It shouldn't require a stash, as the quest is to "gather", not "provide", so I consider it to be a bug if it's impossible to gather (and I'd say that means the flowers/leaves to gather should have its last existence date at least the day the quest would expire). That they mature at different rates is natural, so every plant shouldn't necessarily give you what you need if it's flowers (I think you can get leaves until they wither). Thus, most heathers may have lost their flowers when you get the quest, but there should still be some somewhere.
In fact, it's poor planning on the part of the quest giver to wait until the very last moment to try to get the plant parts, although I guess asking the PC may be considered a desperate last ditch attempt...