A few older yellow leaves are normal on gardenias. It's the plant's way of renewing the foliage and making room for spring growth. But when most of the leaves take on that yellowish look, you have a problem. Gardenias are not a shrub you can plant and forget.
Good care starts by selecting a grafted plant.
Only these are resistant to nematodes, which are microscopic roundworms living in most Florida soils. Nematodes cause root decline of susceptible varieties. Fortunately, gardenias are not too finicky about the garden site. They grow in full sun to light shade. Gardenias tolerate sandy soils, but you can make care a lot easier by enriching the planting site with lots of organic matter.