Chrysanthemums

Chrysanthemums are the signature flower of the autumn garden. As summer fades and the days shorten, they unfurl their full, rounded, many-petalled blooms in warm glowing colours, just when most other flowers have finished. Because they are short-day plants, their flower buds are triggered naturally by the shortening days of late summer and autumn, giving a generous display from September through to November. Their layered, cushion-like flowers make them a favourite for beds, borders and patio pots alike, bringing rich late-season colour exactly when the garden needs it most.

Planting

Start seed indoors in late winter to mid-spring, roughly 6 to 10 weeks before the last spring frost. Sow on the surface of the starting mix and only barely press the seed in. Do not cover it deeply, because chrysanthemum seed needs light to germinate, so keep it in bright light. Germination takes about 10 to 14 days at a soil temperature of 18 to 21 C, and bottom heat helps. Transplant outdoors once frost danger has passed, setting each plant at the same depth it was grown. Never bury the root ball, as this causes poor aeration and root rot. Space plants about 30 to 45 cm apart, allowing more room for spreading varieties. Choose a site in full sun with at least 6 hours of direct light per day, since plants in lower light grow weak and spindly with few flowers. Avoid spots lit by street or porch lights at night, which disrupt the short-day flowering trigger.

Fertilizing

Work a low-nitrogen balanced fertilizer into the bed before planting in spring, at roughly 0.2 kg/m2 of a 5-10-5 blend worked to about 15 cm depth. Feed with a dilute fertilizer several times before the buds set, and give a second, lighter feed of 5-10-5 or 10-6-4 around early August to support plants on poorer soils. Nitrogen has the strongest effect on growth and flowering, but too much of it delays bloom and reduces the number of flowers, so keep it moderate. Once flowering begins, switch container plants over to a high-potassium feed such as a tomato fertilizer.

Care

Keep the soil evenly moist throughout the growing season, but never waterlogged. Chrysanthemums are shallow-rooted, so they need frequent watering in hot, dry spells, and good drainage to avoid root rot. For potted plants, water once the top few centimetres of mix have dried out. To build bushy, well-flowered plants, pinch the tips to force branching: make the first pinch when plants reach about 15 to 20 cm tall, removing roughly 2.5 cm from each shoot tip, then repeat once the new branches grow to about 15 cm. Stop pinching about 3 months before you want bloom. Plants left un-pinched grow tall, leggy and flower poorly. Watch for aphids, leaf miners, leafhoppers, spider mites in hot dry conditions, caterpillars, capsid bugs, earwigs, slugs and snails; controlling aphids and leafhoppers matters because they spread viruses. Reduce disease such as powdery mildew, grey mould, white rust and leaf spot by avoiding overcrowding and shade and by keeping foliage dry. Buds form naturally as the days shorten in late summer and autumn, roughly 3 months after the last pinch, with flowering from September to November.