The plants should be cut back hard after flowering to encourage the new foliage growth and bloom production. Divide every 2 to 3 years. The plants will self-sow freely if the flower heads are not removed after they fade.
Softwood cuttings in summer or by seed in autumn. When sowing the seeds yourself, remember that they need light for germination and will germinate well at a temperature of about 70F.
Pests, diseases or disorders
Mildew and white blister could cause a problem.