Also no cheesing on this one. You really have to harvest five hundred plants. Buying from vendors doesn't count.
Some tips, though!
-> A forest zone on Jemison is a decent place to farm this.
-> craft a couple hundred O2 SHOTS (chem) to replenish your O2 while you run about, and a couple dozen AMPs so you'll always be running fast.
-> having unlocked the personal atmosphere power is also a nice way to keep refilling your O2
With these tips in mind, took me about an hour to get this achievement from scratch
You can find your total in the pause menu>status>exploration.
Good luck!