Growing variegated sweet flag in a pot

Variegated Sweet Flag is well suited to container growing — at about 90 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.

Variegated Sweet Flag likes steady moisture but not waterlogging — a tricky balance in a pot. Use a good peat-free compost with some added perlite for air, make sure the container drains freely, and mulch the surface to slow evaporation.

Containers dry far faster than open ground, so variegated sweet flag in a pot needs checking more often than the general guidance of "Keep permanently wet; grow in standing water or saturated bog soil at all times" — especially in summer, wind, or a heated room. Check the top few centimetres with a finger rather than watering on a fixed day.