Houseplant fertilizer: what to use, when to feed and how much

Feed houseplants only while they are actively growing, roughly March through September: either the full label dose every 2–4 weeks or a solution 8–10 times weaker at every watering. Use a balanced fertilizer such as 10-10-10 or 20-20-20 with micronutrients for foliage plants, switch flowering plants to a higher-phosphorus mix like 15-30-15 when buds form, and stop for most of winter.

The rest of this page is what a label can't tell you: who should not be fed at all, how to measure a dose for one pot, what UK favorite Baby Bio actually is and what to buy instead in the US, and how to rescue a plant that got too much.

When to feed, and when not to

Clemson Extension's indoor plant care fact sheet adds a useful threshold: plants need at least about 200 foot-candles for 12 hours a day to make use of fertilizer. A pothos in a dark hallway can't turn extra nutrients into growth, so the salts just accumulate. Fix the light first, then feed. University of Illinois Extension's houseplant care guide is blunt about the same point: fertilize only when the plant is actively growing, and in low light growth slows and burn risk rises.

How to read an NPK label

The three numbers are the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 20-20-20 is 20% of each; a 1-1-1 has the same ratio but is far more dilute, so you use more of it per gallon.

• N drives leaves and stems: the main need of foliage plants.
• P supports roots and flowering.
• K supports overall vigor, stem strength and bloom quality.

Look for micronutrients on the label too (iron, manganese, zinc, magnesium). Bagged peat- and coir-based mixes run short on them after a few months.

What to pick by plant group

Illinois Extension lists 5-10-5, 10-20-10 and 10-10-10 as typical houseplant ratios; Clemson recommends 20-20-20 for foliage and 15-30-15 for bloomers. All of these work. Too much nitrogen on a flowering plant is the usual reason it is lush but won't bloom.

Baby Bio and its US equivalents

Baby Bio is a long-running British brand, which is why "baby bio houseplant food" and "baby bio concentrate houseplant food" are such common searches. The classic product is a small 175 ml bottle of liquid concentrate. UK garden centers such as Ashwood Nurseries list it at about 10.6-4.4-1.7 NPK, dosed at 5–10 drops per half liter (roughly a pint) of water every two weeks in the growing season. In other words, it is a nitrogen-leaning, drop-dosed liquid feed for foliage plants. Nothing about it is special beyond being easy to measure.

It is sold mostly in the UK and Ireland; in the US it turns up only through import sellers, at a premium. You can get the same result locally by type:

The search "miracle-gro tropical houseplant food" points to the same category in the US. Miracle-Gro's Indoor Plant Food for All Plants is a mild 1-1-1 liquid: one pump on the soil for small pots, two for pots over 6 inches, or four pumps per quart of water, every two weeks. Brands such as Espoma, Schultz and Dyna-Gro make comparable liquids. The best houseplant fertilizer is the one you will actually dose correctly. Brand matters much less than the dose.

Forms of fertilizer

• Liquid concentrate and water-soluble crystals. The easiest to control and the best fit for pots. Clemson favors them because dilute solutions reduce burn risk.
• Slow-release granules or coated pellets. Feed for 2–6 months from one application. Good for busy owners, but you can't switch them off in winter or if the plant gets sick.
• Spikes and tablets. Salts concentrate around one spot and burn the roots next to it. Not recommended for houseplants.
• Organic liquids (fish, seaweed, worm-casting teas). Gentler, but they smell and can draw fungus gnats indoors. If gnats show up, see the fungus gnat removal guide on our sister site.
• Foliar sprays. A short-term fix for micronutrient deficiencies, not a diet; see foliar feeding.

How to measure and apply a dose

Example: the label says 1/4 teaspoon per gallon, and you need to water one 6-inch pot with about 2 cups (1/8 gallon). That's a pinch. It is easier to mix a full gallon, use what you need, and pour the rest on other plants the same day. For drop-dosed concentrates, a 5 ml oral syringe from a pharmacy is the cheapest way to hit the dose.

1. Water with plain water first and wait 20–30 minutes. Fertilizing dry soil scorches roots.
2. Measure the water with a measuring cup, not by eye.
3. Measure the concentrate, mix until fully dissolved.
4. Pour evenly over the whole surface until a little drains into the saucer.
5. Empty the saucer after 20 minutes so salts aren't wicked back up.
6. Note the date and dose.

Clemson gives three workable schedules: full strength on a longer interval, weekly at one-tenth strength, or feeding every seventh watering. Weak-and-constant gives the most even growth but needs a monthly plain-water flush. Never make up for a missed month with a double dose. Extra salt does more harm than a month without food.

Deficiency or something else?

Most yellow leaves come from overwatering, low light or root damage, not missing nutrients. Rule those out before adding fertilizer.

Overfeeding and salt burn

Signs of salt buildup: a white or yellowish crust on the soil and pot rim, crisp brown tips and margins, wilting in wet soil, dropping lower leaves and stalled growth. University of New Hampshire Extension's fertilizing houseplants article explains why: fertilizers are salts, and too much salt makes it hard for roots to take up water and burns root tips and leaf edges.

If the roots are healthy:

1. Stop fertilizing for at least 8 weeks.
2. Scrape off the top half inch of crusted soil and top up with fresh mix.
3. In the sink, slowly pour about two pot volumes of lukewarm water through the pot in 3–4 rounds.
4. Let it drain fully; don't reuse the runoff.
5. Repeat after a week if the crust returns.
6. Resume at half strength once a new leaf appears.

For prevention, Illinois recommends leaching every 4–6 months and UNH about every four months for any plant on a regular feeding schedule. If the roots are already brown and mushy, flushing makes things worse; repot and trim instead.

Common mistakes

• Feeding right after repotting. Fresh bagged mix usually contains starter fertilizer; wait 6–8 weeks. See potting soil.
• Feeding a sick or freshly bought plant to "help it settle in."
• Keeping a summer schedule in a dim winter apartment with the heat on.
• Using spikes or loose granules on the soil surface in a small pot.
• Mixing "a bit extra." Illinois and UNH both say never exceed the label rate.

Safety

Store concentrates out of reach of kids and pets; drop bottles look like medicine. If a child swallows fertilizer, call Poison Control at 1-800-222-1222. For pets, call your vet or the ASPCA Animal Poison Control Center at (888) 426-4435.