Foliar feeding of plants is defined as the spraying of nutrients onto the foliage of plants to increase the surface where nutrients can be assimilated by the plants. Orchids will absorb nutrients both through the roots and through their leaves. Several studies have shown that spraying very dilute fertilizer on the leaves will make them grow better than plants that do not receive such feedings.
In rainforest jungles, orchids catch a variety of nutrients from the air around them and absorb them through openings in their leaves. This ranges from nitrogen dissolved in rain to ammonia and carbonic acids in the form of gases absorbed through the stomata. We can mimic some of these naturally occurring phenomena by spraying fertilizer on leaves and allowing the plant to use the nutrients as it is able and needs. Foliar absorption is not as efficient as that through the roots, but more of an additive feature for the plants.
Commercial growers use sprayers that feed both leaves and roots at the same time since their operations are automated. Many hobby growers, including myself, will spray a very dilute fertilizer on a daily basis in the summer months to provide that extra bit to make the plants grow better. It can also be used to green up plants that are a bit yellowed before a show or other display.
Studies by Texas A&M University have shown that foliar fertilizing on Phals as they initiate inflorescences will make a difference in the growth of the inflorescence and the number of flowers on the spike. Foliar feeding as an adjunct to regular root feedings will help your plants achieve their potential.
For the hobbyist, foliar feeding means using a spray which is your normal fertilizer solution diluted half and half with additional water. Add 1/2 teaspoon of some kind of liquid dish soap per gallon to provide a sticking action on the leaves. Spray with some kind of good sprayer directly on the leaves and roots daily, preferably early morning before the sun gets on your plants. Spray all surfaces of the leaves, bottom and top. If you’ve added a bit of liquid soap you will have the added benefit of killing any insects that you spray.
Be sure that you have good air circulation to dry the leaves fairly quickly so that you do not risk the problem of rot. If you can, have a fan blowing lightly on your orchids to increase the evaporation of the spray. Be very careful of spraying the centers of Phals, Paphs and Phrags because they are susceptible to crown rot. Try not to spray the center of new growths of Dendrobiums since some species are also susceptible to rots.