Dividing Primroses and Polyanthus
Primroses and polyanthus can be divided either immediately after flowering or in early autumn. Dividing in May has the advantage of giving a longer growing season, but exposes the divisions to the stress of summer heat and drought when they are at their most vulnerable. We always divide in September and October, and find that the plants are fully established, flowering freely the following spring. However, if your winters are particularly severe, you will probably get better results by dividing in May.
Lift the plant with a fork, taking enough soil to avoid damaging the roots. Shake off as much soil as possible, washing it off if necessary. Tease the roots apart and cut out large well-rooted crowns for replanting. Plant the small pieces also, - you will probably be lucky ! Discard the old woody centre of the plant.
Before planting, trim the roots on the separated pieces back to about four inches, and also cut off any large leaves to prevent water loss during the early life of the separated plant. It's a good idea to soak the new pieces in clean cold water for a couple of hours before planting, so that they will be 'plumped-up' with water at replanting. Water in the new plantings with a half strength potash solution and keep them moist and shaded until they are established.
Auriculas produce offsets which grow from the main root or 'carrot'. They are best separated in September or October, and rooted offsets should be potted up in a standard peat compost with a little sharp grit for drainage. They should establish in about a month and should be kept cool and slightly dry. They should also be well ventilated and can be planted into their permanent position in the spring.
Sieboldii should be lifted every three or four years when the foliage dies down in autumn. Tease apart the rhizomes, discarding the old centre of the plant, and selecting the younger outer ones for replanting. As the best flowers come after two years, it is recommended that you renew only a third of your plants each year. Plant in a wide shallow hole with the roots spread out around the bud, and cover with about an inch of non-alkaline soil. Mark the spot where the rhizome is planted and wait for the new growth which will not start until the following March or April.