Invasive plants

Unfortunately, several invasive plant species are appearing in the Sønderborg area, such as giant hogweed, Japanese knotweed, etc.

If you have discovered an invasive plant in the municipality, you can report it either via the "Giv et praj" app or by email. If you see an unknown or new invasive plant in the municipality, we would also love to hear from you. New species are given high priority in the fight to prevent them from establishing themselves.

Please note that your inquiry will not necessarily lead to control. The municipality's efforts depend on the species' damage, distribution and resources. Read more about the individual species below.

Giant hogweed (Heracleum mantegazzianum)

Sønderborg Municipality is working purposefully to eradicate giant hogweed on both public and private land.

Giant hogweed is an umbrella plant up to 4 meters tall with a thick stem (up to 10 cm in diameter), green with red spots and stiff hairs. The leaves are large, coarsely toothed and slightly downy on the underside.

 

Combat:

It is important that the plant is controlled before it flowers and sets seeds. Smaller populations can typically be rooted out. For larger populations, methods such as grazing, mowing, soil treatment or mulching are used. Chemical control is generally not recommended for private households.

Cut flower umbels must be disposed of at the recycling center – they must not be disposed of in garden waste, as the seeds can ripen after cutting.

Japanese willow (Reynoutria japonica)

Japanese knotweed is a fast-growing species and very difficult to control. It forms dense stands that crowd out other plants, and its roots can damage buildings and pavements.

The municipality is combating the plant in selected locations, but due to its robustness and costs, we have to prioritize the effort.

Japanese knotweed can grow to over 2 metres tall. It has hollow stems and pointed heart-shaped leaves with a straight cut base. The plant is often confused with giant knotweed or hybrid knotweed – all three are invasive.

Combat:

Effective control requires long-term and targeted efforts:

• Excavation: most effective, but also expensive and resource-intensive.
• Uprooting: weakens the plant, especially in new stands.
• Covering: requires light-tight covering for at least 10 years.
• Chemical control: possible, but must be carried out correctly and only where legal.
• Mowing and grazing: can prevent spread, but rarely eradicates the plant.

All cut or dug up material must be disposed of. Stem and root pieces can sprout from even small fragments.

Poorly carried out control is one of the most common causes of further spread.

Other invasive species

The municipality also keeps an eye on species such as:

• Wrinkled rose
• The glossy leaf is hooked.
• Canadian goldenrod

These are only controlled in special cases where they pose a significant nuisance. However, your knowledge of their occurrence is important to prevent their spread.