Robin2:
To extend what @johnwasser has said, I suggest you use an Uno for developing the project and subsequently implement it on cheaper Arduinos with soldered connections for greater reliability.
Specifically the "Pro Mini" which is missing the USB to serial converter, and thus much smaller (and cheaper). You can of course use it for prototyping and code development, in which case you need to connect a USB to serial converter. With pins soldered along the sides, it can be used in a "solderless breadboard" but without the pins, wires can then be soldered directly.
Robin2:
You will also need to consider protecting the Arduino from excessive heat, humidity and insects. A hot moist greenhouse is not the best of environments for electronic equipment.
Heat: Keep it out of the sun. The MCU board itself will generate very little heat, but the power supply may generate some.
Humidity and insects - a sealed box; you can use a "conformal coating" on all non-moving parts (other than heatsinks).