In addition to their roots, plants are capable of absorbing nutrients through their leaves and stem. It is most commonly used to apply trace minerals to the plants, and sometimes as an emergency treatment if the plant is sick or lacking nutrients. Soluble synthetic fertilizers can be used, as well as compost tea and kelp.
The most common time to apply nutrients to the leaves is in the morning or evening, as the summer heat and sun causes some plants to close their pores during the day, thus reducing the effectiveness of the foliar application.
Some gardeners believe that foliar feeding is effective when tomatoes are flowering, as this may lead to increased fruit production.
Cover Image by Chrissy Wainwright, used under its Creative Commons license.