While light gives plants the energy they require, house plant fertilizer provides the nutrients they need to grow healthy and strong.
The fertilizer you need depends on your plant. Read the labels carefully. They usually list three numbers, such as 10-20-10. These are (in order) the percentages of nitrogen, phosphorus, and potassium (often called potash) that make up the major nutrients in the house plant fertilizer. And here's what they do:
Fertilizers specially formulated for flowering plants will contain less nitrogen and more phosphorus and potassium. You'll even find some labeled for specific plants, such as orchids.
Most house plants need a balanced fertilizer such as a 10-10-10 formula.
In addition to nitrogen, phosphorus, and potassium, plants need small amounts of other nutrients, or trace minerals -- called micronutrients.
You'll find some of these minerals, such as calcium, magnesium, and sulfur in good-quality potting mixes. And even smaller quantities of copper, iron, and zinc. However, plants that aren't repotted very often will need fertilizer to replenish their supply of micronutrients.
Water-soluble fertilizers are the most common and are easy to use. You'll find them in the forms of liquid, powder, or crystals. Manufacturers often suggest to use the maximum amount that a healthy plant can tolerate. This sells a lot of fertilizer, but may be too much for your plant. As a general rule, I use only half the recommended amount.
Slow-release granules (shown at left) are easy to use because you just sprinkle them on top of the soil and they'll dissolve when you water your plant.
Fertilizer spikes (at left) work the same way, but are inserted into the soil. Push spikes fairly deeply into the soil near the edge of the pot to avoid damaging the plant's roots.
Although the roots are responsible for taking in nutrients, the leaves can also absorb them. Foliar feeding is a quick way to revive a plant that has been deprived of nutrients. Most fertilizers will scorch leaves if they are sprayed directly on the leaves, but foliar feeds are made to be applied this way.
Here are some clues:
Whoa, there gardener! Many indoor gardeners mistakenly believe more is better when it comes to fertilizing house plants. The truth is, far more plants suffer from too much fertilizer than lack of it. Excess fertilizer can burn roots and leaves. When in doubt, use less.
How do you know if you've over-fertilized your plant? Look for these symptoms:
If you want to know how often to fertilize your plant, look it up in the House Plants Encyclopedia A-Z.