After the spring rain, the lushness of the garden's vegetation was astonishing.

lushness 是由形容词 lush（茂盛的、葱郁的）加上名词后缀 -ness 构成的名词，表示"繁茂的状态"或"葱郁的程度"。lush 本身常用来形容植物生长旺盛、枝叶茂密的样子。