Plant of the Week
Dortmann's Cardinalflower (Lobelia dortmanna L.)
By Sue Trull
Dortmann's cardinalflower is a perennial herb in the Bellflower family (Campanulaceae). Also known as water lobelia, it is an aquatic plant of shallow, soft water in northern regions. Lobelia dortmanna is named for two people, Matthias von Lobel, a Flemish botanist, and a Dutch pharmacist named Dortmann.
Dortmann's cardinalflower grows in a form referred to as “isoetid”, after the aquatic fern ally, quillwort (genus Isoetes). These aquatic plants are small, forming underwater basal rosettes of thick evergreen leaves. They are quite slow growing, occurring in nutrient-poor lakes (oligotrophic waters), and using their extensive root systems to exchange gases and nutrients in the sediments, as opposed to obtaining these essential chemicals from the water column. The root system can be about 50% of the plant’s biomass.
The isoetid plants also have large air spaces within their tissues, allowing efficient movement of gases such as oxygen and carbon dioxide through the plant. These adaptations enable the isoetids to compete well in lakes where other plants cannot. Conversely, these adaptations make these plants vulnerable to changes in water and sediment chemistry, such as acidification of lakes. With increasing carbon availability in the water, other aquatic plants such as milfoil species (Myriophyllum sp.) can take over and replace the isoetids. Other isoetid species include quillworts (Isoetes species), American shoreweed (Littorella uniflora), and sevenangle pipewort (Eriocaulon aquaticum).
Dortmann's cardinalflower has a basal rosette of leaves, which are two hollow tubes, joined side-by-side, about 3/4 to 3 inches long. The leaf ends are rounded and flare outwards. An erect, hollow stem rises above the leaf rosette, a few inches to about three feet tall. There are tiny leaves along the smooth, unbranched stem. Underground stems connect adjacent plants. The plant has a milky sap.
The stems produce a few, well-spaced, tubular whitish-blue flowers, usually, but not always, on the part of the stem that extends above water. The flowers are two-lipped like other lobelias, and turned upside down, so that the upper lip appears to be the lower lip, which flares into 3 parts, and the apparent upper lip is divided into two smaller parts. The flowers bloom in mid-summer, and are self-pollinating. They produce small seeds in a capsule which gradually breaks down, releasing the seeds to sink to the lake sediments and overwinter before sprouting.
Dortmann's cardinalflower plants usually occur along lake perimeters, but may occur well into a lake if the water is shallow. They may also occur out of water on recently exposed damp sediment.
Dortmann's cardinalflower is listed with Endangered status in New Jersey, Threatened status in Pennsylvania and Washington, Special concern status in Rhode Island, and Exploitably Vulnerable status in New York. Threats include changes in water chemistry; application of herbicides to control aquatic invasive plants; shoreline development; and trampling by lake visitors.