Dahlia × hortensis

Dahlia

At a glance

Light: Full sun
Pot size: from 20 l
Lifespan: Perennial
Hardiness: Cold damage below 0 °C
Water need: Medium

Photo: Diego Delso · CC BY-SA 3.0 · Source

This description was machine-translated.

Description

The Dahlia is a lush, flowering tuberous perennial that blooms from July until the first frost in a wide array of colors and flower forms. It produces excellent cut flowers and thrives in sunny beds. The tubers are not winter-hardy.

Care instructions

Plant the tubers in mid‑May (after the ice‑holy days, week 20) in a full‑sun location with nutrient‑rich soil. Water deeply during dry periods and support tall varieties with a stake. Regularly prune spent blooms to encourage continuous flowering. After the first frost in October, dig up the tubers, keep them dry and frost‑free over winter, and re‑plant in spring.

The care notes were machine-translated from German.

Garden year

SowingApr–May

HarvestJul–Oct

Germination

Soil temperature: from 15 °C, ideally 22 °C
Days to sprout: 7 to 14 days
Light: Sprouts in light or dark
Sowing depth: 0.5 cm
How to start: Start indoors, then plant out

Soil & site

Soil pH

4.0pH 6–78.0

Soil type

humus-rich, well-drained, nutrient-rich, loamy

LightFull sun

Hours of sunat least 6 h a day

HardinessFrost-sensitive

Water needMedium

Dahlias need loose, humus-rich, nutrient-rich soil with good drainage, because waterlogging makes the tubers rot. Loosen heavy loam soils, and improve sandy soils with plenty of mature compost; the soil should warm up quickly in spring.

Feeding

Heavy feeder

Months

Apr–Aug

When planting in spring (from the end of April), work mature compost and horn shavings shallowly into the soil. During the main growth phase from May to July, feed regularly with organic feed, such as compost tea or organic liquid fertiliser. From August, stop the nitrogen-rich feeding and give the tuber only a potassium-rich organic dose, so that it ripens and stores well. At the latest about six weeks before the first frost, stop feeding altogether.

Overwintering

Dahlias are not hardy: after the first frost (the foliage turns black), cut the stems back to about 10 cm and carefully dig up the tubers on a dry day. Clear the soil off the tubers, leave them to dry in the air for a few days, then store them cool (5 to 12 °C), dark and frost-free, for example in a cellar or a frost-free garage; lay them in crates on sand so that they do not touch, and check them occasionally over winter; plant them out again after the Ice Saints in mid-May.

Propagation

Methods

Division
Cuttings
From seed

The most reliable way to propagate dahlias is by tuber division in spring: from March, bring out the overwintered tubers and divide the tuber clump with a sharp, clean knife. Make sure that each piece contains both an “eye” (a visible shoot bud) and a piece of the old crown neck (stem base). If one of them is missing, the tuber stays “blind” and does not sprout. Let the cut surfaces dry briefly before you plant the pieces.

For cuttings, bring the tubers into a bright, warm place (about 16 to 18 °C) from February to March so that they sprout. To do so, lay the tubers half in soil so that the shoot tips stay visible. As soon as the shoots are 8 to 10 cm long, cut them off just above the tuber, ideally taking a small piece of the crown at a slight angle (more rooting surface). Remove the lowest leaves and insert the shoot in moist seed compost under a cover. After about 2 weeks roots have formed (recognisable by resistance when you tug gently). Cuttings give you many plants that come true from a single tuber clump, often 5 to 7 per tuber.

Sowing suits simple dahlia varieties and mixes (e.g. bedding dahlias) that are sold as seed. Sow from the end of February to the end of March under glass at 18 to 20 °C in seed compost, cover the seeds only lightly and keep the substrate evenly moist. Germination takes about 1 to 2 weeks. Important: dahlias are a hybrid complex (octoploid) and cross freely via insects. Bred hybrid varieties (cactus, pompon, decorative dahlias) do not come true from seed. To keep a particular favourite variety, therefore, use tuber division or cuttings; only these give a true clone of the mother plant.

Plant all young plants outdoors only after the Ice Saints (mid-May · week 20).