Humidity sensor for wet environment
I'd consider using Bosch BME280 digital sensor. Although it's a bit expensive, it has very good sensitivity, precision and amazingly fast response for humidity changes (for ex. in my project it detects dry human hand lying in 2" away within a 1-2 seconds).

So its features make possible to put it into an enclosure to avoid direct water contact with water spays and it'll still be very responsive for greenhouse needs, I suppose.
Datasheet warns:


  Liquids shall not come into direct contact with the device. 


Also, I'd use electronic-friendly protective acrylic lacquer (Plasticote 70 for instance) and put two layers to sensor's PCB, avoiding small black tiny vent hole on the chip itself:


UPD: Here's DHT11 with cover removed:


It might be the easiest and cheapest solution to cover all internal contacts and IC (avoiding humidity sensor's stripes) of DHT11 with Plasticote 70.