Why? Are there serious obstructions between the emonPi and the greenhouse - I’m thinking of a thick stone wall, or a wall that’s solid metal or filled with foil-backed insulation, or multiple brick walls? The RFM69 operates on 433 MHz, a much lower frequency than Wi-Fi, so the range should be better, not worse.
The claimed range for those radios is well in excess of your 20 m, approaching an order of magnitude better, given a clear line of sight.
The only way to test is with another transmitter, the emonPi does indicate the received signal strength.
Note that you can improve the efficiency of the aerials with a ground plane.
@Bill.Thomson might offer some better advice - it’s his line of business.