Shift+click to plant multiple spots is nice, but 36 squares is a lot, especially when you're dodging around a few things already growing (e.g. juicy queenbeets). And when you're not trying to unlock seeds, you usually want to seed the whole garden with one plant type.
It'd be -amazing- if, say, shift+ctrl+click on an empty plot planted every empty spot with that seed. Return an error message if you have insufficient cookies. Or, make a "Fill Garden" tool that, when selected, plants the next selected seed in every slot; this way you could easily see the necessary cost in the seed's popup, and the feature would be very visible.