I do at time have projects that will have many next actions that all could be done at the same time .... but don't have to be done at the same time.
I try not to worry myself about this as it's important first and foremost to stay attracted to my lists. And being overwhelmed... is NOT attractive So I'll just take a from my brainstorm and postpone the rest. (I put them in the notes section of the project or leave them in the mindmap until the next weekly review. So using your example for instance, I might just leave 'buy a hose' in my planning notes until I've done the research and I know what produce and plant seeds I may want to buy. That way I just have to make a store trip once and then I'm very clear on what I need to get when I'm there.
So, just because it's a next action doesn't mean it needs to be done right now. I give myself permission to put it in my lists ... to commit to it ... only when I'm ready to see it there.