Which species are alien?

Alien species are those, brought in from outside, which are not naturally distributed in Bulgaria. Their arrival is the result of human activity – transport, tourism, trade, agriculture, gardening, afforestation. Sometimes introduction happens when the natural barriers across countries and continents are breached – through bodies of water, by birds and animals, etc.

Most invasive alien plant species (IAS) possess powerful reproduction and survival mechanisms. They do not have particular environmental requirements or natural pests. IAS plants are easily naturalised and spread without the aid of humans.

Some of the most widespread and dangerous invasive alien plant species in Europe are ailanthus, ragweed, false acacia, desert false indigo, eastern prickly pear, bohemian knotweed, ashleaf maple.


Look through the gallery with images of invasive alien plants in Bulgaria


IAS plants establish themselves quickly and take over meadows, pastures and forests. They extract the nutrients and water from the soil and influence the light regime, thus suppressing the development of native plants and leading to their decline and extinction.

Apart from causing irreversible changes to wild nature, IAS plants cause serious economic damage and threaten the health of people, specifically causing:

  • Loss of agricultural produce and ever larger costs for weed control, due to the spread of ragweed, amaranth, Johnson grass, thorn apple, tall fleabane.
  • Reduction of the area of arable land, pastures and meadows, because of the spread of desert false indigo, false-acacia, ailanthus.
  • Destruction of infrastructure – ailanthus, desert false indigo and bohemian knotweed have powerful root systems with many offshoots. The seeds of numerous species disseminate and grow on buildings, churches and monasteries, causing the destruction of cultural monuments, canals and fortification walls.

Some IAS plants like ragweed and giant hogweed cause allergies, asthma and skin diseases (dermatitis).

Measures for Control and Limiting Invasion

We must limit the spread of IAS plants, by applying specialised measures, in accordance with the biological and ecological characteristics of each species! There are a few main methods, which show some success in the fight against IAS: mechanical, ecological, chemical, and biological. It is best to apply a combination of measures, according to the specific conditions and with the active participation of all interested parties.

Improper eradication of IAS trees and shrubs just exacerbates the problem. When they are cut down or burned IAS plants form an abundance of root offshoots, leading to even more overgrowth. There are dormant seeds in the soil and the destruction of mature shrubs and trees illuminates the ground, stimulating the formation of a large number of new plants.

Herbicide treatment can be successful, however the type and quantity must be precisely determined by an expert, in order to avoid soil and water contamination and the destruction of native plants and animals.

A consistent and tenacious fight until complete eradication of the treated groups must be performed in the course of several years. Each remaining stalk gives an opportunity for the root system to recover and create new offshoots.

The most certain way to protect against IAS is to not allow their entry in the first place and to react immediately, upon the first appearance of new plants. Limiting distribution is the main approach toward fighting invasive alien plants. This can happen only with the support of local communities, living in close proximity to the protected areas and Natura 2000 habitats and everyone with vested interest and responsibilities, connected to them.

Three of the most aggressive invasive tree and shrub species in Bulgaria are ailanthus, false acacia and desert false indigo. They are included in the list of the most dangerous invasive, alien species, threatening Europe’s biodiversity, as well as in the list of the European and Mediterranean Plant Protection Organisation.

Ailanthus, Tree of Heaven
(Ailanthus altissima)

False Acacia, Black locust
(Robinia pseudoacacia)

Desert False Indigo
(Amorpha fruticosa)