There are any number of specific houseplant fertilizers available and most of them, while supplying your plants needs, are much more adept at separating you from your money. If you are growing some specialty items such as African Violets that you are trying to keep in bloom, then there may be some justification for buying a specific fertilizer. In our interior landscaping business we maintained thousands of indoor plants and used only one fertilizer. A good evenly balanced fertilizer such as 20 20 20 will take care of the vast majority of our houseplants. A soluble powder that is applied as part of the regular watering program is the easiest to use and to monitor.
Where Are They Growing?
We ask this question because we have to divide indoor plants into two groups. The first group are those plants that live by a window and get all of their light as natural light. The second group are those plants that receive mostly artificial light that is constant in terms of intensity and duration throughout the year.
Why Do We Care?
Naturally lit plants will have some seasonal variation in their need for fertilizer, while the artificially lit ones will have a regular year round requirement. Plants of either group, that are growing in high light intensities and therefore get watered more frequently, will need to be fertilized more often. Not more fertilizer just a more frequent application.
Rather than give you an easy, “once a month” answer, we have to consider all of the above factors. Nonetheless, houseplant fertilizing is still pretty simple. For artificially lit plants, add some of the soluble fertilizer to their normal water every fourth or fifth watering. This takes a bit of bookkeeping if some of your plants are watered weekly and some have a longer cycle. Naturally lit plants are fertilized on a seasonal schedule. From early November to mid March - no fertilizer. As the quantity of sunlight increases so will those plant’s demand for water. If you add the soluble fertilizer every 3 - 4 waterings then fertilizing houseplants becomes naturally adjusted as the increase in light increases the demand for water.
I have typically used about ½ the amount that the package recommends as its basic requirement. For most of our houseplants we are interested in maintaining them in a healthy state. Rampant growth is usually more of a problem than a desirable feature. A build up of excess fertilizer in the soil is one of the most dangerous conditions that indoor gardeners have to deal with. This is one of the reasons that a good houseplant container has adequate drainage. The waterings that have no fertilizer can leach out any excess through that good drainage.
If in doubt; don’t! Lack of fertilizer has killed very few houseplants.