If I could only grow one cactus this would probably be the one. It was labelled Notocactus schlosseri when I bought it but now it’s been renamed Parodia erubescens. The specimen in the photo was about 10 years old but it flowers just three years after seed germination and blooms annually thereafter, becoming more showy with every passing year. Older plants like the one above produce a magnificent , tightly packed spiral of flowers that bloom for a couple of weeks, then freely set seed which germinates very quickly. They also produce offsets that can be broken off and rooted, which is useful because old plants become top-heavy once they are about 20 centimetres tall and tend to topple over rather easily.