These flowers prefer cooler climates. They produce flowers from early summer to mid-autumn.
Before the beginning of spring, you need to prune them down to the ground level. To improve their flowering potential remove the dead flowers. This will encourage the plant to produce more flowers.
Bellflowers can produce a lot of seeds and take over the entire area that they have been planted. So keep this property of the plant and plan in advance where you want them in your garden and whether other plants can be affected by their vigorous growth.
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