The park and the huge, beautiful greenhouse (a space used for events, not for plants) are worth a visit with or without the flower show or other events. The only thing is that Giardino dell’Orticultura a little bit outside the city center so do go if you have plenty of time to explore the city or something else to do in that area.
Two times a year Società Toscana di Orticultura organizes flower shows. There’s the spring show, from the 25th of April until the 1st of May. Then there’s the fall show on the first weekend of October. The park fills up with all kinds of plants and flowers and it becomes a very nice social event. Of course you can buy the flowers as well.
As I mentioned, the greenhouse also hosts other happenings during the year but unfortunately I don’t have a list of those events and they are often announced on short notice. In the summer there’s a bar in the garden and a DJ playing music in the evenings. The crowd is young and local and a nice change from the touristic city center.