Invasive plants are non-native plants that cause negative environmental and economic impacts.

Many of the plants we know were transported from their natural habitats to the most diverse parts of the world, where they are considered exotic plants.

In Portugal, for instance, there are numerous of these plants coexisting with plants that are native to Portugal, or autochthonous, in a balanced way.

However, there are other plants that develop very quickly and uncontrollably, becoming harmful and creating an imbalance in the ecosystem – they are the invasive ones.

There is a set of criteria for considering an invasive plant. These have, above all initially when they appear in the country, several advantages, however, we quickly realise that they are a problem for several reasons:

• They are one of the main threats to biodiversity;
• Economic impact, as they invade production systems such as agriculture or forests;
• Impact on public health, if they are toxic or pest vectors, for example;
• Decrease in water availability in groundwater;
• Impact on the balance of ecosystems;
• Consumption of essential resources for other species, namely native species;
• Difficulties in controlling their expansion.

In 1999, Portuguese legislation recognised this problem and its seriousness, with the creation of Decree-Law No. 565/99, of 21 December, which regulated the introduction of non-indigenous species into nature, identifying around 30 invasive species.

In 2019, the diploma was revoked and replaced by Decree-Law 92/2019, of 10 July, which lists more than 200 species as invasive in Mainland Portugal and/or Madeira.

One of the most invasive plants in Portugal is the Acacia, and there are several species of Acacia that are invasive in the country. This plant appeared abundantly in the country, perhaps because of the beauty of its yellow flowers. Right now, it appears in an uncontrolled way pretty much everywhere.

Another known species is the Arundo donax, that we are used to seeing in our landscapes, usually near water lines.

An invasive aquatic plant in Portugal, especially in the center of the country, is the Hyacinth, Eichhornia crassipes, which creates authentic green carpets on water levels, like the Fermentelos pond (Pateira de Fermentelos).

Loved for interior spaces and floral arrangements, the pampas weed is also a very much present invader on our horizon. Its feathers are a real trend, but it is in them that the seed that effectively propagates through the wind is located, causing an increasingly uncontrolled invasion phenomena.
The control of the growth and propagation of each of these plants is done by very specific methods, more or less natural, suitable for each species. They all involve specific means and have associated costs.

As weeds are a threat to biodiversity, more and more campaigns are emerging that seek to control or eradicate this problem.

These plants are exceptionally well adapted to the environment, which makes them also the target of investigations and reflections on the future, sustainability and ecosystems.

Amália Souto de Miranda

Landscape Architect