The high content of nectar in Common Knapweed flowerheads attracts insects like the Painted Lady butterfly
Nature on our doorsteps: Purple Knapweeds
Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into the natural heritage around us and the beautiful biodiversity of the plants and creatures.
The Knapweeds bring a purple-coloured haze to meadows and roadsides from mid-summer until early autumn. They are very important flowers for pollinating insects, as they produce very high volumes of nectar and pollen.
Common Knapweed is generally a plant of semi-natural meadows and grassy roadside verges.
Its stiff, branched stems can grow to 80cm high, and these begin to rise up through the meadow grass during June.
By the end of July and early August the plant’s deep purple flowers are in peak bloom.
The plant is a member of the Aster (or composite) family of plants, and so its flowerhead represents a collection of tiny flowers all crowded together on a sturdy flowerhead. For Common Knapweed, these can measure 2.5cm wide.
This crowded arrangement of flowers, or florets, on a single flowerhead gives small flowers a better chance of being visited by pollinating insects, as the insects can land on a firm platform and feed efficiently from the many nectar-rich flowers.
Greater Knapweed has an elegant ring of ray-like florets around the edge of the flowerhead
Common Knapweeds are sometimes mistaken for Thistles, but while both plants have purple flowerheads shaped like a shaving brush, the Thistle has plenty of prickles on its stem and leaves to distinguish it from Knapweed.
The Greater Knapweed also occurs in Ireland. This is a larger and more robust plant than Common Knapweed.
Its bigger flowers can grow up to 5cm wide, and the florets on the outer edge of its flowerheads are much longer and ray-like, adding to the plant’s more spectacular look.
Greater Knapweed is also a plant of meadows and roadside verge, but because it prefers lime-rich soils, it occurs more typically towards the centre and the south of Ireland.
Interestingly, however, both species occur here in South Dublin County, in Tymon Park.
This is due to both the presence of lime-rich esker soils that occur in Tymon and the way that the park’s meadows are managed.
This allows Common Knapweed to occur throughout the deeper soils of the flower-rich meadows, while Greater Knapweed occurs on the thinner, more lime-rich soils of the esker itself.
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