When, how, and how much to feed houseplants is a perennial puzzle. The proper application of the right fertilizer at the right time, however, will greatly improve the health of your plants.

Fertilizing houseplants regularly is important because the available nutrients in most potting mixes are usually depleted quickly. Because the ratio of root mass to soil volume is much higher than that of garden plants, the roots have fewer nutrients to consume. And every time a houseplant is watered, the nutrients wash out of the soil a little at a time. Although the indoor environment might seem comfortable to you, it can be extreme for houseplants: Low humidity, dry, dusty air, and too little or too much sunlight can overpower a plant that doesn't have the nutrients it needs.

There are many different forms of fertilizers for houseplants, from powders to spikes to premixed liquids. Two of the most convenient and popular types are slow-release pellets and water-soluble crystalline plant food. Slow-release pellets are effective and easy to use: Mix them in with your potting soil, and every time you water, a small amount of fertilizer is released. Pellets come in 2-, 4-, and 9-month-release timetables. They should be your choice if you don't want to have to remember when and how much to fertilize each plant.

Water-soluble crystalline plant food is a powdery substance that you mix with water and pour into the plant's soil (never pour fertilizer on the leaves of a houseplant). A complete fertilizer will have nitrogen, which encourages green, leafy growth, and phosphorus and potash, which enhance vigor and overall health.

The frequency of plant feedings depends on the time of the year and the composition of the mix. In fall and winter, a houseplant won't need quite the support that it does in spring and summer, because it won't be growing as quickly and thus will not be using its nutrients as quickly. If the fertilizer is very potent, frequent feedings can harm the plant. If you want to feed the plant each time you water, then dilute the mixture considerably, so it is the right potency, and follow the manufacturer's recommendations for use.