Very similar to Perforate St John’s-Wort (H.peforatum) except this species is distinguished from the other by blunter sepals and heart-shaped basal leaves. Also the stems of the slender variety are smooth, whereas the perforate has stems marked with 2 raised lines. It likes a reasonably dry soil and grows in heaths and grassy places, sometimes on roadsides, all over Scotland.
The stems can appear reddish, and are glabrous, upright and branching into two shoots whenever a pair of leaves appear, while the main stem continues upwards ending in a small branching cyme. This spreading arrangement is another way Slender St John’s-Wort can be distinguished.
The stalk-less leaves grow in opposite pairs and clasp the stem. They are slightly fleshy-looking, narrow ovals, while the lower leaves, though still sessile and stem-clasping, are more heart-shaped. Leaves are 5 to 20mm in length and dotted with translucent glands.
The flowers are reddish yellow with 5 petals, and these are dotted with tiny black glands around the edges, as are the 5 rounded sepals. In the centre the flower has a pin cushion of many yellow stamens with orange anthers.