Why Giant Rhubarb proved a little problematic
Here at Rosemoor, we have many thousands of different plants but not many are quite as recognisable in late summer as the genus Gunnera, also known as Giant Rhubarb. It is mostly known for its huge, exotic looking leaves that are visible from far away as the leaves can often grow to a monstrous size, reaching about 4m in height.
They really need wet ground to thrive, so they are often found by lakes and rivers, or in boggy areas where the ground is consistently wet. There are two main species that are often found in public gardens such as Rosemoor, namely Gunnera manicata and G. tinctoria. Or so we thought!
Through research conducted by the RHS, it turns out that what we believed was G. manicata was actually a hybrid of the two species tinctoria and manicata called Gunnera x cryptica. The problem is that G. tinctoria has been listed as an invasive species since 2019 and this includes any hybrids of tinctoria which includes x cryptica. The reason for them being listed as invasive is their vigour and their ability to easily spread into natural habitats, causing potential problems for ecosystems across the country.
Gunnera is non-native to the UK and the two species were introduced in the 1800s and slowly over time, they bred to form the hybrid, where x cryptica eventually replaced manicata because it was selected as a hardier and more reliable version. This managed to elude the horticultural world for many years which may seem strange but it’s understandable as they look extremely similar and are hard to tell apart.
So what does this mean if you already have Gunnera at home in your garden? Well, quite simply it means it can no longer be bought, sold or exchanged in the UK but it is okay to keep if you already have it in the garden. However, you cannot intentionally plant it or cause it to spread. If you have it next to a water course or on boggy ground where it can easily spread, it would be wise to consider getting rid of it or at least removing the seed heads before they mature so it cannot spread by seed.
The good news is that if you still want to grow Gunnera in your garden there are other, less well known species that are just as interesting. G. magellanica is a much smaller species that is only a few centimetres across per leaf and forms a great ground cover for wet ground with red fruits towards the end of the season.
There is also G. prorepens which is a similar size and also fruits well but has a more unusual leaf, being more elongated and a grey-green colour. Furthermore, consider G. perpensa which can be a bit more thuggish in wet ground but can be grown in a slightly drier spot instead.
Another option is to grow a completely different genus that looks a bit like Gunnera and will still give you that lush, exotic look in your garden. Darmera peltata is a great choice. In early spring, tall heads of pale pink flowers appear, soon followed by the leaves which look similar to Gunnera and are 50cm across.
Another option is Rheum ‘Great Bere’, especially if you want really giant leaves! The plants can reach 3m across and 3 - 4m in height, so you really need lots of room in your garden for this giant. The caveat for the above plants is you need moist ground for them to be at their best. It doesn’t have to be soaking wet but as long as there is consistent moisture that is fine.
Gunnera can be combined with other moisture loving plants in your garden such as Primula bulleyana, Iris ensata and Carex elata ‘Aurea’ to create a colourful display in your garden with a lengthy season of interest. If you feel like popping down to RHS Rosemoor to see these wonderful plants and more, consider coming to the Rosemoor flower show on the 16 - 18 August, where there will be lots to see and do.
Gardening tips for July/August
- Dead head repeat flowering perennials to ensure continuous flowering.
- Clear algae, blanket weed and debris from ponds.
- Harvest apricots, peaches and nectarines.
- Give Wisteria a summer prune.
- Collect seed from garden plants.
- Feed the soil with green manures.
©RHS/Tom King