I can't think of any American equivalent in terms of size or prestige.
At least in the northern states, flower shows happen in late February or early March. They are basically trade shows open to the public for a small admittance fee. In exchange for seeing all that green and bloom at the worst part of Winter, one is bombarded with salespeople attempting to market everything from patio furniture to tools to paving.
They're a lot like car shows but for a very different demographic.