Life History

Although Eurasian watermilfoil produces seeds, most reproduction is asexual from stem fragments and root crown buds. Propagating root crowns typically break dormancy in the spring when water temperature and light intensity increase. Stem growth is limited by water depth, with growth occurring in water up to nine feet deep. However, Eurasian watermilfoil can grow in water up to 24 feet deep if it is very clear with high-light penetration. When stems reach the water surface, they branch abundantly forming a canopy near the water surface. The dense tangled mats can support frogs and wading birds. Lower branches and leaves constantly slough, adding nutrients to the water column throughout the growing season. The release of nitrogen and phosphorus can be rapid, and can be a significant source of internal nutrient loading to a lake. Plants flower mostly in late July to September. The female flowers ripen as the inflorescence spikes emerge from the water, well ahead of stamen development, favoring cross pollination. Fruits have a stony surface that prevents seed germination. Seed dormancy is prolonged (seven years), germination is erratic, and seedlings are considered rare in nature. Growth slows in the fall and a protective case over buds (hibernacula) may develop. Eurasian watermilfoil can maintain a large biomass throughout the winter which aids in rapid and early seasonal growth. It produces phenolic compounds that deter herbivores.

Stems fragment by natural wave action and by recreational activities such as boating. Differing from native milfoil species, Eurasian watermilfoil stems are adapted to self fragment by the development of specialized abscission stem cells, which typically occurs after flowering. One study found the growth of naturally produced fragments was better than artificially broken stems, and the naturally fragmented stems had higher total non-structural carbohydrate which enabled improved winter survival. Colonization is greatest during late summer in shallow (two feet) water on rich organic sediments. Mortality is greatest in deep water with calcareous, nutrient poor sediments during early autumn. In lakes in Wisconsin, South Carolina and Tennessee, annual biomass production varied from one to five tons per acre. Eurasian watermilfoil is adapted to use bicarbonate as a carbon source for photosynthesis, and although it is a C3 plant, it has structural adaptations similar to C4 plants that allow more efficient carbon utilization in the low carbon, submersed environment. Total non-structural carbohydrate in Eurasian watermilfoil may be highest (20%) in October and in plants under ice in the winter, and declines rapidly during spring flush to the lowest level (5%) in early summer.

Stem fragments allow for rapid colonization. For example, in New Jersey, researchers cleared a one by four meter area on the bottom of a heavily infested lake in mid-July. By early August, 15 rooted stems of Eurasian watermilfoil had established and by early September the number of stems had increased to 80. The following April, the cleared area was indistinguishable from the rest of the infestation, and there were no other plant species present.

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Last Modified: 02/13/2009