This may not help too much, but: I've got a Merc/Nature at 25th right now. Mercs being a bit squishie as they are, I found Nature to be slow going at first, but that might be my fault as I was also chasing the pet powers and some other power pools and may have slept on getting the good Nature powers as early as I should have. Don't make my mistakes!
I've heard that there's little skippable in Nature and so far it seems like it, they're all pretty decent (personally I might skip Wild Growth, but I haven't tried slotting it up much yet either; apparently once it is slotted up later, it's great) and what I've been lead to believe from these forums agrees with that.
Life-giving Spores was a game changer for me for the set, so grab it as soon as you can.
The heal Regrowth struck me as sort of weak and it takes time to get used to the cone nature of it, so be ready to spam it and to get used to learning how to cone it properly.