Sunflower Fertiliser Guide: NPK Ratio, Soil Prep & Feeding Schedule

Sunflowers are more than just cheerful garden favourites—they’re also heavy feeders that demand proper fertilisation to grow tall and produce large, healthy blooms. Whether you’re growing giant sunflowers for height, branching varieties for cut flowers, or dwarf hybrids in pots, giving your plants the right nutrients is essential for fast, vigorous growth and stunning flower heads.

This in-depth guide will teach you how to feed sunflowers effectively from seedling to bloom, covering:

The best NPK ratio for sunflowers at each growth stage
When and how to fertilise sunflowers in Australian conditions
How to prepare soil and choose the right fertiliser type
Tips for pot-grown sunflowers and commercial growers

Sunflowers (Helianthus annuus) are known for their rapid upward growth, large leaves, and nutrient-hungry nature. Their tall stalks and big blooms require strong root systems and consistent access to key macronutrients like nitrogen (N), phosphorus (P), and potassium (K). Without adequate feeding, sunflowers become stunted, with smaller heads, pale leaves, and poor seed formation.

Whether you’re sowing in garden beds, paddocks, or large pots, this guide gives you everything you need to fertilise sunflowers effectively—so they grow bold, bright, and healthy all season long.

Sunflower Fertiliser Requirements: CompleteGrow Feeding Plan

Sunflowers are rapid-growing annuals with high nutrient demands. To produce tall stems, broad leaves, and vibrant flower heads, you need to feed strategically at different stages—especially with phosphorus and potassium as the plant transitions from growth to bloom.

Best Fertiliser for Sunflowers

CompleteGrow NPK 4-18-38 + Cal-Mag is the ideal all-in-one solution for sunflowers, providing a phosphorus and potassium-rich formula that promotes strong root growth, tall stems, vibrant blooms, and healthy seed development. Its added calcium and magnesium also help prevent stem weakness and flower drop, ensuring a full display of healthy sunflower heads.

How Often to Fertilise

Seedling stage: Apply a diluted solution every 2 weeks to establish strong roots.
Vegetative stage: Feed every 7–10 days to support leaf and stem growth.
Flowering stage: Continue every 10–14 days to maximise bloom size, colour, and seed set.

Container-grown sunflowers should be fed slightly more frequently, as nutrients leach faster. In rich garden soil, you can space applications further apart while still maintaining healthy growth.

Micronutrients for Bigger, Healthier Blooms

Our formula includes essential trace elements that enhance sunflower performance:

Magnesium – Promotes deep green leaves and efficient energy production.
Boron – Essential for flower and seed development.
Zinc – Supports stem strength and growth regulation.

These micronutrients ensure complete plant health and vibrant flowering, especially during hot Australian summers.

Sunflower Feeding Tips

Always water the soil before applying fertiliser.
Use a hose-end sprayer or watering can for even coverage.
Avoid feeding during midday heat—mornings or late afternoons are ideal.

With a consistent feeding schedule using CompleteGrow NPK 4-18-38 + Cal-Mag, your sunflowers will grow tall, produce bold blooms, and set healthy, well-filled seeds for a stunning garden display.

Sunflower Soil Prep, Nutrient Deficiencies & Growing Tips in Australia

Fertiliser works best when paired with the right soil and growing environment. Sunflowers thrive in full sun, warm conditions, and loose, well-draining soil that allows deep root penetration. Poor drainage, compacted ground, or low nutrient content can limit their growth—no matter how good your fertiliser is.

Best Soil for Sunflowers

Soil type: Well-drained loam or sandy-loam enriched with compost
pH range: 6.0–7.5 (neutral to slightly acidic)
Preparation: Add aged compost and dig through 30–40 cm depth for deep rooting

Sunflowers have taproots that can reach 1–2 metres in depth. To support that, avoid shallow or heavy clay soils unless amended with gypsum and sand for better structure. In raised beds, use premium planting mix combined with slow-release organic matter and fortnightly liquid feeding.

Common Nutrient Deficiencies in Sunflowers

Yellowing lower leaves: Often caused by nitrogen deficiency
Purple leaf veins: Indicates phosphorus deficiency
Weak stems or small flower heads: Usually a sign of potassium shortage

If your sunflowers are growing slowly, forming small heads, or showing uneven colouring, the solution is usually a consistent feeding schedule—not more fertiliser at once. Regular liquid feeding with trace elements solves most deficiencies within 7–10 days.

Australian Growing Tips

Best time to plant: Spring to early summer once soil temps reach 15°C
Sunlight: Full sun (6–8+ hours daily) is essential for large blooms
Watering: Deep watering once or twice a week to encourage strong taproots
Spacing: 30–60 cm between plants depending on variety

In regions like Queensland, NSW, and WA, sunflowers perform well in both backyard gardens and small-scale farms. They grow quickly in warm conditions and bloom best when planted in nutrient-rich soil and supported by a proper feeding schedule.

With consistent care and quality fertiliser, your sunflowers will grow tall, bloom bright, and reward you with beauty, pollinator activity, and even edible seeds if left to mature.