Dwarf St. John's Wort
St. John's Wort family (Hypericaceae)
Description: This native wildflower is either a short-lived perennial or summer annual. It is usually a short bushy plant about 4–18" tall, although larger specimens have been reported. The much-branched stems are light to medium green, glabrous, sometimes glaucous, 4-angled or terete, and sometimes narrowly winged. The opposite leaves are about ½–1½" long, ¼–¾" across, and sessile; they are medium green, broadly oblong to oval in shape with 1-5 prominent veins, smooth among their margins, and glabrous. The upper stems terminate in small clusters (cymes) of flowers. Individual flowers are ¼" across, consisting of 5 yellow to yellow-orange petals, 5 green sepals, a light green pistil with 3 styles, and 5-15 stamens. The petals and sepals are about the same length; the petals are oblong, while the sepals are linear-oblong. The peduncle and pedicels of the flowers are light to medium green, slender, and glabrous; sometimes there are small scale-like bracts near the area where two pedicels diverge. These bracts are linear-lanceolate in shape and much smaller in size than the leaves. The blooming period occurs from mid-summer to early fall and lasts about 2-3 months. Usually, only a few flowers are in bloom at the same time. Each flower is replaced by an ovoid seed capsule that becomes up to 1/3" tall at maturity; this glabrous capsule has remnants of the styles at its apex and it is 3-celled. Each cell of the capsule contains numerous tiny seeds that are dark-colored, narrowly oblongoid, and somewhat flattened. The persistent sepals become enlarged as the seed capsules mature; the sepals are usually about the same length or a little longer than full-sized capsules. This wildflower reproduces by reseeding itself.
Cultivation: The preference is full sun to light shade, wet to moist conditions, and acidic soil that is either sandy or rocky.
Range & Habitat: Dwarf St. John's Wort is occasional in most areas of Illinois, otherwise it is uncommon or absent (see Distribution Map). Habitats include sandy forests in floodplain areas, sandy swamps, wet to moist sand prairies, gravelly seeps and springs with an acidic bedrock (e.g., sandstone), damp depressions in sandstone glades, damp depressions along sandstone cliffs, low sandy areas along rivers and ponds, damp depressions in sandy paths, and abandoned sandy fields. This wildflower is found in both disturbed and little-disturbed habitats.
Faunal Associations: The flowers are cross-pollinated primarily by small bees, which collect pollen. Some flies may visit the flowers to feed on the pollen. Nectar is not available as a floral reward. There are a small number of insects that feed on various parts of Hypericum spp. (St. John's Wort species). These insect feeders include the caterpillars of several moths, some leaf beetles, the aphid Brachysiphum hyperici, and the caterpillars of the butterfly Strymon melinus (Gray Hairstreak). See the Insect Table for a listing of these insect species. Mammalian herbivores usually avoid consumption of the foliage because of its toxicity. The foliage contains hypericin, which produces a photosensitive reaction to sunlight, particularly in light-skinned animals.
Photographic Location: A damp depression in a sandy path at the Iroquois County Conservation Area in Illinois.
Comments: This wildflower resembles a dwarf version of the better-known Hypericum spp. (St. John's Wort species). Other small-flowered Hypericum spp. (flowers 1/3" across or less) usually have more narrow leaves and therefore are easily distinguished from Dwarf St. John's Wort. An exception is Hypericum boreale (Northern St. John's Wort), which is rare in Illinois. Generally, Northern St. John's Wort can be distinguished from Dwarf St. John's Wort and other small-flowered species in this genus by its leafy floral bracts, which resemble small leaves. It also has purple seed capsules that are as long or longer than its sepals. Dwarf St. John's Wort, in contrast, has tiny scale-like floral bracts and its seed capsules usually remain green for a longer period of time.