May. They have been traditional corsage flowers for many years. Floridians like to pick flowers and float them in shallow bowls indoors. Of course, the flower fragrance of the gardenia is also a major attraction. They have a very sweet smell like some jasmine flowers, hence the Latin name jasminoides. Gardenia is happy in southern zone 8, all of zone 9 as well as South Florida's zone 10.
Some species are dwarf and grown as houseplants as well.
Various varieties can do different work for your landscape, especially 'Miami Supreme' which has the largest flowers of all. The primary key to gardenias is to start with a highly acid soil. This seems excessive but it always gets results with excellent foliage and fewer pests. Nematodes are a common problem so fresh soil helps a lot. Provide reasonable drainage, water regularly but not excessively so as not to strain your gardenia.