noun, noun which may take the genitive case particle 'no'
growth period; period of rapid growth
Refers to a time when someone or something is growing quickly, often used for children, adolescents, or plants.
うちの子は今、伸び盛りで、服がすぐ小さくなる。
Our child is in a growth spurt now, so clothes quickly become too small.
この植物は伸び盛りだから、水をたくさんあげ
This plant is in a period of rapid growth, so give it plenty of water.
成長期 is a more formal or technical term for a growth phase, while 伸び盛り has a more vivid, everyday feel emphasizing the visible rapid growth.
Compound of 伸び (nobi, 'growth') + 盛り (zakari, 'peak' or 'height'), literally 'peak of growth'.