How to fertilize houseplants (without burning them)

Fertilizer has a reputation as a plant's dinner. It is closer to a vitamin. A plant makes its own food from light, air and water, and fertilizer supplies the minerals it cannot make: the nitrogen, phosphorus, potassium and trace elements that potting soil slowly runs out of.

That matters for how you use it. Fertilizer will not rescue a plant that is short of light, rotting from overwatering or covered in pests, and it can harm a plant that is already struggling. For a healthy plant that is growing, a little at the right time makes a real difference, and too much is one of the easiest ways to damage roots.

N-P-K in plain words

Every fertilizer label has three numbers, such as 10-10-10 or 20-10-20. They are the percentages, by weight, of three main nutrients:

N, nitrogen: drives leafy, green growth. Foliage plants use a lot of it.
P, phosphorus: supports roots and flowers.
K, potassium: helps the plant's overall health and its handling of water.

A balanced fertilizer, where the three numbers are close, is a sensible all-round choice for most houseplants. For foliage plants such as pothos, monstera or ferns, a mix a little higher in nitrogen is common. For flowering plants such as African violets or orchids, products made for them usually carry more phosphorus and potassium. If a bottle is labeled for houseplants, it is built for this job.

Many fertilizers also list trace elements such as iron, magnesium, calcium and manganese. They are needed in small amounts and help prevent odd yellowing, so a houseplant fertilizer that includes them is worth a look.

When to feed

Feed when the plant is growing. For most houseplants in most homes that means spring through early autumn, when the days are long and new leaves are appearing. In winter, with short days and low light, growth slows or stops, and a plant that is not growing cannot use the extra nutrients. They build up in the soil instead.

Some useful rules of thumb:

New growth is the signal. If the plant is putting out leaves, it can use food. If it is sitting still, hold back.
Water first. Never fertilize dry soil. Dry roots take up concentrated fertilizer too quickly and burn. Water lightly first, or apply the fertilizer with a normal watering.
Skip sick plants. If a plant is wilting from root rot, dried out, newly bought and stressed, or fighting pests, fix that first.
Wait after repotting. Fresh potting mix usually has some nutrients in it. A month or two without feeding is typical. See when and how to repot.
Plants under grow lights, or in a warm, bright room, may keep growing through the winter and can be fed lightly then.

How weak to mix it

The most important habit is to go weaker than you think. Fertilizer is made of salts, and salts draw water out of roots when the mix is too strong. That is what "burning" means. Plants in pots cannot spread their roots away from a strong dose the way plants in the ground can.

Follow the label as a ceiling. Many growers mix at half the stated strength, and some go to a quarter strength and feed more often. The idea is a steady, gentle supply instead of a sudden jolt.

Plants that are slow growers or low in light, such as snake plants, ZZ plants, succulents and cacti, need much less food than fast-growing leafy plants. When in doubt, use less.

How often

There is no universal number, but these are reasonable starting points for the growing season:

Liquid fertilizer at half strength every 2 to 4 weeks, or at quarter strength every other watering.
Slow-release pellets or sticks as often as the package says, commonly every few months.
Succulents and cacti: a few times a season at most.

Write down the date when you feed. It is surprisingly easy to lose track, and the plants that suffer are usually the ones fed "just a bit extra" by two different people.

Liquid or slow-release?

Liquid fertilizer is mixed into the water. You control the strength each time, it works quickly, and it is easy to dilute. The downside is that you have to remember it.

Slow-release pellets or spikes release nutrients over weeks or months as you water. They are convenient. The downside is that you cannot take a dose back, and spikes concentrate nutrients in one spot, which can burn roots nearby. Mixing pellets evenly into the top layer of soil is gentler.

Organic options, such as worm castings, compost or fish-based fertilizers, feed more slowly and are harder to overdo, though they can smell, and the strength varies. Follow the instructions on the product.

Pick one method and use it consistently. Stacking several, such as a liquid feed on top of slow-release pellets, is how plants end up with far more than anyone meant.

How to apply it

Check the plant is growing and the soil is not bone dry.
Mix the fertilizer into water at the strength you chose, and stir well.
Pour it onto the soil, not over the leaves, and let it drain from the bottom.
Empty the saucer so the roots do not sit in the salty runoff.
Note the date.

Signs of too much fertilizer

Too much is more common than too little, and the signs are easy to miss:

A white or yellowish crust on the soil surface, or a white ring on a terracotta pot.
Brown, crisp leaf tips and edges.
Wilting even though the soil is moist.
Slow or stunted growth, or a sudden flush of weak, leggy leaves.
Yellowing, dropping leaves.

Signs of too little

Growth is slow during the growing season.
New leaves come in smaller or paler than older ones.
Older leaves fade to pale green or yellow.

Be careful here: light and watering cause these same signs more often than hunger does. If a plant is pale and the room is dim, more light will do more good than more fertilizer. The yellow leaves guide helps tell the causes apart.

If you gave too much: flush the soil

If you see a crust, burnt tips or a plant that seems worse after feeding, flush the pot:

Take the plant to a sink or outdoors.
Slowly pour lukewarm water through the soil, using several times the volume of the pot, and let it all drain out through the holes.
Let the pot drain completely, and empty the saucer.
Scrape off any crust from the surface.
Do not fertilize for a month or so, and let the plant show you new, healthy growth before you start again, at a lower strength.

If the roots look brown and damaged, or the plant keeps declining, repotting into fresh mix is the more reliable fix. Trim away dead roots as you go.

The short version

Feed only while the plant is actively growing.
Water first, then feed, and never on dry soil.
Go weaker than the label and more gently.
Use one method, and write down the date.
If a plant looks unwell, check light, water and pests before you reach for fertilizer.