Hemp requires a lot of moisture. Measurements indicate the crop needs 300-400 mm (10-13 in.) of rainfall equivalent. Since that amount of rainfall may or may not occur during your growing season, it is important to make use of early soil moisture and to obtain early ground cover to reduce surface evaporation, as well as to maintain good weed control.
About half of this moisture is required during flowering and seed set in order to produce maximum grain yields. Drought during this stage reduces seed set and produces poorly developed grain heads. Continued drought results in low yields of light grain.
During the period of vegetative growth, hemp responds to daytime high temperatures with increased growth and increased water needs. It is said that after the third pair of leaves develops, hemp can survive daily low temperatures as low as -0.5°C. for 4-5 days.