A word for a period covering two years
Specifically the phrase "two years".
Just like how you can say two moons for “two months”.
I've thought of: "two periods" but that doesn't seem right, and "two seasons" but not quite sure if it is correct.
Poetic or not as long as it would still be understandable to some extent.
The phrase goes something like this...
For it [flower] has seen [two something].
Biennium: a period of two years
With your added context, I suggest "For it has seen two summers...". Or whatever specific season you want, really, since they occur once a year.