Campanula -- also called bellflower and Rainier's bell -- is a herbaceous flowering perennial that blooms in late spring and summer. It comes in many varieties, from the small alpine campanula to species that grow 45 cm (18 inches) tall. Though it's pretty, with purplish-blue blossoms on delicate stems, campanula will also spread rapidly in the right environment. A notorious self-sower, campanula can be propagated easily by seed or through root division of mature plants every three to four years.
Allow mature campanulas to go to seed in late summer. They will produce pods that contain three seeds apiece. Break the pods open when they become dry and brittle. Collect the seeds, put them in a paper envelope, and store in a cool, dry place for the winter.
Sow campanula seeds directly into the garden when all danger of frost is past and the soil is warm, usually mid-spring. Pick a sunny spot and scatter the tiny seeds on the ground, and sprinkle water on them if the ground is dry. Do not cover seeds with soil, because they need light and air to germinate. Thin seedlings to 30 cm (12 inches) apart when they sprout in 14 days or so.
Allow campanulas to self-sow if you prefer to let nature take its course. Watch for seedlings and thin them when they appear.
Propagate by division every three to four years, after spring blooming. Put on gloves and dig a garden spade into the centre of the plant, effectively cutting it in half. Dig one half out, and check to see that you've got a healthy amount of the rhizome, or root. It should be firm and sprouting from several areas on the root.
Replant divided campanulas 30 to 45 cm (12 to 18 inches) apart, or pot divided plants and give to friends or neighbours.
Clip off flower and stems as they fade to encourage continuous blooming in summer.
Campanulas will escape garden borders due to their seeding ability. Watch for seedlings and pluck them as they appear.