Water-Starworts (Callitriche spp.)
Water-Starworts - Callitriche spp - are mamed after the rosette / star shape of their leaves. They are most common in slow-flowing waters where it is muddy and silty, although they will grow in still and fast-flowing conditions up to 1 m deep.
They are intolerant of inorganic pollution. They have slender stems and long, thin submerged leaves which expand when they reach the surface. The submerged leaves are
characterised by the notched ends. They are polymorphic which means they can take on different leaf shapes in different environmental conditions. Classification and
distinction between species is often only possible by
examination of the flowers and seeds.
The Water-Starworts should not be confused with Crassula helmsii, which has similar shaped leaves to the submerged type exhibited by Callitriche species but no notches in the leaf tips.
They seeds sink to the bottom when released and can lie dormant in the mud for up to five years. Often this accounts for their sudden appearance in waters which have recently been cleared or dredged. Darkness inhibits the germination of the seed.
The Water-Starworts cause problems by blocking flow in drainage systems and by dominating slow-flowing ponds and canals. In such conditions they form dense
mono-specific stands. They are important as food and shelter for fish, as an invertebrate habitat and as food for ducks.
Control
Mechanical control - Cutting and removal using our Truxor gives good control for this species. Care should be taken to restrict the spread of viable fragments out of the area in which the plant is controlled. Cutting can be carried out at any time during the season but should be delayed until after July if longer term control is required.
Cutting must be carried out carefully because of the high viability of the fragments and the risk of spreading the plant to unaffected areas.
Being perennial, they retain their leaves overwinter and rooting internodes bearing lateral buds can remain in the mud overwinter and regrow the following spring. This characteristic makes mechanical control late in the season less likely to produce a good long-term reduction in plant density.
Chemical control - This is not an option as there are no approved herbiced for this underwater plant.
Biological control - Grass carp will control the growth of Callitriche species. Wildfowl will also eat the submerged plant material. Increasing wildfowl numbers may reduce the amount of weed growth, but may promote algal growth due to higher nutrient enrichment from the wildfowl.
Environmental control - Deepening the water to below 1 metre will make the conditions unsuitable for the growth of this plant. Shading is another possibility, although deep shade will be required.