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Best Fertiliser for Magnolias in Australia
Quick answer
The best fertiliser for magnolias in Australia is a balanced liquid NPK 20-20-20 through the growing season, then NPK 4-18-38 with calcium and magnesium from bud swell through flowering. Mix the 20-20-20 at 4 mL per litre (2 mL for pots), feed from early spring to autumn, and don’t feed deciduous magnolias in winter.
- Everyday feed: NPK 20-20-20 liquid at 4 mL per litre
- Flowering feed: NPK 4-18-38 + calcium & magnesium, from bud swell
- Young trees and pots: weekly in spring and summer (pots at 2 mL per litre)
- Mature trees: fortnightly from early spring to autumn
- Avoid: high-nitrogen lawn fertiliser near the roots, which cuts flowering
What Fertiliser Do Magnolias Need?
Magnolias need a balanced fertiliser with nitrogen, phosphorus and potassium in similar amounts, plus calcium, magnesium and trace elements such as iron and manganese. Here is what each one does:- Nitrogen (N): drives leafy growth, especially in young trees. Too much means lots of leaves and few flowers.
- Phosphorus (P): supports root development and flower bud formation.
- Potassium (K): improves flower colour and size, and general resilience.
- Calcium & magnesium: strong cell walls and deep green leaves.
- Trace elements (iron, manganese): prevent yellowing between the leaf veins (chlorosis), which is common in sandy or alkaline soils and in pots.
CompleteGrow’s view: magnolias don’t “hate” phosphorus
You’ll read online that magnolias are sensitive to phosphorus. That advice belongs to Australian natives like grevilleas and banksias, which genuinely are. Magnolias aren’t in that group. What does cause problems is too much of anything, and too much nitrogen in particular, which gives you a big leafy tree and very few flowers. So we don’t avoid phosphorus. We time it: balanced feed for growth, and a phosphorus- and potassium-rich feed only while the tree is setting buds and flowering.
Which Complete Grow Fertilisers to Use on Magnolias
Magnolias do best on two fertilisers used at different times: a balanced feed for growth, and a bloom feed while the tree is budding and flowering.- Everyday feed: Complete Grow All Purpose Liquid Fertiliser (NPK 20-20-20). Use it from early spring to autumn for balanced growth, healthy leaves and root strength. It also carries iron, zinc, manganese, boron, copper and molybdenum, the trace elements magnolias need to stay deep green.
- Flowering feed: Complete Grow Advanced Formula NPK 4-18-38 + Calcium & Magnesium. Use it instead of the everyday feed from when the flower buds start to swell and through flowering. The extra phosphorus and potassium support bud set and bloom quality, and the calcium and magnesium keep leaves and flowers strong.
How Much Fertiliser to Mix for a Magnolia
NPK 20-20-20 liquid: 4 mL per litre of water (40 mL in a 10 L watering can). For pots, newly planted trees or a stressed tree, use half strength: 2 mL per litre (20 mL per 10 L).
NPK 4-18-38 + Calcium & Magnesium: per 10 L of water, use 8 g of the NPK 4-18-38, 8 g of Calcium Nitrate and 4 g of Magnesium Sulphate (one scoop is about 6 g). For pots, use the lighter rate of 6 g, 6 g and 3 g.
Important: mix and apply the three parts separately, and rinse the can between them. Calcium reacts with phosphate and sulphate at high concentration and drops out of the solution, so it would never reach your tree.
When & How Often to Fertilise Magnolias
Through the Year
- Early spring: start the 20-20-20 as new growth begins: weekly for young and potted trees, fortnightly for mature trees. Switch to the 4-18-38 + CaMg, on the same rhythm, once flower buds start to swell.
- Summer: keep going: weekly for young and potted trees, fortnightly for mature trees. Water deeply after every feed.
- Autumn: drop every tree to fortnightly, then stop once growth slows before winter.
- Winter: don’t feed deciduous magnolias while they’re dormant. Evergreens like ‘Little Gem’ in warm areas can have a fortnightly half-strength feed if they’re still actively growing.
How Often, by Type of Tree
Newly planted
When: wait 4–6 weeks after planting, then feed fortnightly for the first season.
Mix: 20-20-20 at half strength (2 mL per litre).
Young tree in the ground
How often: weekly in spring and summer, fortnightly in autumn.
Mix: 20-20-20 at 4 mL per litre.
Mature tree in the ground
How often: fortnightly from early spring to autumn.
Mix: 20-20-20 at 4 mL per litre. Switch to 4-18-38 + CaMg from bud swell through flowering.
In a pot
How often: weekly in the growing season, fortnightly in autumn. Nutrients wash out of potting mix quickly.
Mix: 20-20-20 at half strength (2 mL per litre). Flush with clean water every 6–8 weeks.
How to Fertilise, Step by Step
- Water the soil first. Feeding dry soil risks burning the roots.
- Mix at the right rate (see the mixing tip above). Don’t round up.
- Apply around the drip line. That’s the ring of soil under the outermost branches, where the feeder roots are. Magnolia roots spread wide and shallow, so fertiliser against the trunk is wasted and can cause root burn.
- Water it in well so the nutrients reach the roots.
- Mulch. A 5–7 cm layer of composted bark keeps moisture in and slowly improves the soil. Keep it clear of the trunk.
