Chrysanthemum Seeds

The chrysanthemum, or garden "mum" (Chrysanthemum morifolium), is one of the great stars of the autumn garden. Unlike spring and summer flowers, it is a short-day plant: its buds are set naturally by the shortening days of late summer and autumn, so the showy blooms open just as most other flowers fade, roughly from September to November. Sown the same year, plants flower that same autumn in a generous display, making them a favourite for sunny borders, mass plantings and patio pots alike.

Planting

Start seed indoors from late winter to mid-spring, roughly 6-10 weeks before the last spring frost. Because chrysanthemum seed needs light to germinate, sow it on the surface of the starting mix and only barely press it in, never burying it deeply, and keep it under bright light. At a soil temperature of 18-21 C (bottom heat helps) germination takes about 10-14 days; sown cooler at around 15 C it still sprouts within roughly two weeks. In Egypt, sow in trays in late winter (January-February) while nights sit around 15-21 C, then transplant out to the field in February-March in the Nile Delta once any hard frost risk has passed. Set plants at the same depth they grew in the tray, without burying the root ball, since burying it causes poor aeration and root rot. Space plants about 30-45 cm apart, allowing the wider end of that range for spreading varieties. Give them full sun, at least 6 hours of direct sunlight a day in summer, in well-drained soil; plants in low light grow weak and spindly with few flowers. Keep them away from street or porch lighting at night, as artificial light disrupts the short-day trigger that sets their flowers.

Fertilizing

Before planting in spring, work a low-nitrogen balanced fertilizer into the bed, at roughly 0.2 kg/m2 of a 5-10-5 blend mixed into about the top 15 cm of soil. Feed with a dilute fertilizer several times before the buds set, and on poorer soils give a second, lighter feed (such as 5-10-5 or 10-6-4) around early August. Nitrogen has the strongest effect on growth and flowering, but go easy: too much nitrogen delays bloom and reduces the number of flowers, so keep feeding moderate. Once the plants come into flower, container-grown mums benefit from switching to a high-potassium feed, such as a tomato feed.

Care

Keep the soil evenly moist all season but never waterlogged. Mums are shallow-rooted, so they need frequent watering during hot, dry spells, while good drainage is essential to avoid root rot; for pots, water once the top few centimetres of mix have dried out. Pinch the plants to force branching and more flowers: make the first pinch when they reach about 15-20 cm tall by removing roughly 2.5 cm from each shoot tip, then repeat once the new branches reach about 15 cm, stopping around mid-summer (about three months before the desired bloom). Plants left un-pinched grow tall, leggy and flower poorly. In the Egyptian summer, protect young plants from the harshest June-August heat with light shade and frequent watering, as heat and drought stress also favour spider mites. Watch for aphids, leaf miners, leafhoppers, spider mites, caterpillars, capsid bugs, earwigs, and slugs and snails; aphids and leafhoppers also spread viruses, so keeping them in check matters. Avoid overcrowding, shade and wet foliage to limit diseases such as white rust, powdery mildew (favoured by cool, humid autumn nights, common in the Delta), grey mould, Septoria leaf spot and viral diseases. As the autumn days shorten the buds form and the plants bloom, roughly three months after the last pinch, giving Egypt its main flush of colour in about October-December.