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Last updated 14/05/20 10:00.
Gunnera - Care Guide
Caring for Gunnera manicata
Commonly known as ‘Giant Rhubarb’
Gunnera manicata can be both loved and admired but also feared! The key thing to remember is that the more feed and the more access to water you can give them the larger the rhizomes will grow and the bigger the leaves will become. These are not aquatic plants which grow in water. They are bog plants which like their roots in rich wet bogs which remain wet all year round. They happily colonise alongside lakes and streams where their gigantic umbrella-like leaves reflect splendidly in the water. Do not try them as umbrellas though unless you have a sharp knife to first remove the prickles off the stems!
There are several huge clumps growing in permanent bogs beside streams at Caerhays. The individual leaves can be 12-15ft tall by August and the chubby rhizomes 5-6ft long. In May gunnera produce fat chubby flower spikes 2-3ft in height which will produce red fruits by autumn. Our plants have seeded themselves widely in the garden and odd ones appear hundreds of yards from the main clumps. Some grow in full sun on hot banks with leaves of only 3-4ft.
Some fear gunnera as being too huge and invasive but the leaves can readily be chopped off if they get in the way on a path with no ill effect to the main rhizome. In winter the huge rhizomes are easily prized from the ground where they sit. The roots die off back to the rhizome so it can be rather like lifting a rotted tree trunk. Rhizomes can thus be moved intact, chopped into pieces with a spade and moved or potted. Pieces of rhizome will readily shoot away even if you do not have the old crown. Invasive – yes! Containable – yes as well!
The second great and largely misguided worry about growing G. manicata is that it is not frost hardy. This is nonsense and gunnera will readily survive -12°C of frost. In the Valley Gardens at Windsor they gardeners used to cover the rhizomes in leaf mould and leaves for winter protection. This time consuming business is achieved far more quickly and effectively today by cutting off the green leaves in late September or October and simply laying them on top of the rhizomes themselves. Even this is probably unnecessary except in a very severe winter. Even then it is unlikely that all of all the rhizomes will actually be killed by frost.
What can be a problem is late frosts in April or May as the leaves are emerging and developing. Gunnera tend to grow in frost pockets near water and certainly do with us. If you know frost is coming you can use fleece protection. However, in the main, there is no cause for concern as these hugely vigorous plants will simply grow through a bit of browning and dieback to their first leaves. They will quickly grow more or the damaged leaves will develop fully with a little browning to the edge of their leaves. We get a few slightly hysterical calls in the nursery about this problem at Chelsea time but the problem is short lived and transient.
Examples of how to protect your gunnera over winter are below.
We stock G. manicata as small plants and huge specimen plants in pots of virtually any size you might want. Just ring and ask but remember gunnera can only be sent out by mail order when dormant in winter or early spring.
Protecting from Frost Damage
Further reading and images in the Caerhays Garden Diary - Gunnera.