Dahlia Fertiliser Guide: NPK Ratio, Soil Health & Seasonal Feeding

Dahlias (Dahlia pinnata) are one of the most rewarding flowering plants you can grow in Australian gardens—but only when fertilised with precision. These tuberous perennials produce vibrant, long-lasting blooms and dense foliage, yet they’re also heavy feeders that require a consistent supply of balanced nutrients to thrive. This guide explains how to fertilise dahlias for strong stems, lush foliage, and spectacular blooms throughout their growing season. You’ll learn:

The best NPK ratio for dahlias from sprouting to flowering
When to feed and how much fertiliser to apply in Australia
Soil preparation and drainage tips for tuber health
Fertilising dahlias in pots, raised beds, and cutting gardens

Dahlias thrive in rich, free-draining soil with a slightly acidic to neutral pH (6.2–7.0). They are especially sensitive to over-fertilising with high nitrogen, which leads to excessive foliage and poor flowering. Instead, they need a balanced or flower-leaning fertiliser regimen that supports root establishment early on and promotes bloom density later in the season.

NPK Ratio and Feeding Plan for Dahlias

Dahlias have three key nutritional phases: early growth, bud formation, and peak blooming. Matching your fertiliser to each stage ensures optimal growth and flowering.

1. Early Growth (Weeks 1–4 After Sprouting)

Start with a balanced fertiliser high in nitrogen to support stem and foliage growth while roots develop.

CompleteGrow NPK 20-20-20 Powder Fertiliser
Use this water-soluble formula weekly at 1/2 strength. This all-purpose feed supplies equal parts nitrogen, phosphorus, and potassium—ideal for initial leaf and root development.

2. Bud Formation (Weeks 5–7)

Reduce nitrogen slightly and increase potassium to help strengthen stalks and prepare the plant for flowering.

NPK 4-18-38 + Cal-Mag Fertiliser
Apply this low-nitrogen formula every 7–10 days to enhance bud development and support calcium and magnesium uptake for strong cell walls and vibrant petal colour.

3. Blooming (Week 8 Onward)

Once flowers appear, keep feeding with the same 4-18-38 + Cal-Mag blend to promote repeat blooms and healthy tuber formation. This will also enhance vase life if growing dahlias for cutting.

*Tip:* Always water deeply before and after feeding. Avoid feeding during hot midday temperatures.

Soil Preparation, Tuber Care & Container Growing

Soil Preparation for Dahlias

Dahlias grow best in nutrient-rich, well-aerated soil. Before planting:

Work in composted bark or perlite to improve drainage—especially important for clay-heavy soils
Add a light pre-planting dose of NPK 20-20-20 Powder one week before tuber placement
Ensure pH is between 6.2–7.0; dahlias dislike alkaline soils

Feeding Dahlias in Pots & Raised Beds

Container-grown dahlias need more frequent feeding due to nutrient leaching. Use a liquid blend like:

CompleteGrow Liquid Indoor & Outdoor Plant Food
This balanced liquid concentrate (suitable for container ornamentals) can be applied every 10 days to keep nutrients available in pots. Ideal for patios, balconies, and small gardens.

Overwintering Tubers

After flowering, stop feeding and allow foliage to yellow naturally. Once foliage dies back:

Lift tubers if frost is expected and store in dry sawdust
For warm zones (QLD, NSW North Coast), you may leave tubers in-ground with mulch cover
Resume fertilising with NPK 20-20-20 in early spring once new shoots appear

By tailoring your dahlia fertiliser routine to the plant’s growth stages, you’ll enjoy stronger stems, more blooms, and healthier tubers season after season.