When and how do I divide my Gunnera?
“Divide large types before growth starts into single crowns in midspring.” (Source: American Horticultural Society Plant Propagation, ed. by A. Toogood, 1999, p. 198)
“Many gunneras are huge and so are impossible to dig up and divide in the conventional sense. For propagation, cut pieces from the edge of the clump. Pot up in a large pot and in a fibrous medium and keep moist.” (Source: The Complete Book of Plant Propagation, ed. by C. Heuser, 1997, p. 40)
The sources I looked at indicate that you should divide your Gunnera in April or May. Because of the size of the plant, it may be impractical to divide the rootball. You can use a spade or pitchfork to cut sections from the main clump, and then plant those divisions in pots, keeping them moist. Gunnera is a tough plant, and should take well to this kind of division, as long as there is adequate moisture.
Should you need to protect your Gunnera over the winter months, you can cut the leaves and use them like a tent to protect the crown of the plant during the coldest months.