I'm picturing someone using it as a utility for gardening, maybe cutting vines or chopping stems. Maybe a kid took it out to play in the garden. Then it was left there and forgotten as the grass and weeds grew over it over and over, until it was buried for someone to find.

I suspect the previous owner had a knife she didn't know how to sharpen and wanted to supplement the iron in the soil for the nasturtiums.

Or it was a murder weapon.

Seems a lot less likely. She's a well-known actress; I doubt she ever killed anyone, even in self-defense. There just aren't enough murders around here to make it a reasonable guess.

Yeah, don't think it's a given (and you have more context to determine that). It's just what comes to mind when hearing/reading about a buried knife (i.e. something you don't typically bury & could be evidence in a crime case).