They should be able to work if you're standing close enough to and then click on a valid plant, so any that get brown leaves for the watering can (as long as there's water in it, which there would be if you haven't been able to use it yet), or any that have started sprouting visible weeds for the trowel. But nothing happens unless you're interacting with a plant that actually needs something. Sorry for any confusion! I probably should have spent less time doodling pumpkins, and more on making the mechanics a little more intuitive and user-friendly