Application of a water-soluble salt of chitosan to the foliage of growing plants increases the yield of vegetables, tubers, cereal grains, fruits and blossoms. Plants so treated are healthier, sturdier, more resistant to drought and many varieties enjoy an extended period of production. The plants may be effectively and conveniently treated by spraying the foliage with a solution containing 0.01-1.5% wt. chitosan salt using conventional agricultural equipment and techniques.