I have done lots of work for agricultural facilities that contain greenhouses. They are definitely damp locations, if not a wet location with the use of sprinkler systems verses bed watering tub systems. The other concern is temperature derating of the current-carrying capacity of the conductors used to feed equipment. Sealtight (Article 356) and PVC (352) has always been the best choice for me. W/P connectors, boxes, and devices are also a must. Good Luck!
Bryan P. Holland, ECO. Secretary - IAEI Florida Chapter
I also use PVC in green houses. Every thing that can be - conduit, boxes etc. Inuse covers for outlets that are 6' or below. I also cord and plug connect the luminares and support them from the structure. Temp derating is necessary. Also support the PVC conduit at least every 3' I use the plastic mini's. If you keep the conduit and boxes sealed, you will not have a problem with chemicals. But since you will almost always have a condensation problem you should arrange the conduit run to drain.
I am planning on using emt w/ compression fittings. The last house had emt w/ set screw and they were going strong for 20yrs. Till the roof come down, literally come down. 3' of snoww will do that. This place is more retail then, growing operation. The put the plants in and keep them alive till they sell. Very limited fertilzation, Etc.