Growing cosmos sulphureus 'klondike' in a pot

Cosmos sulphureus 'Klondike' is well suited to container growing — at about 60 cm mature it is comfortably pot-sized. A container around 15-20 cm (6-8 in) across gives it room without leaving the compost sitting wet, which is the usual mistake when people over-pot a small plant.

Aim for a container 15-20 cm (6-8 in) across. Pot on when roots show through the drainage holes or growth slows noticeably — usually every 1-2 years for a plant this size. Move up one pot size only; jumping several sizes leaves wet compost the roots cannot reach, and that is what rots them.

Cosmos sulphureus 'Klondike' needs sharp drainage, so this matters more in a pot than in the ground. Use a free-draining mix (add perlite, grit or bark to standard compost), always pick a container with drainage holes, and never let it stand in a saucer of water.

Containers dry far faster than open ground, so cosmos sulphureus 'klondike' in a pot needs checking more often than the general guidance of "When the top 3-5 cm of soil is dry, roughly every 7-10 days once established" — especially in summer, wind, or a heated room. Check the top few centimetres with a finger rather than watering on a fixed day.