The phrase "be in flower" suggests the state of blooming and flourishing. The opposite of this would be the antonym "wither" or "fade". When a flower withers, it loses its vibrance and vitality. It becomes dull and lifeless. Another antonym could be "dormant" or "inactive" which signifies a lack of growth and development. When a flower is dormant, it is not showing any signs of blooming or thriving. In conclusion, the antonyms for "be in flower" would be "wither," "fade," "dormant," and "inactive.