of the following are perennial plants and the seed can be sown in spring, summer or early autumn.

Although the occasional one may flower in its first year, usually they flower in their second and subsequent years.

Most perennials will survive for several years, many dying down completely in the winter and appearing again each spring. 

These plants are also perennials but they need a period of cold before they will germinate.  These are best sown in autumn either into the ground or in trays/pots/modules but left outside.