Growing dahlia 'mystery day' in a pot

Yes — dahlia 'mystery day' grows well in a container, but it needs a substantial one. At about 1.3 m mature it wants a pot around 30-40 cm (12-16 in) across to stay stable and hold enough compost to buffer moisture. Go too small and it dries out constantly and blows over.

Aim for a container 30-40 cm (12-16 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.

Dahlia 'Mystery Day' 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 dahlia 'mystery day' in a pot needs checking more often than the general guidance of "When the top 2-3 cm of soil is dry, roughly every 5-7 days in summer" — especially in summer, wind, or a heated room. Check the top few centimetres with a finger rather than watering on a fixed day.