Once settled, colchicums can last for decades and will form large clumps. When they become overcrowded, divide in early summer, when their corms are dormant, or just after blooming. Carefully separate the corms or detach the cormlets and replant them in new locations 10 to 12 centimetres apart.
Colchicums can also be propagated by seed but will take three to five years to flower. Seeds are found in the large seed pods at the base of the leaves after they mature in mid- to late spring; plant them immediately. Note: Colchicums are very toxic plants, especially the seeds and corms. Keep them away from children.
- Choose a location in full to partial shade.
- Dig a hole 20 to 25 centimetres deep in moist, fertile, well-drained soil.
- Add a layer of 10 to 15 centimetres of sandy loam, compost or leaf mould to the bottom of the hole. Slightly acidic soil is preferred, but many colchicums are fine in alkaline soil.
- Place corms with new shoots pointing upwards, if visible, 10 centimetres below the surface and 10 to 12 centimetres apart. If new shoots are not visible, plant the more elongated, pointed end facing up.
- Press soil firmly around corms and water well.
- Water moderately during growing period; avoid wetting flowers and foliage. Don’t let the soil dry out completely.
- Apply low-nitrogen fertilizer when leaves start growing in the spring. Stop fertilizing once corms go dormant over the summer.
- Though relatively pest- and disease-free, colchicums can be affected by botrytis or grey mould. To control, remove infected flowers or leaves; do not compost.
Photo: C. autumnale ‘Alboplenum’, a very old, rare cultivar.