Q: I see so many different kinds of houseplant fertilizer advertised. There seems to be a different one for every kind of plant. Do I really need to buy so many? What about plant hormones and root stimulants and mycorrhizal fungi? Are they necessary?
A: There certainly has been an explosion of specialized fertilizers, many of which have the same ingredients but different labels. I see ones made for everything from orchids to fiddle leaf figs. All plants need certain chemicals, and most kinds of plants want higher levels of one or another. Too little of one of the elements prevents the plant from growing at its best, but just as with humans, too much food can sometimes cause serious problems. More fertilizer will not make a plant grow better or bloom more often. If the nutrient is missing fertilizer will help. If it is not, it does no good.
Plants need micronutrients along with the big three, nitrogen (N), phosphorus (P) and potassium (K), which are the three numbers on a plant label. Some of the specialized fertilizers contain micronutrients, but most just have varying percentages of N-P-K. And different manufacturers can have wildly different numbers for the same type of plant.
Most of us can get by with an all-purpose houseplant fertilizer. Try to find one with the numbers as equal as possible. We call that a “balanced” fertilizer. If you use reverse osmosis, distilled or rain/snow water on your houseplants (which is a good idea if you have a water softener, as salt-softened water can kill plants pretty quickly), look for one that says it contains micronutrients. Micronutrients are usually already found in well and city tap water. Fertilize at half the label’s recommendation. Again, more fertilizer is not helpful. If it were, the company would be more than happy to tell you to use more. Some plants like succulents only need to be fertilized a few times each year.
As far as the stimulants and hormones, there is no science behind them. We may want them to work or think our plants grow better with them, but when tested in controlled settings they did nothing.
Mycorrhizal fungi are crucial to plant growth, but they have a knack of already being where they’re needed. There are lots of kinds and the odds of the one you buy being right for your plant (and missing from the soil) are pretty remote. Also, most of the ones for sale, either in a separate package or mixed in potting soil, are already dead. They won’t hurt, but I wouldn’t pay any extra for the addition.
Send your questions to firstname.lastname@example.org.