How to grow dahlia
Overview
Dahlia is a herbaceous perennial grown for its spectacular, diverse flowers in a range of colours, forms, and sizes. It is a staple of summer and autumn borders, providing continuous cut flowers and seasonal colour. Growers must plant tubers in warm, well-drained soil after frost risk has passed.
Varieties
- Bishop of Llandaff — Dark bronze foliage with bright red single flowers; a favourite for Australian borders.
- Karma Choc — Chocolate-brown double blooms on strong stems, excellent for cutting in subtropical gardens.
- Moor Place — Large, pale pink decorative blooms with sturdy stems; ideal for temperate climates.
- Verrone's Obsidian — Near-black, fully double flowers with a compact habit; popular in Mediterranean gardens.
Soil Preparation
Dahlias thrive in slightly acidic to neutral soil (pH 6.0-7.0) with good drainage. Enrich with well-rotted manure or compost before planting, and add a handful of bone meal per hole for strong root development. Avoid heavy clay; if present, plant in raised beds or mounds.
Growing Guide
Plant dahlias in a position with full sun and rich, well-drained soil. Space tubers 45-60 cm apart, depending on variety, and set them 10-15 cm deep with the eye (growth point) facing up. In cool districts, wait until soil temperature reaches 15°C. Staking is essential for tall or large-flowered types; insert stakes at planting time to avoid damaging tubers. Water deeply once or twice a week, more in dry spells, and apply a 5-10 cm layer of organic mulch to retain moisture and suppress weeds.
Care
Feed dahlias monthly with a low-nitrogen fertiliser (e.g., 5-10-10) once flowering begins to encourage blooms over foliage. Pinch out the central tip at 30 cm to promote bushiness, and deadhead spent flowers regularly to extend the flowering season. In autumn, cut back stems to 15 cm after frost and lift tubers for storage in frost-prone areas.
Companion Planting
Dahlias pair well with sunflowers, zinnias, and cosmos for a vibrant summer display. Avoid planting near potatoes or other nightshades, as they share pests like slugs and snails. Good companions include marigolds to repel nematodes and lavender to attract pollinators.
Common Pests & Risks
Slugs and snails are the main pests, especially on young shoots; use iron-based pellets or beer traps. Earwigs can damage petals at night; trap in rolled-up newspaper or apply diatomaceous earth. Powdery mildew appears in humid, crowded conditions; improve air circulation and apply a milk spray (1:9 with water) or sulfur-based fungicide.
Harvesting
Harvest dahlias for cut flowers when the blooms are fully open but still firm, usually in the morning. Cut stems at an angle and place in warm water immediately to prolong vase life. For tuber storage, lift after first frost blackens foliage, dry in a shed, and store in cool, dry conditions.
Troubleshooting
Yellow leaves: overwatering or poor drainage; reduce water and improve soil. Brown edges on leaves: sun scorch or salt burn; provide afternoon shade or flush soil. Stunted growth: root knot nematodes; rotate with marigolds or solarise soil. Buds fail to open: thrips or heat stress; spray with insecticidal soap and mulch to cool roots. Powdery mildew on leaves: poor air circulation; space plants wider and apply fungicide.