Senecio is a very large genus with over 1000 species dispersed around the world. Some are lovely, and some, like the groundsels and ragworts, are noxious weeds. Some, like Dusty Miller, are tender perennials. This profile covers the succulents. There are about 100 succulent Senecios. Some are oddities and not suited to the garden, but they are certainly interesting.
There are some large shrubs, but many are small, either trailing plants or spreading ground covers. As with most succulents, they are very drought and heat tolerant and not very cold tolerant. Many Senecio species are toxic to animals. Use care and do not plant where animals or kids might be tempted to munch them.
Cold Hardiness: Most fall somewhere around USDA Hardiness Zones 9 – 11. A few can tolerate brief periods of cold or dampness, but prolonged exposure will turn them to mush.
Soil: Senecio doesn't seem to be particular about soil pH. Something in the neural range (6.0 – 7.0) would be fine. More importantly, make sure the soil is on the sandy side and well-draining soil. Plants will rot if left damp.
Senecio can be grown from either seed or cuttings. Seeds prefer warm temperatures and constant moisture to germinate. Cuttings are easier and faster. Cut during the growing season, early spring to fall—root in sandy soil in containers.
Established plants are extremely drought tolerant. They do need some water, during the summer, but do not leave the soil wet for prolonged periods. Allow the soil to dry out between waterings in the winter, when they are somewhat dormant. Since they are growing in sandy soil, nutrients will need to be replenished. Fertilize annually, but lightly. Too much fertilizer will cause a lot of leggy growth.
Taller varieties can get floppy. You can prune them back to where the stem is firm, in very early spring. You can even root the cuttings.
Plants can be divided or repotted in early spring. If you are growing them in containers, they enjoy spending the summer outdoors. Wait until there is no danger of frost and move them back indoors in the fall.
Pests and Problems
Few pests bother Senecio. They can occasionally be affected by scale and mealy bugs. Rabbits found my String of Pearls quite tasty.