CompleteGrow’s view: why we prefer liquid for magnolias
Magnolia feeder roots sit close to the surface and spread well past the canopy. A liquid feed watered evenly over the drip line reaches all of them at once, at a strength you control. Granules tend to sit on top of mulch or get dropped in a pile near the trunk, which is exactly where you don’t want them. Controlled-release granules can work, but with liquid you know what the tree got and when.
And one rule we’d never break: don’t feed a thirsty or heat-stressed tree. Water it deeply, wait a day or two for it to recover, then feed.
Feeding a ‘Little Gem’ Magnolia
‘Little Gem’ is a compact, evergreen form of Magnolia grandiflora, popular in pots, hedges and small gardens. Because it keeps its leaves and flowers over a long season from late spring into summer, it benefits from steady feeding:
- In the ground: 20-20-20 at 4 mL per litre weekly in spring and summer while young. Once established, fortnightly is enough.
- In pots: 20-20-20 at 2 mL per litre weekly in the growing season, and flush the pot with clean water every 6–8 weeks to clear salt build-up.
- For flowers: switch to 4-18-38 + CaMg as the buds form and through flowering. In pots, use the lighter 6 g / 6 g / 3 g rate.
- In winter: cut back to fortnightly at half strength in warm areas, and only if it’s actively growing. In cold areas, stop.
Common Magnolia Feeding Mistakes
1. Using High-Nitrogen Lawn Fertiliser
Lawn fertilisers are made for leaves, not flowers. Around a magnolia they produce lush foliage and very few blooms. Keep lawn feed away from the root zone, which on a mature tree can reach well past the edge of the canopy.
2. Feeding at the Wrong Time
Magnolias only take up nutrients well while actively growing, from early spring to early autumn. Feeding a dormant deciduous magnolia in winter, or pushing soft new growth late in the season, wastes fertiliser and can stress the tree. The flowering feed also has a window: it helps while buds are forming, not after the flowers have finished.
3. Fertilising Against the Trunk
Feed the drip line, not the base. See the step-by-step above.
4. Ignoring Trace Elements
Yellowing between the leaf veins is usually an iron or magnesium shortage, most common in pots and sandy soils. Complete Grow 20-20-20 carries iron, manganese and other trace elements, and the flowering kit adds magnesium, so a regular program helps prevent and correct this.
5. Overfeeding, Especially in Pots
Too much fertiliser builds up salts in potting mix and leads to burnt roots, leaf drop and stalled growth. In pots, use 2 mL per litre every 3–4 weeks and flush every 6–8 weeks. The flowering feed goes in place of the everyday feed, never on top of it.
CompleteGrow’s view: a magnolia that won’t flower is rarely underfed
The instinct is to feed more. Before you do, look at what’s already reaching the roots. A magnolia growing in or beside a lawn often gets plenty of nitrogen from the lawn feed, and that alone can stop it flowering. Stop the nitrogen first, then switch to the 4-18-38 + CaMg as the next buds form. And check the pruning: cutting a magnolia back after its buds have set removes next season’s flowers, whatever you feed it.
Magnolia Problems Linked to Feeding
- Yellow leaves: often a nutrient shortage or waterlogged soil. Check drainage first, then apply a balanced feed. More on yellow magnolia leaves.
- No flowers: too much nitrogen is the usual cause. Stop any lawn feed nearby and use the flowering feed as the buds form. Why a magnolia won’t flower.
- Buds dropping: usually stress from dry spells or a sudden change. Keep watering even while buds develop. Why magnolia buds drop.
- Slow growth: improve the soil with organic matter and keep to a regular feeding schedule.
- Leaf scorch in summer: mulch to hold moisture, water deeply, and feed at half strength while the tree recovers.
Magnolia Fertiliser FAQs
What is the best fertiliser for magnolias in Australia?
A balanced NPK 20-20-20 with trace elements for everyday feeding, then NPK 4-18-38 + Calcium & Magnesium from bud swell through flowering.
How much Complete Grow do I mix for a magnolia?
20-20-20 liquid: 4 mL per litre (40 mL in a 10 L can), or 2 mL per litre for pots and new plantings. 4-18-38 kit: per 10 L, 8 g NPK, 8 g Calcium Nitrate and 4 g Magnesium Sulphate, each mixed and applied separately.
When should I fertilise magnolias?
From early spring, as new growth starts, through to early autumn. Don’t feed deciduous magnolias in winter.
How often should I fertilise magnolias?
Young trees every 4–6 weeks in the growing season. Mature trees 2–3 times a year. Potted magnolias every 3–4 weeks at half strength.
What’s the best fertiliser for a Little Gem magnolia?
The same two-step program: 20-20-20 for everyday feeding and 4-18-38 + CaMg as it comes into flower. In pots, feed at half strength every 3–4 weeks.
Is high-phosphorus fertiliser bad for magnolias?
Not at the right time and rate. Phosphorus sensitivity is an issue for many Australian natives such as grevilleas and banksias, not for magnolias. Use a phosphorus-rich feed while buds form and during flowering, and a balanced feed the rest of the season.
Can I use lawn fertiliser on magnolias?
No. Lawn fertilisers are too high in nitrogen and cut flowering.
How do I fertilise a newly planted magnolia?
Wait 4–6 weeks after planting, then start with 20-20-20 at half strength (2 mL per litre).
Should I fertilise magnolias in winter?
Not deciduous ones: they’re dormant. Evergreens like ‘Little Gem’ in warm areas only need an occasional half-strength feed if they’re still growing.
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