Just having a lot of hobbies you pick up and put down doesn't make you bipolar. That said, I am diagnosed bipolar, and I've done a lot of the things you describe - get really into or excited about a project or hobby, research it in depth, plan everything out, look for deals...but either it fades quickly or I set it aside after a while. Sometimes I'll go back and pick it up, sometimes not. Right now I have about $200 worth of yarn sitting in storage for a big project I planned out in great detail, but never even started. It's about great intentions that don't get followed through.

These are some factors that might help you work it out:

Do your purchases leave you in financial difficulty?
Do you have a house that looks like a Home Depot demonstration area because of all the unfinished projects?
Do you feel out of control, or feel pressure to do these projects RIGHT now?
Do you make a lot of impulse buys?
Do you eventually (and frequently) wonder what in the world you were doing spending that money on these projects, after a little time has passed?
Do people around you act like you're doing something strange when you start or drop these hobbies, or tell you it's not a good idea?

The thing is, excessive spending/poor impulse control is just one factor to consider. Anybody could get into those patterns. If you're doing these things while you're also not sleeping, having racing thoughts, talking a lot more than usual, thinking you're going to develop a new strain of strawberry that will revolutionize the gardening world, being unusually sexually active, on a hair-trigger temper, feel pressured like you can't slow down...

Then this might warrant additional consideration as a bipolar thing. But bipolar hypo symptoms aren't standalone. You'll see a pattern of similar behaviors during that time.

Hope that helps. Good luck with your strawberries!

ETA: One surefire way to test this is to set yourself a three-day waiting period. If, at the end of three days, suddenly making that purchase isn't so important or interesting, then it may have been a mood thing.