The game has a dedicated "Farm" unit category
Garden Tower Defense's own wiki tags 7 units as "Farm" — units whose job is generating income rather than fighting: Bonsai Tree, Gemspire, Gingerbread Seedmill, Money Tree, Rainbow Money Tree, Seedmill, and Spiritwood. Two of these have been individually verified so far.
Money Tree (Rare) — the accessible pick
- Obtainable now — 11% drop chance from the Classic Summon crate (the starter crate).
- Income: +20 → +65 per tick, and the interval between payouts shortens from 5s to 3s as you upgrade it — both the amount and the speed improve.
- Placement Cost: 400, Placement Limit: 6.
- Per the wiki: "the first ever farm unit added to the game."
Seedmill (Legendary) — no longer obtainable
- Was purchasable in the Spring Bundle (125k in-game currency or a real-money price); that bundle is gone, so this unit can't currently be acquired.
- Its ability: after being placed for at least 180 seconds, it adds a 5% boost to your final Seeds income, and this stacks with other players' Gingerbread Seedmill units.
- This is the third time in this wiki-building project a heavily-recommended unit turned out to be discontinued (see Golem and Astraflora on the Best Units page) — always check "Currently Obtainable" before planning around a unit.
Unconfirmed
- The remaining 5 Farm-tagged units (Gemspire, Gingerbread Seedmill, Rainbow Money Tree, Bonsai Tree, Spiritwood) haven't been individually verified yet for stats or obtainability.
- No verified guide or tested video was found for an optimal money-farming route or map.