Batten Blooms
Dahlia
Dahlia pinnata

Dahlia

8a–11bHardiness zone
SummerFall
Peak bloom windowZone 6b · frost-offset weeks
Winter
Not in bloom
Spring
Not in bloom
Summer
Peak bloom
Fall
Peak bloom
Peak bloom
In bloom
Gallery
Dahlia
Dahlia
Dahlia
Dahlia
Background

Dahlias are the showpiece of the late-summer and autumn garden. Their colours deepen as the nights cool, hitting their richest just before the first frost ends the season in a single night, and they come in every size from 2-inch pompons to dinner plates, on plants 2 to 5 feet tall. For cutting, the ball, decorative and waterlily forms hold up best.

Plant the tubers after last frost, when the soil is above 60°F, 4 to 6 inches deep and 18 to 24 inches apart, with the stake set at the same time. They are slow to start: do not water until the first shoots show, then water deeply and consistently. Pinch the growing tip at 12 inches to encourage branching and more stems, and cut or deadhead constantly, since every stem taken brings more. Slugs eat the new shoots and earwigs hide in the flowers.

Dahlias are perennial only in zone 8 and warmer. Elsewhere lift the tubers after the first killing frost blackens the foliage, cure them for a week in a dry, airy place, and store them packed in barely damp vermiculite or peat at 40 to 50°F; divide the clumps in spring, making sure each piece has an eye. The open-centred types feed bees and butterflies; the full doubles do not. They are native to the mountains of Mexico.

Growing dahlias? Read the Dahlia Growing Guide - pinching and disbudding for more stems, the flower forms, and the winter tuber storage method, timed to your frost dates.
Good companions
BasilCosmosMarigoldZinnia
In these starter beds
All-Summer Cutting Bed
Care guide
SunFull sun - 6–8 hours daily
WaterDeep watering 2–3 times per week once established
SoilRich, well-draining; pH 6.5–7.0
Spacing18–24 inches
Height2–5 feet depending on variety
Zone8a – 11b
WinterLift after first frost; store tubers at 40–50°F in barely-damp vermiculite.
Seasonal tasks
winter
overwinterPack for winter storage: Once cured, pack the clumps in barely-damp vermiculite, peat, or sawdust inside a ventilated box or paper bag, and hold them at 40–50°F - a cool basement or unheated garage that never freezes is ideal.
buyOrder tubers - about two months before planting; popular cultivars sell out early
spring
buyOrder tubers - about two months before planting; popular cultivars sell out early
plantPlant tubers into warm soil: Wait until the soil is consistently above 60°F - dahlias planted into cold, wet ground rot before they ever sprout.
cutStake before it's needed: Drive the stake in at planting time, not after the plant is 3 feet tall and flopping - going in later risks spearing the tuber you can no longer see.
cutPinch the growing tip: Once the plant reaches about 12 inches, pinch out the growing tip just above a set of leaves.
summer
cutPinch the growing tip: Once the plant reaches about 12 inches, pinch out the growing tip just above a set of leaves.
watchWatch for aphids and slugs: Soft new growth draws aphids, and slugs and earwigs go straight for tender shoots and dark foliage overnight.
watchCheck for aphids on new growth - knock off with water
cutDisbud and deadhead through summer: For bigger show blooms, pinch off the two smaller side buds flanking each main bud, leaving one to develop.
cutCut for the vase: Cut in the cool of morning, just as a bloom fully opens - dahlias are one of the few cut flowers that will not keep opening in the vase once severed.
fall
cutLift tubers after first frost; cure 1 week before storing
overwinterLift and cure the tubers: In any zone colder than the dahlia's hardiness (most of the country), lift the clump within a few days of the blackening frost, before hard ground makes digging difficult.
overwinterPack for winter storage: Once cured, pack the clumps in barely-damp vermiculite, peat, or sawdust inside a ventilated box or paper bag, and hold them at 40–50°F - a cool basement or unheated garage that never freezes is ideal.
watchPack cured tubers in barely-damp vermiculite and store at 40–50°F
cutCut for the vase: Cut in the cool of morning, just as a bloom fully opens - dahlias are one of the few cut flowers that will not keep opening in the vase once severed.
watchWatch the forecast: Unlike most of the garden, dahlias don't fade gradually - one hard frost blackens the foliage overnight and ends the season in a single night.
Common problems

Powdery mildew

Symptoms

White or grey powdery coating on leaves - usually starting on older growth in humid conditions or when nights cool.

Treatment

Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.

Botrytis (grey mould)

Symptoms

Grey fuzzy mould on petals and stems, worst in cool wet conditions.

Treatment

Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.

Mosaic virus

Symptoms

Yellowing, mottled, or distorted leaves. No cure - spread by aphids.

Treatment

Remove and destroy infected plants. Control aphid populations to prevent spread. Do not propagate from infected tubers.

Tuber Rot

Symptoms

Stored or in-ground tubers turn soft, brown, and mushy, often with mold growth. Affected tubers fail to sprout in spring.

Treatment

Lift tubers only after foliage dies back, cure them in a dry airy spot before storage, and store in a cool dry location with breathable packing. Discard damaged or soft tubers and avoid waterlogged soil during the growing season.

From the field journal
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