- In cold and wet gardens, border dahlias are best lifted and overwintered under cover once their leaves turn black. If you live in a mild area, the tubers can be left in the ground for the winter: cut down the tops and spread a mulch of bark chippings around them.

- Move troughs of alpine plants into an unheated greenhouse to prevent possible damage from winter waterlogging. If you do not have a greenhouse, place the trough in a sheltered corner of the garden.

Reader’s question

Do I need to protect a gunnera through late autumn and winter?

J Dobson, by e-mail

Gunnera manicata does need frost protection. Remove the spent leaves and surround the crown with dry straw or a thick mulch of bark chippings. Lay a couple of the old leaves over the straw and crown, and hold in place with a few twiggy prunings. Remove in spring.

gardenexpert@sunday-times.co.uk

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