How to Fertilize Grape Plants in Australia

Healthy grapevines need a balance of nutrients to produce strong canopies, reliable flowers, and sweet, heavy bunches of fruit. In Australian soils — often sandy, alkaline, or nutrient-poor — grape plants quickly show stress without consistent feeding. The best approach is to use a combination of
CompleteGrow NPK 20-20-20 during early growth and
CompleteGrow NPK 4-18-38 + CaMg as fruit begins to set.

Why Fertilising Grapes Matters

Unlike ornamental plants, grapevines pull enormous reserves from the soil each season. Every cluster of grapes requires large amounts of potassium, calcium, and magnesium. Without supplementation, vines produce small, sour fruit, yellowing leaves, or poor fruit set. Balanced feeding keeps vines productive year after year.

NPK 20-20-20: Kickstarts spring growth, supports canopy expansion, and builds strong roots.
NPK 4-18-38 + CaMg: Boosts flowering, enhances grape size and sweetness, and prevents calcium/magnesium deficiency.

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Seasonal Feeding Guide for Grapevines

Grapes respond best when feeding matches their growth cycle. Here’s how to fertilise at each stage in Australia:

Spring (September–November): As buds break and new shoots appear, apply
NPK 20-20-20 every 3–4 weeks. This fuels leafy growth and prepares vines for flowering.
Summer (December–February): Switch to
NPK 4-18-38 + CaMg as flowers open and fruit sets. High potassium drives grape sweetness and colour, while calcium strengthens fruit skins against splitting.
Autumn (March–April): Apply a lighter dose of NPK 4-18-38 to support late-ripening varieties and help vines store reserves for next year.
Winter (May–August): No fertiliser is needed — vines are dormant. Use this time for pruning and soil conditioning.

Application Tips

Apply fertiliser around the drip line (outer edge of the canopy) where feeder roots absorb nutrients best.
Water deeply after each feed to wash nutrients into the root zone and reduce salt stress.
For potted grapevines, halve the dosage and feed more frequently (every 2–3 weeks).

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Common Fertiliser Issues in Grape Plants

Yellow leaves: Usually a nitrogen or magnesium deficiency. Start with NPK 20-20-20, then use 4-18-38 + CaMg for correction.
Poor fruit set: Often linked to low phosphorus. Ensure NPK 4-18-38 is applied during flowering.
Sour, small grapes: Potassium deficiency. Consistent summer feeding fixes this.
Excessive leaf growth but few grapes: Too much nitrogen alone. Always use balanced fertilisers, not lawn feed.

FAQs: Fertilising Grape Plants

What is the best fertiliser for grapes? Use a two-stage approach: NPK 20-20-20 for spring growth, followed by NPK 4-18-38 + CaMg during fruiting.

How often should I fertilise grapevines? Every 3–4 weeks in the growing season. Potted vines may need more frequent, lighter feeds.

Do grapes in pots need different care? Yes. They exhaust nutrients quickly, so use diluted liquid fertiliser more often and flush pots monthly with clean water.

Can I just use compost? Compost improves soil texture but doesn’t supply the full balance grapes demand. Always pair it with a complete fertiliser program.

By combining early-season feeding with NPK 20-20-20 and fruiting support from NPK 4-18-38 + CaMg, your vines will deliver full, sweet bunches every summer.