The dahlia: growing, overwintering the tubers and varieties

Daniel Platteau

jpg/dahlia.jpg

The dahlia flowers non-stop from July until the first frosts, then collapses all at once: the first real frost blackens its foliage. That is when many gardeners lose their plants, not in summer but in winter, for want of having put the tubers under cover. This guide covers growing dahlias from A to Z, with particular attention to that tricky moment.

A Mexican plant in our gardens

The dahlia belongs to the Asteraceae family, like the daisy and the sunflower. The genus has 42 species according to the botanical reference of Kew Gardens, all native to an area stretching from Mexico to Colombia. They are tuberous perennials: swollen roots, clustered at the base of the stems, which store reserves through the dry season of their native region.

The hollow stems grow from 30 cm to over 2 m depending on the variety. The leaves are opposite, green or purple in some varieties such as 'Bishop of Llandaff'. What we call "the flower" is in fact a flower head: a central disc of small tubular florets surrounded by ray florets that act as petals. In double varieties, almost all the florets of the head have become ray florets.

From the Aztecs to the European horticultural craze

Long before the Spanish arrived, the peoples of Mexico knew the dahlia by Nahuatl names such as acocoxóchitl, which refer to its hollow stems. They used it among other things as a remedy, and in the 16th century the Spanish physician Francisco Hernández already depicted a double form, a sign of ancient selection.

In 1789, seeds sent from Mexico reached the Royal Botanical Garden in Madrid. Its director, Antonio José Cavanilles, described the genus in 1791 and named it in honour of the Swedish botanist Anders Dahl, a pupil of Linnaeus. The dahlia then spread across Europe in the 19th century, where breeders created thousands of varieties. In Mexico, it was declared the symbol of national floriculture in 1963.

jpg/Cavanilles_R01.jpg Antonio José Cavanilles, Spanish botanist (Valencia, 1745 – Madrid, 1804).

Planting at the right time

The dahlia does not tolerate frost, neither in leaf nor as a tuber. In Belgium, it is planted out after the last frosts, in practice from mid-May (the "Ice Saints" are a useful marker, but the local weather decides), or even early June in cold regions such as the Ardennes.

To gain several weeks of flowering, you can start the tubers under cover from February to April: in a pot or tray of barely moist potting compost, in the light and frost-free. The plants go out into the garden when the weather allows, after a few days of hardening off outdoors during the day.

The dahlia likes:

• full sun, at least six hours a day;
• rich, deep, well-drained soil, improved with well-rotted compost;
• space, especially for the larger varieties, so that air circulates between the plants.

Plant the clump of tubers lying down, with the crown (the point where the stems leave the tubers) just at soil level or slightly below. Water very little until the shoots appear: a leafless tuber in waterlogged soil rots easily.

Staking, pinching, deadheading: summer care

• Stake early. Put the stake in at the same time as the tuber, so as not to damage it later. A sturdy bamboo cane is enough for most varieties; for a clump, four canes in a square with twine around them. Giant varieties need one post per stem.
• Pinch out when the main stem reaches around forty centimetres: remove the tip just above a pair of leaves. The plant branches and produces more flowers.
• Deadhead every week, cutting above a pair of leaves. With no seeds to produce, the dahlia keeps flowering until the frost.
• Water generously in dry spells, at the base, rather than a little every day. In pots, a potassium-rich liquid feed (tomato-type fertiliser) every two weeks from July to early September; in the ground, a good mulch of organic matter is usually enough.

Overwintering: lift or leave in the ground?

This is the big autumn question. There are two options, and the choice depends mainly on your soil.

Leaving the tubers in the ground

This is the easiest option, but it is a gamble. Only try it in light, well-drained soil, in a mild region. After the first frost, cut the stems down to ground level and cover the clump with a mulch at least 15 cm deep: compost, dead leaves, shredded stems or bark. A very cold winter, or simply a very wet one, can still kill the tubers: in heavy clay soil they rot long before they freeze.

Lifting and storing under cover: the safe method

This is what we recommend for most Belgian gardens, and certainly for the varieties you care about.

1. Wait for the first frost. The foliage blackens overnight: that is the signal. The tubers have then made the most of the whole season to build up their reserves. If autumn stays mild, do not wait beyond the end of November; in heavy, waterlogged soil it is even better to lift earlier.

2. Cut the stems to 5 to 15 cm from the crown, and label each clump with the variety name or colour. In spring, without flowers, all tubers look alike.

