Dahlias are vibrant and versatile flowering plants celebrated for their stunning variety of shapes, sizes, and colours. Belonging to the Asteraceae (daisy) family, they are closely related to sunflowers and chrysanthemums. With thousands of registered cultivars, dahlias offer an extraordinary diversity of bloom forms — from small, globe-shaped pompons to enormous dinner-plate varieties spanning 30 cm or more across.

Flowers come in virtually every colour of the rainbow, including shades of red, pink, yellow, orange, purple, and white, as well as striking bi-coloured and multi-toned varieties. Petal forms range from smooth and rounded to spiky, cactus-like, or intricately ruffled, giving gardeners an almost limitless palette to work with.

Dahlias typically bloom from midsummer through to the first autumn frosts, providing months of spectacular colour in garden beds, borders, and containers. Plant heights vary dramatically — compact bedding types grow as little as 30 cm tall, while giant border varieties can exceed 1.5 m.

They thrive in well-drained, fertile soil in a sunny position. In frost-free climates, tubers can remain in the ground year-round. In regions with cold winters, tubers should be lifted after the first frost, dried, and stored in a cool, frost-free location until the following spring. Dahlias are widely grown both as garden ornamentals and as cut flowers, prized for their long vase life and bold visual impact.

How To Grow

Growing Dahlias

Starting Tubers

Dahlias are most commonly grown from tubers. Wait until soil has warmed to at least 15°C (60°F) and all risk of frost has passed before planting outdoors. Plant tubers 15 cm deep with the growing eye facing upward, covering with 5–8 cm of soil initially and backfilling as growth emerges.

Starting from Seed

Some bedding dahlia varieties can be raised from seed. Sow indoors 6–8 weeks before the last expected frost date, at a soil temperature of 18–21°C. Transplant seedlings out once conditions are frost-free.

Soil and Position

Choose a sunny spot receiving at least 6–8 hours of direct sun daily. Prepare well-drained, fertile soil with a pH of 6.0–7.5. Incorporate compost or well-rotted organic matter before planting. Avoid waterlogged soil, which can cause tuber rot.

Spacing and Support

Space taller varieties 60–90 cm apart; compact bedding types 30–45 cm apart. Insert sturdy stakes at planting time for tall varieties to avoid disturbing roots later.

Watering and Feeding

Water well after planting but avoid overwatering until shoots emerge. Once growing, water deeply two to three times per week in dry weather. Feed with a low-nitrogen, high-potash fertiliser every three to four weeks once flowering begins. Avoid high-nitrogen feeds, which encourage leafy growth at the expense of blooms.

Pinching Out

When plants reach approximately 30 cm tall, pinch out the central growing tip to encourage bushy, multi-stemmed growth and more flowers.

Deadheading

Remove spent blooms regularly to prolong flowering and maintain plant vigour throughout the season.

Overwintering

In frost-prone climates, lift tubers after the first frost blackens the foliage. Allow to dry for a few days, then store in boxes of dry compost, sand, or vermiculite in a cool, frost-free, dark location (4–10°C) until replanting in spring. In mild, frost-free climates, tubers can be left in the ground with a protective mulch.

Harvesting

Harvesting Dahlia Flowers

When to Cut

Harvest dahlia blooms when the flowers are fully open and petals are firm and vibrant, but before they begin to fade or drop. Unlike many cut flowers, dahlia buds cut too early will not open satisfactorily after cutting. The best time to harvest is early morning or late afternoon when temperatures are cooler and the plant is well-hydrated.

Cutting Technique

Use sharp, clean scissors or secateurs to make a clean, angled cut on the stem, just above a leaf node or pair of leaves. Cutting at an angle increases the surface area available for water uptake. Choose the longest stems possible to allow maximum flexibility in arranging.

Immediate Aftercare

Place cut stems immediately into a bucket of clean, cool water. Remove any foliage that would sit below the waterline to prevent bacterial build-up. Allow flowers to condition in a cool, dark place for several hours or overnight before arranging — this significantly extends vase life.

Vase Life

Change vase water every one to two days and re-trim stem ends each time. With proper conditioning, dahlias typically last 5–8 days in a vase.

Encouraging More Blooms

Regularly cutting flowers — combined with deadheading spent blooms — actively encourages the plant to produce more flowers throughout the season. The more you cut, the more dahlias will bloom.

Harvesting Tubers

At the end of the season, after the first frost kills the foliage, dig up tubers carefully using a fork. Shake off excess soil, allow to dry in a well-ventilated space for several days, then store in a cool, frost-free environment over winter for replanting the following year.

Common Problems

Common Dahlia Problems

Slugs and Snails

Young dahlia shoots are highly attractive to slugs and snails, which can demolish emerging growth overnight. Use barriers such as copper tape, grit, or wool pellets around plants, or apply an approved slug control. Check regularly after wet weather.

Aphids

Greenfly and blackfly can colonise soft growing tips, weakening growth and spreading viral diseases. Knock off with a strong jet of water, or apply an appropriate insecticidal soap or neem oil spray. Encourage natural predators such as ladybirds and lacewings.

Earwigs

Earwigs can chew ragged holes in petals, particularly on decorative varieties. Trap them using upturned pots filled with dry straw placed on canes near the plants — empty and dispose of trapped insects regularly.

Powdery Mildew

A white, powdery coating on leaves and stems, particularly in late summer. Caused by dry roots combined with humid air. Improve air circulation by spacing plants adequately and water consistently at the base. Remove and dispose of heavily affected leaves.

Tuber Rot

Excessive soil moisture, especially in heavy or poorly drained soils, can cause tubers to rot. Ensure well-drained soil and avoid overwatering. During storage over winter, inspect tubers regularly and remove any that show signs of rotting before they affect healthy tubers.

Dahlia Mosaic Virus

Causes mottled, distorted, or yellowing leaves. Spread by aphids. There is no cure — remove and destroy affected plants promptly to prevent spread. Always source tubers from reputable, disease-free suppliers.

Botrytis (Grey Mould)

A grey, fuzzy mould that develops on leaves, stems, and flowers in cool, damp conditions. Improve ventilation, avoid overhead watering, and remove affected plant material promptly.