Couch potatoes give 3,000 XP at elder. They also reseed, not sure if key limes do. My personal choices for high XP at elder are maelstrom snap dragons, couch potatoes and king parsley. If you're trying for amber, put a plant near your king parsley that attracts pixie along with the housing items they like. I've gotten 4 amber from my last few elder king parsley harvests thanks to the star chalice and pixie.
What rank of gardener are you? You can farm for Evil Magma Peas, Couch Potatoes, Deadly Helephant Ears, White Tiger Lilies, Key Limes, and Red Huckleberries. These all give 300 xp at each regular harvest and 3000 XP at and elder harvest. If you don't want to farm or spend Crowns, you can buy Deadly Fly Traps for gold (in Nordilund). They're a lot more work to keep alive than the Crowns seeds, though, and require you to be at least gardening level 7. There are a few other high XP seeds that you can SOMETIMES find in the bazaar, but they're so rare that it would probably be easier just to farm for Couch Potatoes or Deadly Helephant Ears than wait for one to show up in the bazaar.
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What rank of gardener are you? You can farm for Evil Magma Peas, Couch Potatoes, Deadly Helephant Ears, White Tiger Lilies, Key Limes, and Red Huckleberries. These all give 300 xp at each regular harvest and 3000 XP at and elder harvest. If you don't want to farm or spend Crowns, you can buy Deadly Fly Traps for gold (in Nordilund). They're a lot more work to keep alive than the Crowns seeds, though, and require you to be at least gardening level 7. There are a few other high XP seeds that you can SOMETIMES find in the bazaar, but they're so rare that it would probably be easier just to farm for Couch Potatoes or Deadly Helephant Ears than wait for one to show up in the bazaar.
key limes are way to go if you want to rank from 1-10 in a single harvest with just a few plants. I had the middle ring size with pots in my dorm room got to rank 10 when they all hit elder :-)
11-15 just do normal gardens for regent/megas etc. it will come :-)