3. Lift carefully. Push the garden fork in well away from the stem and lift the clump from below, without pulling on the stems: the "neck" linking each tuber to the crown breaks easily, and a tuber detached from the crown will not grow again.

4. Clean and dry. Shake off the soil and remove damaged or soft tubers. Then leave the clumps to dry upside down, stems pointing downwards, for a few weeks in an airy, frost-free place: the water held in the hollow stems drains away instead of rotting the crown. Keep the clumps whole; division is easier and safer in spring.

5. Store cool, dry and dark. Place the clumps in crates or shallow boxes, surrounded by a material that insulates them and stops them drying out: dry used potting compost, sand, vermiculite, wood shavings, straw or newspaper. The ideal is a room between roughly 4 and 10 °C, frost-free: garage, cellar, insulated garden shed. In severe cold, add a blanket or horticultural fleece.

6. Check once a month. Remove any tuber that is rotting or mouldy before the problem spreads. Tubers that shrivel have dried out too much: move them closer to a slightly moist material. Tubers that go mouldy are too damp, poorly ventilated or stored too warm.

Pelargoniums, fuchsias and other tender plants need the same precautions: see our article on overwintering garden plants (in French) and our fuchsia guide.

Propagating your dahlias

• Dividing clumps in spring, when you start them into growth again. The buds (the "eyes") are then visible on the crown. Cut the clump with a clean knife so that each piece keeps at least one healthy tuber and a piece of crown bearing an eye.
• Cuttings. Start the tubers under cover in late winter; when the shoots reach 8 to 10 cm, take them with a small heel and root them in a light compost, in a closed, humid environment. Each cutting reproduces the variety exactly.
• Seed. Mostly used for dwarf dahlias and single-flowered mixtures, sown in warmth in late winter. Seed from a hybrid variety does not give plants identical to the parent: expect surprises, good or less good.

Pests and diseases

• Slugs and snails: the main danger in spring, on young shoots.
• Aphids: they distort young shoots and transmit viruses.
• Earwigs: they nibble petals at night; an upturned pot filled with straw, set on a cane, traps them.
• Red spider mites: dull, speckled foliage in hot, dry weather.
• Powdery mildew: white felting on the leaves in late summer, especially when plants lack air.
• Viruses: mottled or distorted leaves, stunted plant. No treatment: pull up and throw away the plant, without composting it.
• Rot in storage: almost always caused by insufficient drying or a store that is too warm and damp (see above).

Forms and varieties

The International Dahlia Register, kept by the British Royal Horticultural Society, classifies varieties into 14 groups according to flower shape. The most common in gardens:

1. Single and anemone-flowered: a single row of ray florets around a visible centre. The most useful for pollinators.
2. Collerette: a row of flat ray florets, with a collar of shorter petals around the centre.
3. Waterlily: double, flat, regular flowers, reminiscent of a water lily.
4. Decorative: very double, with broad ray florets; these reach the largest sizes, sometimes over 25 cm across.
5. Ball and pompon: spherical flowers with ray florets rolled into tubes, excellent in bouquets.
6. Cactus and semi-cactus: narrow ray florets, rolled along their length, with a spiky look.

A few must-haves: 'Café au Lait' (giant decorative in cream and coffee tones), 'Bishop of Llandaff' (semi-double scarlet flowers over purple foliage), 'Thomas Edison' (deep purple decorative).

Tubers are sold in garden centres mainly in spring, and online from specialist nurseries, particularly in the Netherlands. Expect to pay from a few euros for a common variety to 15 euros or more for a novelty.

Test yourself

Sources

• Mera Ovando, L. M., & Bye Boettler, R. (2006). La dahlia, una belleza originaria de México. Revista Digital Universitaria, 7(11). https://ru.tic.unam.mx/bitstream/123456789/1203/1/nov_art90_2006.pdf
• Miller, D. (n.d.). Storing dahlias. American Dahlia Society. https://www.dahlia.org/wp-content/uploads/2022/11/STORING-DAHLIAS-by-DHM-edit-2014-pdf.pdf
• Plants of the World Online. (n.d.). Dahlia Cav. Royal Botanic Gardens, Kew. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:325940-2
• Royal Horticultural Society. (n.d.). How to grow dahlias. RHS. https://www.rhs.org.uk/plants/dahlia/growing-guide
• Royal Horticultural Society. (2025). The International Dahlia Register: 30th supplement. RHS. https://www.rhs.org.uk/plants/pdfs/plant-register-supplements/dahlias/30th-dahlia-supplement