growth period; growing season
Refers to a period of growth, whether biological (e.g., adolescence, plant development) or economic.
Children need a lot of nutrition during their growth period.
The growing season for this plant is from spring to summer.
成長期 is the formal, biological term for the growth period, while 育ち盛り is the warmer everyday word emphasizing a child's active growing and hearty appetite.
Compound of 成長 (seichō, 'growth') + 期 (ki, 'period').