Forsythia is the first shrub to break winter and the most cuttable of all woody plants — long arching stems covered entirely in yellow flowers before a single leaf appears. In Zone 6b it typically blooms in late March to early April. It is vigorous and unfussy, tolerating poor soils and neglect.
For cutting and forcing, harvest bare stems in January or February — well before outdoor bloom — and bring them inside into a warm room. They will open in 2–4 weeks, providing a vivid yellow branch arrangement in the depths of winter. Once the buds begin to swell outdoors, cut flowering branches freely. Prune immediately after flowering on the shrub, not in autumn, or you will cut off next year's flower buds.
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.
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.
Forsythia Twig Blight
Dieback of branch tips, wilting shoots, and dark sunken lesions on stems. Affected twigs may show small fruiting bodies in wet weather.
Prune out affected twigs several inches below visible damage and dispose of them. Avoid overhead watering and improve air circulation by thinning crowded stems. Disinfect pruning tools between cuts.