Rhizomes such as border irises can be divided as other herbaceous
perennials. Again this becomes necessary to get rid of the developing
bare patch where the rhizomes grow away from their starting point.
Normally about 10 cm of non-woody rhizome is cut off with each
fan of leaves. The leaves are reduced to a third to reduce water loss
and wind rocking. Some species, such as Bergenia, are flowering in
the early spring so it is best to lift the plants in mid-winter and remove
the rhizomes. These should be washed and the dormant buds found.
Sections with a bud can be taken off and rooted in potting compost in
trays by burying them horizontally to half their depth. It is advantageous
to provide ‘bottom heat’ by standing the trays on soil warming cables.
The plants are not ready to go out until the fibrous roots have emerged
from the bottom of the tray and have been ‘hardened off’.