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Fertilising basics

Also known as Feeding houseplants, N-P-K, Plant food

Growing know-howDepends
Granular fertiliser (diammonium phosphate) Photo: Suyash Dwivedi · CC BY-SA 4.0

In short

Fertiliser supplies mineral nutrients; it is not food in itself, since plants make their own sugars by photosynthesis. Houseplants need modest feeding only while actively growing, and too much fertiliser builds up salts that burn roots and leaf tips.

How it looks

Fertiliser labels give three numbers, N-P-K: the percentages of nitrogen, phosphorus (as phosphate) and potassium (as potash). A balanced general fertiliser such as 20-20-20 suits most houseplants. Forms include liquids, soluble crystals, granules, spikes, tablets and slow-release pellets.

Signs

Underfeeding: pale older leaves and slow growth during the growing season. Overfeeding: brown, dead leaf tips and margins, white crust on the mix, leaf drop, dead root tips, wilting and soft, leggy growth.

What favours it

Feed only when the plant is growing actively; in temperate regions that is roughly spring to early autumn (Kansas State suggests March to September). Plants in low light, in winter or in a rest period use very little and unused fertiliser accumulates as salts. In warm tropical homes many plants grow year round, so feed according to actual growth.

Prevention

  • Follow the label; general garden fertilisers can be diluted to half or quarter strength for houseplants, which grow slowly.
  • Feed only when there is active new growth; stop during dormancy or slow periods.
  • If in doubt, feed less often (for example every second or third watering) rather than more.
  • Leach pots every 4–6 months to flush accumulated salts.
  • Do not feed newly bought or struggling plants heavily while they settle.

Treatment

Start with the first step and move down only if the problem persists.

  1. For overfeeding, flush the mix with plenty of plain water several times, or repot small plants into fresh mix.
  2. Remove badly burnt leaves once new growth appears.
  3. For suspected deficiency, check light, watering and roots first; then resume light, regular feeding in the growing season.

Pros and cons

Pros

  • Regular light feeding keeps foliage colour good and supports steady growth in pots, where nutrients are limited.

Cons

  • Too much causes salt damage and weak, leggy growth.
  • Feeding cannot compensate for poor light or root problems.

Look-alikes

Salt and fertiliser burn
The damage from overfeeding: brown tips and margins, white crusts.
Nutrient deficiency
Pale, yellowing or patterned leaves during active growth may mean too little feeding.
Etiolation
Leggy growth from low light is not cured by fertiliser.

Typical uses

All container plants; carnivorous plants and many epiphytic orchids need very dilute or specialised feeding.

Sources

  1. How often should I fertilize houseplants? — Iowa State University Extension
  2. Fertilizing Houseplants — K-State Research and Extension, Wild West District
  3. Houseplant Care — University of Wisconsin–Madison Extension
  4. Watering Indoor Plants — University of Maryland Extension

Written in our own words from these sources. Check a product label before using any treatment.