High phosphorus fertiliser gets talked about like a shortcut to better growth and more flowers. For most indoor plants, it's neither.
What phosphorus actually does
Phosphorus is the P in NPK — the middle number on any fertiliser label. It's most active when a plant is flowering, fruiting or setting seed. That's the part that actually matters here.
A fertiliser with an NPK of 10-15-5 is considered high phosphorus. A low-phosphorus formula — like our Indoor Plant Food — keeps that middle number deliberately low, because most foliage plants aren't trying to flower in your living room.
Most indoor plants don't need it
The majority of indoor plants are grown for their foliage — Monstera, Philodendron, Pothos, Devil's Ivy, ZZ Plant, Snake Plant. They're putting energy into leaves, roots and stems, not reproduction.
For foliage plants, excess phosphorus can lock out other nutrients — particularly iron, zinc and manganese — leading to deficiencies even when you're feeding regularly. It can also shift how a plant allocates energy, which isn't always what you want when you're after growth rather than flowers.
Our Indoor Plant Food is formulated with this in mind — lower phosphorus, higher nitrogen, designed for the foliage plants that make up most indoor collections. It's what we use, and a good starting point for most indoor setups.
If you grow a lot of aroids specifically, our Aroid Food takes this further — a urea-free, nitrate-based formula with calcium and magnesium built in.
Learn more about Aroid Food
Nutrients only work as well as the conditions around the plant allow. If light is low and growth slows, adding more phosphorus won't change that — it'll just sit in the mix, building up without being used. The same goes for watering — a plant sitting in soggy mix can't absorb nutrients effectively regardless of what you're feeding it. Getting light and water right first gives fertiliser something to actually work with. We've blogged about this here
When it actually helps
There are situations where a phosphorus boost is genuinely useful:
- Flowering indoor plants — Peace Lilies, Anthuriums, orchids and other blooming plants benefit from higher phosphorus during their budding and flowering stage. It supports bud formation and prolongs blooming. This is where high-P fertiliser actually earns its reputation.
- Newly planted or transplanted seedlings — phosphorus supports early root establishment, so a higher-P formula can help a new plant settle in faster.
- Root vegetables — if you're growing carrots or beetroot in pots, phosphorus encourages healthy root development. Less relevant for most indoor setups, but worth knowing if you grow edibles.
One exception worth calling out: Australian natives
Australian soils are naturally old, weathered and low in phosphorus — and our native plants have evolved accordingly. Many are highly sensitive to elevated phosphorus and can be damaged or killed by standard fertilisers.
If you grow Grevillea, Banksia, Waratah or Hakea — indoors or out — use a fertiliser with phosphorus below 3%. Many are specifically labelled "native-safe."
How to read the label
NPK is straightforward once you know what you're looking for. For most indoor foliage plants, you want nitrogen leading and phosphorus kept low. If the middle number is the highest, it's probably not right for your foliage plants.
- High phosphorus: middle number above 10 (e.g. 10-15-5)
- Low phosphorus: middle number below 3 (e.g. 4-1-3)
- Balanced: roughly equal across all three (e.g. 5-5-5)
The bottom line
High phosphorus fertiliser has its place — but for most indoor plants, it's not your default. If your plants are putting out healthy new growth, you're better off with a low-phosphorus, foliage-focused feed and focusing on the conditions the plant is growing in. Light, watering and a decent potting mix will do more for your foliage than chasing a higher P number.
Feed every two weeks during active growth, pull back in winter, and let the plant tell you if something's off.
Further Reading
How often should you water indoor plants
Not sure which fertiliser is right? Start here
Leave a comment