We're sorry about the obvious pun in calling The Secret Garden's language flowery, but, well, it really is. Not only are there tons of literal flowers in the novel—forget-me-nots, roses, delphiniums, and so on—but the novel also uses a lot of exclamation points and adjectives when it wants to convey something particularly beautiful or important. Take, for example, the kids' earliest discussion of Magic in Chapter 22:
They always called it Magic and indeed it seemed like it in the months that followed—the wonderful months—the radiant months—the amazing ones. Oh! the things which happened in that garden! If you have never had a garden you cannot understand, and if you have had a garden you will know that it would take a whole book to describe all that came to pass there. At first it seemed that the green things would never cease pushing their way through the earth, in the grass, in the beds, even in the crevices of the walls. Then the green things began to show buds and the buds began to unfurl and show color, every shade of blue, every shade of purple, every tint and hue of crimson. (22.26)
This passage can't say red when it could say "every tint and hue of crimson." It also uses a lot of repetition ("every shade" and "the green things "), which emphasizes the huge number and varieties of plants and blossoms beginning to grow through the soil and bloom. What's more, the months can't just be "wonderful"—they also have to be "radiant" and "amazing."
Frances Hodgson Burnett also uses plenty of short, direct words in this passage to vary the tone and to keep it from becoming too sweet and dense to tolerate. At the same time, The Secret Garden's passages of description always include multiple adjectives to describe similar things, which makes the prose seem both highly decorative and, well, flowery.