The standard for hydroponic growing. Slow bolting and tolerant to tipburn, Rex performs extremely well for spring and summer greenhouse production. Packet: 30 seeds. Days to Maturity or Bloom: 50 CULTURE: Lettuce is hardy and can be planted as early as the soil can be worked. It is a cool weather crop and makes its best growth at temperatures of 60-65°F (16-18°C). Careful variety selection is important for hot weather crops. Sow every 3 weeks for a continuous supply of fresh lettuce.