Fetching details from the dictionary...

to become vigorous; to gain force; to become more active
to be in heat; to be in rut; to copulate
「盛る」 has another reading with different meanings.
ご飯をお茶碗に山盛りに盛ってもらったよ。
I had rice heaped high in a bowl.
祭りの日は町全体の活気が盛った。
On the day of the festival, the whole town's energy surged.
春を迎えて山の木々の緑が盛っている。
With the arrival of spring, the greenery on the mountain is flourishing.