The longer days not only lift our spirits, but spur renewed growth in our houseplants.

In winter, most houseplants languish a bit under less-than-ideal conditions. Our homes tend to be too warm and dry for houseplants, many of which hail from the tropics, and the natural light they receive often is too scant to make them grow actively.

But once spring rolls around, plants start to perk up and send out tender new sprouts. New growth for them means it's time for us to resume a regular fertilizing schedule.

Food for growth

Plants actually make their own "food" by converting light energy to sugars and other carbohydrates through the process of photosynthesis. As days grow longer and brighter, more light falls on a plant's leaves. That increases the rate of photosynthesis, which produces extra carbohydrates for plant growth and maintenance.

So, why do we need to feed plants if they make their own food?

Houseplant fertilizers supply a combination of important minerals that supplement the carbohydrates that plants make on their own. These minerals -- primarily nitrogen (N), phosphorus (P) and potassium (K) -- are called major or macro-nutrients because plants need them in significant quantities to be healthy.

Nitrogen is associated with stem and leaf growth; phosphorus with root development and flowering; potassium with stem strength and disease resistance. All three are important on a cellular level. If they're missing from the soil, plant growth will be poor.

Regardless of whether a fertilizer is designed for vegetables, grass or houseplants, its contents are always represented in the same order on the label: N,P,K. A typical houseplant fertilizer with the numbers 15-30-15 means the contents are 15 percent nitrogen, 30 percent phosphorus and 15 percent potassium.

Some houseplant fertilizers also contain micro-nutrients or trace elements, minerals that plants need only in tiny amounts. Trace elements include boron, copper, iron, manganese, molybdenum and zinc. Usually there are enough trace elements present in the soil and water to satisfy a plant's needs, but because many houseplants are in soil-less potting mixes, it's a good idea to use a fertilizer containing trace elements, especially if you haven't repotted the plant in a while.

Mix master

Houseplant fertilizers are available in several forms. My favorites are water-soluble fertilizers.

No matter what kind of fertilizer you choose, it's a good practice to mix it at half the strength recommended on the label. Then, water it thoroughly into the soil every three weeks or so when the plant is growing actively, in spring, summer and early fall. If you prefer to fertilize each time you water, cut the amount of fertilizer to one-eighth the recommended strength.

Timed-release pellets can work well, too. Scratch them into the surface of the potting soil. A small amount of nutrients will be released into the soil every time you water. Just remember not to add them in autumn. That's when it's time to cut back on fertilizing houseplants.

Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.