Knotgrass Polygonum aviculare
The sort of plant which catches the attention of herbicides, in the name of botanical cleanliness. But Wait! Stop before you spray!
Not just a weed, this is a bare ground coloniser, taking nature's first steps to fill a vacuum. It has noted wildlife value: as the 'aviculare' part of the name suggests, birds such as finches and sparrows feed avidly on the seeds.
|Knot overlooked ... supporting wildlife on Cathedral Green|
Added to which, the long flowering period can support insects from May - November. Right now Knotgrass clumps are flowering in profusion around Cathedral Green and St David's church, in the worn bare areas among the grass.
The tiny clusters of 1-6 flowers emerge at the bases of the upper leaf stalks. The three inner stamens are bent over so that pollen falls directly on the styles; the others point outwards for any passing insects. But this isn't all: there are also concealed flowers under the membraneous sheaths (the ochreae) around the stem 'joints' (the joints give the plant the other part of its scientific name: Polygonum 'many - kneed'). These hermetically sealed flowers do not open (or in technical terminology, are cleistogamic), which, along with the arrangement of inner stamens, ensures some self-pollination at least. It is said Knotgrass may produce similar cleistogamic subterranean flowers underground as well, among its roots and runners, alongside which the tap root can penetrate up to 45cm deep into the ground.
As a multi-pronged pollination and propagation strategy, these would be useful adaptations for an annual plant, which dies off every year, but for which the seed bank must persist biding its time in the soil for years, until the next unpredictable opportunity brings seeds to the surface where they can expeditiously germinate.
As well as a pioneer species to kickstart vegetation recovery on bare ground, Knotgrass is a host plant for over a hundred different invertebrates: some 9 beetle species, 2 flies, 8 bugs, and 93 moths, especially among the the Geometer moths.
Tolerating a bit of Knotgrass in the bare patches and nooks and crannies among the concrete would be small, simple and effective action to help our urban wildlife.