What Do You Fertilise Hibiscus With?

Fertilise hibiscus with a balanced, complete NPK during active growth, then emphasise potassium and calcium–magnesium to support heavy blooming and leaf strength. As a simple, proven program: use CompleteGrow NPK 20-20-20 as your routine feed, and step up to Advanced Formula NPK 4-18-38 + CaMg when setting buds, chasing colour, or correcting deficiency symptoms.

NPK for Hibiscus: What Each Number Does

N (Nitrogen) drives green foliage and canopy building. Too much N = lush leaves, fewer flowers.

P (Phosphorus) supports root development and early bud initiation (especially important after pruning or transplant).

K (Potassium) is key for bud formation, colour intensity, and bloom longevity; also improves stress tolerance.

Ca & Mg (Calcium & Magnesium) underpin strong cell walls and chlorophyll production. Ca/Mg shortfalls show up as tip burn, weak leaves, or interveinal yellowing on new growth.

Micros (Fe, Mn, Zn, B) fine-tune colour and growth; high pH or waterlogging can lock them out.

Which Ratio When?

Establishment & recovery (post-planting/prune): A balanced, complete feed such as NPK 20-20-20 encourages root regrowth and steady foliage without pushing soft, lanky shoots.
Vegetative framing (spring flush): Keep 20-20-20 to build a healthy framework of shoots and leaves that will carry flowers later.
Pre-bloom & in-bloom (late spring–summer): Shift emphasis to potassium and Ca/Mg with 4-18-38 + CaMg to support bud set, colour, and petal integrity while preventing leaf chlorosis.
Yellowing new leaves / weak buds: Prioritise Ca/Mg and consistent K—use 4-18-38 + CaMg until colour deepens and growth normalises, then resume your regular cadence.

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Feeding Schedule & Application (Australia)

Spring: Resume feeding every 2–3 weeks as buds swell. Start with 20-20-20 to rebuild canopy after winter/light pruning.
Summer: Maintain the 2–3-weekly cadence. Alternate as needed: continue 20-20-20 for balanced growth, or run blocks with 4-18-38 + CaMg during heavy bud/flower cycles.
Autumn: Taper frequency and dose as growth slows; avoid pushing soft growth into cooler nights.
Winter: Pause fertiliser; water sparingly to prevent stem shrivel. Restart in spring.

Pots vs In-Ground & Product Formats

Potted hibiscus: Prefer liquids/solubles for precise dosing and quick uptake. For 20-20-20, use your standard mix of 4 mL per litre of water (maintenance), adjusting slightly with heat and plant size. For 4-18-38 + CaMg, mix to label grams per litre and observe plant response.
In-ground hibiscus: Liquids or soluble powders both work. Water in thoroughly after feeding to carry nutrients into the root zone.
Flush salts seasonally: Run clean water through pots until it drains freely to prevent tip burn and yellow margins from salt build-up.

Seven Application Rules for Reliable Blooms

Feed on moist media; avoid applying to bone-dry soil.
Soak thoroughly after feeding to distribute nutrients.
Maintain a wet–dry rhythm (deep water, then partial dry-down).
Prioritise sunlight (6–8+ hrs); fertiliser can’t replace light.
Keep pH ~6–7; high pH locks out Fe/Mg—use CaMg support if new leaves pale.
Rotate to 4-18-38 + CaMg during heavy bud set or when chasing colour/flower count.
Back off nitrogen if you see lots of leaves but few buds.

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FAQs: Hibiscus Fertiliser & NPK Ratios

What NPK ratio is best for hibiscus overall?
A balanced formula like 20-20-20 is a dependable baseline for leaf health, root strength, and general flowering. When buds are forming or colour is your priority, shift to 4-18-38 + CaMg to increase potassium and stabilise leaf colour with Ca/Mg.

Is a high-phosphorus “bloom booster” better than balanced feeding?
Not for long-term plant health. Consistent balanced nutrition with adequate K (and Ca/Mg support) produces more reliable flowering than occasional high-P spikes.

How often should I fertilise hibiscus in pots?
Every 2–3 weeks through spring–summer. Use 4 mL/L for 20-20-20 and follow label grams per litre for 4-18-38 + CaMg. Flush pots each season.

My leaves are yellow but veins are green—what should I use?
That pattern suggests Mg/Fe issues. Use a CaMg-fortified feed—4-18-38 + CaMg—and ensure pH and drainage are on point.

Can I alternate 20-20-20 and 4-18-38 + CaMg?
Yes. Many growers run 20-20-20 for canopy and general health, then switch or alternate with 4-18-38 + CaMg during heavy bud set and peak bloom.

Should I fertilise in winter?
No. Pause feeds during dormancy; water just enough to prevent stem shrivel and resume in spring.