Ask Mr. Smarty Plants

Since our specialty is plants native not only to North America but to the area in which they are being grown, we don't know much about African violets, which are native to (of course) Africa.  Adiantum capillus-veneris (common maidenhair) is native to California and, in fact, is shown as growing in the vicinity of Contra Costa County by this USDA Plant Profile. Ordinarily, we don't consider it necessary to do much fertilizing of plants native to the area, because they are already accustomed to growing in the climate and soil. However, if you are growing them as container plants, they should probably have a yearly application of a 10-20-10 garden food; that is, 10% nitrogen, 20% phosphorus and 10% potassium. We did check on what was recommended for African violets, and they apparently prefer a 20-20-20 plant food, much more frequently applied. All you have to do is read the label of any commercial fertilizer preparation you pick up, and it will tell you what the proportions are. We doubt that the African violet fertilizer you are using now would hurt the maidenhair fern, but don't overdo it!