The likes of CREE COB chips can be very expensive and that’s before you add heatsinks and drivers etc and to be honest i wouldn’t want the things to be overheating in an already hot and humid greenhouse.
You could always go cheap, and i mean real cheap 
https://www.ebay.co.uk/itm/2X-30W-12V-LED-Floodlights-Outdoor-Garden-Wall-Security-Flood-Lights-Waterproof-/173269282180