The secret to fertilizing your houseplants lies in moderation.
Rule Number One: Less is more
The number-one rule for fertilizing plants bears repeating: When it comes to fertilizing your plants, less is more. Go ahead and fertilize your plants, but never give them as much fertilizer as the manufacturer's label suggests. Fertilizer companies want to encourage you to use as much fertilizer as possible (you use it up quicker, you buy more often). The dosage on the label usually represents the largest amount of the fertilizer that a healthy plant growing under ideal conditions can tolerate without feeling ill effects.
Under less than ideal growing conditions (like those in the average house or apartment), plants won't absorb large amounts of fertilizer. If a plant lacks light and humidity, it doesn't synthesize the fertilizer as quickly because it isn't operating at peak performance. Therefore, fertilizer builds up in the potting mix unused.
Unless you're certain that you are providing your plants with absolutely perfect growing conditions, never apply more than half the recommended dose of fertilizer.
Rule Number Two: Never fertilize a weak plant
A plant that's in bad shape, such as one suffering from insects or disease, recovering from a bad shock (such as a spill to the floor), or struggling with root damage, simply can't utilize fertilizer properly. Wait until you see healthy new leaves appear or note other obvious signs of recovery before you fertilize the plant again.
Rule Number Three: Some plants don't live by the rules
Some plants do require more fertilizer than others. Flowering plants and plants grown for fruit require more light, more water, and more humidity than other plants. If you boost the amount of growing essentials (and you have to if you want them to perform), it stands to reason that they require more fertilizer as well.
Just don't go overboard: It's easier to add a little more fertilizer if necessary than to remove excess fertilizer from a plant that you've pretty much poisoned by overfertilizing.
So, when's the best time to fertilize?
Wait for a month or so before fertilizing newly purchased or freshly repotted plants. Not only does their mix usually already contain fertilizer, but also the last thing they need as they acclimate to their new pot or home is an extra dose of fertilizer. (Remember: Never fertilize a weak plant. Plants adapting to a new environment qualify as weakened.)
Fertilize plants only during their active growth phases. Most plants grow most strongly from spring through summer and need the most fertilizer at that time. Begin reducing the fertilization rate in the autumn (an excellent time to apply a bit of tomato fertilizer, which is rich in potassium, to help the plant through the dark days of winter). You may want to apply fertilizer at half the recommended rate in spring and summer and then cut back to a quarter of the rate in autumn.
Most plants grow slowly, if at all, in winter. Give them either no fertilizer during this time of year, or only a weak dosage. Never fertilize a plant that is completely dormant.
