Square-stalked St John's-wort - Hypericum tetrapterum
Short to tall erect hairless plant. Stems square, with four narrow wings, spreading and rooting at the base. Leaves opposite, rounded to oval with translucent dots. Flowers pale yellow, 9 to 10 mm, many in a spreading panicle.
Other St.John's-worts. See ID Aids below.

Petals pale yellow; small flowered; stems with 4 broad wings
The St John's Wort (Hypericum) family are attractive, yellow-flowered perennials. They can look confusingly similar but can be distinguished by careful examination of key features. A hand-lens may be required (depending how good your eyesight is!). The leaf perforations are visible as translucent dots if the leaf is held up to the light.
A photo of the plant in its habitat
Damp places, marshes, pond and river margins.
June to September.
Perennial.
Fairly common throughout Britain.
Fairly common in Leicestershire and Rutland. In the 1979 Flora survey of Leicestershire it was found in 310 of the 617 tetrads.
Leicestershire & Rutland Map
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MAP KEY:
Yellow squares = NBN records (all known data)
Coloured circles = NatureSpot records: 2020+ | 2015-2019 | pre-2015