Thalia, a problem solved?
For years now, the natural mud ponds at the Beth Chatto Gardens have contained several magnificent clumps of the powdery alligator flag, Thalia dealbata. This strongly architectural plant, native to the marshes and wetlands of the southern U.S states and Mexico has proved remarkably hardy in the UK and has become a real feature of our Water Garden during the summer months. Even during winter, the brown frosted foliage remains standing, until cut down by the gardeners below water level in readiness for fresh growth to emerge, which does, alarmingly late in spring.
Thalia dealbata in full flower August 2019
Beth likened the plants foliage to a Zulu’s shield and the flower spikes to a spear, valuing, as ever, the foliage and form of the plant. Often, not by design but as consequence, many of the plants Beth and her gardeners chose, tend to attract an abundance of insects and the Gardens diversity of wildlife has increased enormously as a result.
The slender flower spikes topped with a cluster of violet flowers are no exception and noticeably attract a wide range of insects. As with many plants, it is a two-way arrangement, a partnership, the insect obtains nectar and the plant gets pollinated.
Perished insects on thalia flowers… Image Chris Gibson
But the powdery alligator flag has a rather nasty bite! Over the years it has become apparent that the plant can trap certain “weaker insects”, many of which are extremely important pollinators, hoverflies, lacewings, bees, wasps and blow flies, amongst others. Larger insects seem to be able to struggle free of the flower’s mechanism. The plant has no reason to kill its visitors – it does not digest them like a truly carnivorous species: it seems that the flowers have an elastic style, used in explosive pollination which can and does trap insects.
The issue has been recently highlighted by the excellent blogs of Chris Gibson, a local, experienced and enthusiastic all-round naturalist, who sees our gardens with a different eye and has led us to question growing this plant, rather than blindly enjoying its beauty. https://www.chrisgibsonwildlife.co.uk/
Essentially, we are introducing a non-native alien plant into our gardens with consequences! Ninety nine percent of such introductions are fine and add to the diversity of flower and wildlife. Occasionally, there is a problem and such plants have been identified and legislation has been put into place to manage that particular issue. For example, it is now illegal to sell plants of the yellow American skunk cabbage, Lysichiton americanus. But it is not illegal to grow the plant in your garden, so long as a management plan is put in place. The issue with this plant is that, in some climates, especially in the south and west of the UK it can seed and spread into water courses and infestations can dominate large areas, crowding out native species in important habitats such as wet woodlands. Simply removing the seed heads and taking care when disposing of any plants removed prevents this issue.
Back to the powdery alligator flag! … Our pollinators are precious, even blow flies! What do we do? Remove the plants completely? As gardeners, we would miss the plants architectural beauty. Beth grew thalia as much, if not more, for its foliage as its flowers, so this year we have decided to remove all of the flowers as they appear throughout the summer, not a straightforward task, as the gardeners have to don their waders to reach the clumps. As a result, the clumps look equally as imposing and perhaps tidier and less prone to flopping.
Flowers removed August 2020
We will still offer the plant for sale and would encourage all potential buyers to remove the flowers. Perhaps other sellers, both retail and the larger wholesale aquatic nurseries could inform their own customers to adopt the same management plan in order to enjoy this plant, safe in the knowledge that you are doing no harm at all to our own native population of pollinators.
Oh, and just in case you are wondering why it is called the powdery alligator flag? As its name suggests, it lives comfortably alongside the alligator in bayous and backwaters. If you see this flag flying, it is best to keep a sharp eye for one of its usual companions…. you have been warned!
Dave Ward, Beth Chatto's Plants & Gardens