Every Thursday morning you can buy beautiful plants and flowers at really good prices in Florence. This may not seem like something speciel to you, but in Italy flowers are generally really expensive, so when you have the chance to buy them at a good price you should really make use of it. The weekly flower market takes place near Piazza della Repubblica in Via Pellicceria under the "portici" (The arcades). The selection is very extensive and you can find everything from herbs to plants for your terrace or balcony, flowers for your home and even Christmas trees when it's that time of the year.
The market starts early in the morning and continues until lunch time. Go as early as possible in order to get the real atmosphere.
Via Pellicceria (Under the arcades behind Piazza della Repubblica).