A greenhouse business is a farming business and should be filed on Schedule F.
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Actually, there's a mitigating factor here to determining if it's a SCH F or SCH C business.
If the land you are doing this on is "in fact" classified by your local property appraiser as agricultural or farm land, then it's a SCH F business hands down.
If you are basically running a nursery business from your residence property (that is not classified as above) or as a business separate from your primary residence on land that is classified by the appraiser as business or commercial user property, then it's a SCH C business.