Invasive plants blocking Dutch waterways

Exotic plants brought to the Netherlands by people are rapidly spreading through the country, according to Statistics Netherlands on Friday. Water plants in particular are causing problems in Dutch waterways. According to calculations by the statistics office, these so-called invasive alien plant species are present in 160 "kilometer grid squares" - areas of one kilometer by one kilometer. Before 1997 there were only invasive plants in 25 grid squares. 

Water pants like the water pennywort, Myriophyllum Aquaticum [parrot's feather] and Crassula Helmsii [swamp stonecrop] are giving local waterworks grey hairs. They grow super fast in stagnant and slowly moving water, eventually covering the surface of entire bodies of water. These invasive species displace all native plants, impedes water flow and accumulates at pumping stations and dams, eventually blocking them. According to Statistics Netherlands, water pennywort alone increased by a factor of 150 since 1990. 

All three these species are banned in the Netherlands, but were still sold in garden centers until a few years ago. They ended up in nature through ponds and aquariums. 

Invasive plant species also cause problems on land, but less so than in the water. Known problems are Canadian dogwood, Japanese knotweed and yellow mask flowers.