Baking soda does not have a direct effect on the growth of plants. Applied on and around plants, however, it deters pests, disease and weeds.
A mixture of half baking soda and half flour sprinkled on cabbage, kale and broccoli plants kills cabbage worms that dine on the leaves of these plants. Application of a thick layer of baking soda eliminates weeds between cracks in a sidewalk or in small areas of a lawn or garden. One teaspoon of baking soda dissolved in a quart of water and sprayed on plants prevents the growth of fungus. Powdery mildew, which is problematic for squash, cucumbers, zinnias and lilacs and other plants, is treated by combining a gallon of water with a single teaspoon each of baking soda, vegetable oil and dishwashing liquid and spraying it directly onto the vegetation.