Songbell Seeds provide you with Motes of Harmony, which provide Spirits of Harmony. In short, the most profitable thing to plant is usually Songbell Seeds or Enigma Seeds. The reputation seeds for leather, ore, cloth, or enchanting mats are generally lower yield than simply getting Harmonies and trading those for what you need, and that's generally less than you'd get by planting Enigma Seeds, selling everything you get, and using that gold to buy leather, ore, cloth, or enchanting mats. This is only true because everyone else already farms them, and they're all you can farm until you get farther into the Tillers reputation grind.

Generally, the least profitable thing to farm is vegetables. There is plenty of opportunity to improve this, though!

So what is the most profitable thing to plant on your farm? Matt assumes correctly that the majority of people simply plant what they can use to avoid having to buy off the Auction House. So what's the start and end of the equation? I don't raid and can't stand dailies, so I only have patterns available that are vendor trained, bought with spirits of harmony, or learned off a cooldown. I, however, am a chronic alt-oholic and have every profession maxed and available to me. What's the most profitable thing to plant in a farm? I'd assume in most cases people plant for their profession or whatever raw material sells for the most.

Golden lotus farming guide