Fertilising Your Bonsai: When, What, and How Much
Because a bonsai lives in a tiny volume of fast-draining soil that gets flushed every time you water, it cannot forage for nutrients the way a garden tree does — so feeding is not optional, it is how the tree eats. This guide covers what the numbers on the bottle mean, when to feed through the year, and the situations where feeding does harm instead of good.
Because a bonsai lives in a tiny volume of fast-draining soil that gets flushed every time you water, it cannot forage for nutrients the way a garden tree does — so feeding is not optional, it is how the tree eats. This guide covers what the numbers on the bottle mean, when to feed through the year, and the situations where feeding does harm instead of good.
Why bonsai need feeding at all
A tree in the ground sends roots out to find nutrients. A bonsai has a confined root ball in an open, inorganic-heavy soil designed to drain freely and hold plenty of air. That same free drainage means every thorough watering washes dissolved nutrients out of the pot. Add the fact that the tree is pushing repeated flushes of growth in a small space, and it becomes clear the tree relies almost entirely on you to replace what is lost. Skip feeding and you get pale leaves, weak short growth and a tree that slowly runs down.
NPK basics: reading the numbers
Every fertiliser lists three numbers, the N-P-K ratio:
- N (Nitrogen): drives leaf and shoot growth and green colour. The main fuel for foliage.
- P (Phosphorus): supports roots, flowering and fruiting.
- K (Potassium): overall vigour, hardiness and disease resistance; helps the tree ripen wood.
For general bonsai use, a balanced feed (e.g. roughly equal numbers like 10-10-10 or 6-6-6) works well most of the year. Two useful tweaks:
- A slightly higher-nitrogen feed in spring pushes strong early growth.
- A lower-nitrogen, higher-potassium feed in late summer/autumn discourages soft new growth and helps hardy trees ripen and harden off before winter.
- For flowering and fruiting species, a higher-phosphorus feed as buds set improves the display.
Organic vs chemical: which to use
Both work; they behave differently.
- Organic feeds (rapeseed/cottonseed cakes, seaweed, fish emulsion, pelleted chicken manure) release slowly as soil microbes break them down. They are gentle, hard to overdose, and improve soil life — but they smell, can grow mould, and act slowly. Solid cakes or pellets placed on the soil surface (often under little mesh baskets to stop them washing away) are a low-effort classic.
- Chemical/mineral feeds (soluble powders and liquids) are fast, precise and clean, giving you exact control over the ratio and timing — but they are just as easy to over-apply, and overdoing it burns roots.
A common practical approach: slow-release organic cakes as a baseline, topped up with a dilute liquid feed during the strong growth of spring and summer. Whatever you choose, follow the label and, if anything, err weaker.
A feeding schedule by season
Feed in step with growth. A dormant tree is not eating, so feeding it just leaves salts in the soil.
- Early spring (buds moving): begin light feeding as growth starts. Hold off on newly repotted trees (see below).
- Late spring to midsummer (strong growth): the main feeding window. Feed regularly — for liquids, typically every 1–2 weeks at label strength or weekly at half strength; refresh solid organic cakes roughly monthly.
- High summer heat: if a tree stalls in extreme heat, ease off slightly and resume as it picks up. Keep watering.
- Late summer to autumn: switch to a lower-nitrogen, higher-potassium feed to harden hardy trees for winter. Ease off as growth slows.
- Winter: stop feeding outdoor hardy trees that are dormant. Tropical/indoor trees that keep growing can be fed lightly through winter, but at reduced frequency because light and growth are lower.
How much: less is more
The most common feeding mistake is overdoing it in the belief that more food means faster growth. It does not — excess simply builds up as salts that scorch fine roots (over-fertilising), leaving leaf edges brown and crispy despite the pot being full of feed.
- Start at half the recommended strength and build up only if the tree clearly wants more.
- Never apply liquid feed to bone-dry soil; water first, then feed, so the solution spreads evenly and does not burn dry roots.
- More frequent, weaker feeding beats occasional strong doses.
- If you suspect a build-up of salts, flush the pot with plenty of plain water several times to leach it out.
When NOT to fertilise
There are clear situations where feeding actively harms the tree:
- Just-repotted trees: repotting cuts roots, and fresh root tips are easily burned by fertiliser salts. Wait about 3–4 weeks — until you see new growth confirming the roots have recovered — before feeding.
- Sick, weak or stressed trees: a struggling tree cannot use the food, and feeding a plant with damaged or rotting roots makes things worse, not better. Diagnose and fix the underlying problem (watering, light, pests, root rot) first. Feed does not cure illness.
- Fully dormant trees: no growth means no uptake; feed just accumulates as salts.
- Newly bought trees still settling in: let them acclimatise and show active growth first.
The rule to remember: feed a growing, healthy tree — never a stressed, dormant or freshly repotted one.
A quick starter routine
- Use a balanced feed as your default.
- Feed only when the tree is actively growing, starting at half strength.
- Water first, then feed, and never onto dry soil.
- Ramp nitrogen up in spring, dial it down in late summer for hardy trees.
- Pause after repotting and whenever a tree is unwell.
Get this rhythm right and you will see denser foliage, better colour and stronger, more even growth season after season.