The plural of gardenium is not gardenia.
Though a gardenia grows from a garden, the word gardenia did not grow from the word garden —not the generic word, anyway. If I were a bullshitternet “Did You Know!!!!?” guy, I’d explain that the floral name gardenia comes from the Latin word gardenium (“domesticated plant”), and was actually originally a plural. But it wasn’t. There’s no such word as gardenium.
Granted, a smidgeon (or two smidgeia) of Latin exists in the name of the plant—in the sense that gardenia puts into a Latin “form” the last name of one coincidentally named Alex Garden, an American naturalist, who was honored by having the plant named after him in 1760. And for all you etymology hoaxers out there, just as prostitutes did not take one of their slang names from a general named Hooker (see page 86), flower and vegetable truck farms did not take their name from Mr. Garden. The word garden was not home-grown; it came to us in the 1300s from Old North French.
Because of its horticultural roots, I wonder why it wasn’t restricted by customs.