'Bishop of Llandaff' is the dahlia that made dark foliage fashionable. Introduced in Wales in the 1920s, it carries single to semi-double scarlet flowers over near-black bronze-purple leaves, and the contrast is what every later Bishop was bred to copy. It reaches 3 to 4 feet and flowers from midsummer until frost.
The open flowers are the other reason to grow it. Unlike the full doubles, the centre is exposed, so bumblebees and butterflies work it all afternoon. Give it full sun, rich soil that drains, and a stake set at planting time; the stems are strong but a loaded plant catches wind. Pinch the growing tip when the plant is a foot tall for a bushier shape and more flowers, and deadhead to keep it going.
It is tender. North of zone 8 lift the tubers after the first frost blackens the foliage, dry them for a few days and store them cool and frost-free in peat or vermiculite, then plant out after the last frost. In mild zones a thick mulch is enough. It also comes easily from cuttings taken from sprouting tubers in spring.
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.Powdery mildew
White or grey powdery coating on leaves - usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Tuber Rot
Stored or in-ground tubers turn soft, brown, and mushy, often with mold growth. Affected tubers fail to sprout in spring.
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.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.
Aphids
Aphididae →Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.
Spider Mites
Tetranychus urticae →Fine stippling or bronzing on leaves, faint webbing on undersides, foliage drying out during hot, dry weather.
Spray foliage with a strong jet of water to dislodge mites. Apply insecticidal soap or horticultural oil to leaf undersides; repeat every 5–7 days as needed.