The long, thin, and flexible stems sometimes form an almost impenetrable mass, which makes it practically impossible for humans to move in such areas. Stems entangle oars, boat propellers and even swimmers’ legs to such an extent that it is like hitting a wall! However, aquatic creatures, such as fish and small invertebrates thrive in the thick growth, which provides them with both shelter and nourishment.

Four watermilfoil species are found in the Baltic Sea

Along the Finnish coastline, there are four species of watermilfoil: the Eurasian-, the Siberian-, the whorled-, and the alternate. The most common are the red-stemmed and spiky Eurasian- and the coarse-leaved Siberian watermilfoils, which, as eutrophication has progressed, have established themselves in the aquatic communities of protected bays. The whorled watermilfoil mainly occurs in riverine estuaries, while the sensitive alternate watermilfoil tolerates eutrophicated waters very poorly.

Hornworts live in the same areas as watermilfoils

Watermilfoils are fast-growing and durable aquatic plants with flexible stems, and their feathery leaves are easy to distinguish from pondweeds and other aquatic plants. The only species that resembles watermilfoils and occurs in similar places is the bushy hornwort.

The growth rate of the hornwort may be extremely rapid due to eutrophication. It may not be properly rooted in the bottom sediments but instead may be only loosely attached to a fine layer of surface silt or mud.

While both water milfoils and hornworts thrive in quite turbid water, the lack of light penetration due to eutrophication first causes the milfoils to disappear and eventually the latter species also. Hornworts are usually the last species to survive in a silted-up sea bay.

Water milfoil and hornwort species found in Finland:

• Eurasian- or spiked watermilfoil (Myriophyllum spicatum)
• Siberian watermilfoil (Myriophyllum sibiricum)
• Whorled or whorl-leaf watermilfoil (Myriophyllum verticillatum)
• Alternate- or alternate-flower watermilfoil (Myriophyllum alterniflorum)
• Hornwort (Ceratophyllum demersum)