- Name also: Dortmann’s Cardinalflower (USA)
- Family: Bellflower Family – Campanulaceae
- Growing form: Perennial water plant. Rootstock short, tuber-like, predominantly white, almost unbranched secondary roots.
- Height: 20–90(–150) cm (8–35(–60) in.). Stem unbranched, hollow, glabrous.
- Flower: Irregular (zygomorphic). Corolla pale blue–almost white, fused, bilabiate, 15 mm (0.6 in.) long. Upper lip with 2 lobes, lower lip with 3. Calyx 5-lobed, lobes linear, round-tipped. Stamens 5. Pistil of 2 fused carpels. Inflorescence a lax, one-sided, nodding raceme that lacks a terminal flower.
- Leaves: Basal leaves a rosette, stalkless. Blade linear, slightly fleshy, flat on top, rounded underneath, tip hooked downwards. Stem leaves alternate, sparse, small.
- Fruit: Conical base, rounded tip, slightly flat, capsule opening from tip.
- Habitat: Nutrient-poor, clear-water lakes with sand and gravel bottoms, sometimes in running water or slightly salty brackish water. Usually 20–150 cm (8–60 in.), sometimes up to 3 m (10 feet) deep.
- Flowering time: July–August.
Water lobelia grows a rosette on the lake bed but raises its inflorescence above the surface of the water from up to a metre (40 in.) deep. This is a legacy from a time when they grew on land, because the flowers self-pollinate before they even open. Apart from the seeds the species also spreads via the small rosettes that form along the lateral branches.
Water lobelia demands clear and clean water so that the sun shines onto the submerged leaf rosette. The species doesn’t usually grow at depths greater than 1.5 metres (5 feet), but in especially clear water it can be found growing up to 3 metres (10 feet) deep. On the other hand the species is susceptible to freezing and drying, so it stays clearly below the water line. Water lobelia usually grows on sandy lake-beds, because loose mud would easily bury the short leaf rosette. The species has suffered from the eutrophication and oxygenisation of lakes. It favours eskers and sandy areas with clear water, and is in fact the type plant for this kind of lake. Visitors to summer cottages welcome the sight of water lobelia flowers because it means that the water is clean and is good to swim in.
Water lobelia can give a slightly distorted view of genus Lobelia: only two of the hundreds of species in the genus are water plants. Edging lobelia (name also garden lobelia, trailing lobelia) from South Africa is a more typical representative of the genus. It is an annual but can sometimes be found as an escape in the Finnish wild.