I was in Boston last month, and bought a "shovel" in Chinatown. It is a 20" spatula-like metal kitchen utensil used in Chinese restaurants. No sharp edges or points. They made me check my luggage with the shovel. It could be used as a weapon, I was told. I also bought two professional stainless steel fryer baskets (each 26" long and 7 pounds). They let me take these as carry-on; I used a black garbage bag to conceal them.
I think I could do more harm with the fryer baskets, but they didn't fit in my suitcase, so I'm glad I got to carry them on the plane.
As your trowel is plastic, I think you're safe as long as they don't see it. Don't pack it with any metal, in case they decide to inspect the metal. Good Luck!
Linda