- Family: Primrose Family – Primulaceae
- Growing form: Perennial herb. Rootstock long, with stout runners.
- Height: 20–70 cm (8–28 in.). Stem often ascending from base, usually unbranched, usually almost glabrous, brownish red.
- Flower: Corolla regular (actinomorphic), wheel-shaped, yellow, approx. 5 mm (0.2 in.) wide, fused, short-tubed, 5–7-lobed; lobes narrow, tip with dark dots. Calyx lobes narrow. Stamens 5(–6), longer than corolla. Pistil a fused carpel. Inflorescence long-stalked, short, dense axillary raceme.
- Leaves: Opposite, sometimes whorled, stalkless, slightly amplexicaul. Blade on lowest leaves scaly, upper leaves lanceolate–linearly lanceolate, with entire margins, densely dark-spotted, underside sparsely haired.
- Fruit: Spherical, 5-valved, shorter than calyx, 1.5–3 mm (0.06–0.12 in.) long capsule.
- Habitat: Shores, streams, ditches, swamps, bogs. Often in shallow water.
- Flowering time: June–July.
Tufted loosestrife has spread across almost all of Finland – it gets rarer towards the north and doesn’t grow at all in the most remote corners of Lapland. It is quite undemanding in relation to its habitat, and doesn’t turn its nose up at poor soil. Tufted loosestrife is quite wild in Finnish wetlands and can be found on shores and bogs and in ditches, pits, broad-leaved forests and even water. It does not grow on hard shores however. Shore silt does not usually have much oxygen, especially during the hot summer. In order to guarantee the rootstock a sufficient supply of oxygen, many members of the Primrose family (like many other aquatic plants) have special loose cells for breathing called aerenchyma at the base of their stem.
Tufted loosestrife flowers are so small and located in such dense, cylindrical racemes that it is difficult to conceive of the plant as a relative of large flowered yellow and dotted loosestrife and creeping Jenny. The species’ scientific name thyrsiflora means roughly ’conical flower group’, which refers to the form of its dense, spiked racemes. The species sometimes flowers sparsely, but its branchless, erect shoots are easy to recognize from its narrow, taper-tipped leaves.
Tufted loosestrife is a perennial which is erect and has stout runners, and it grows in wet places. Its stem is reddish brown and it favours sheltered places such as bays and ditchsides, and it is not demanding with regards to soil. Stands usually expand vegetatively through runners.