How to divide dwarf sweet flag

Dwarf Sweet Flag forms a clump that thickens each year, and dividing it every few years keeps it vigorous — old clumps often die out in the middle and flower less. Lift the whole thing and split it into decent-sized pieces, each with a good share of roots and several shoots. Bigger divisions re-establish far faster than tiny ones.

Early spring as growth restarts is the safest window; after flowering also works for many plants.

Lift the clump, shake off loose soil, then prise it apart — two forks back to back for a tough clump, hands or a knife for a softer one. Keep the vigorous outer sections and discard the tired centre.

Pot each division into fresh compost at the same depth it was growing before, water it in well, and keep it somewhere bright but out of direct sun for two to three weeks while new roots take hold. Do not feed straight away — damaged roots take up fertiliser poorly and it can scorch them.