Here’s an easy fertilizing tip for container plants, either indoors or out. Rather than fertilize monthly, the frequency usually recommended for soluble fertilizers, why not fertilize with each watering, but at a lower dose? For many plants, that would be about once a week.
The dose needs to be low, about 1/4 the usual requirement. So, if the fertilizer label recommends diluting one teaspoonful of fertilizer in a quart/liter of water per month, just add about one quarter of a spoonful of fertilizer before your weekly watering. You could even leave out an appropriately-sized measuring spoon near where you store your watering can just for that purpose.
The main advantage of this method is that it quickly becomes a routine. Unlike a once-a-month treatment you’re likely to forget half the time, it’s soon part of your normal watering process and you’re unlikely to forget.
Also, plants actually grow better under a constant fertilization program (which is what professionals would call this). Monthly feeding tends to encourage the plant to grow by spurts; it bursts into growth after its monthly feeding, then slows do as minerals become rare after two or three weeks. This can result in an unequal appearance. With constant fertilizing, there are always minerals to be had and plants grow more evenly.
Obviously, you should only fertilize plants that are in active growth and most plants have a seasonal growth habit, from spring through early fall, so, in most cases, you’d stop fertilizing houseplants weekly weakly in late fall and only start again in spring. Under plant lights, where it’s always summer, you can fertilize weakly weekly all year long.
There are other methods for fertilizing container plants (slow-release fertilizer, for example, tablets or fertilizer spikes), but many gardeners find the weakly weekly method perfectly suited to their habits.