35-40 Days to maturity. Brassica rapa Chinensis group. San Fan Pak Choi Seeds. Annual, Hybrid. Cool-Warm Season. This popular green pak choi, also known as Shanghai pak choi hybrid, has thick, dark green leaves and light green petioles that are shaped like an hourglass. It is an early maturing plant with uniform plant growth and excellent tolerance to heat. It also adapts well to most areas provided temperatures stay in the 50-80°F range. A tender, easy-to-grow variety with excellent flavor and texture. ~ 10,000 seeds per oz.