How do they spread ?
Invasive plants have a high growth rate and high dispersal capacities : they grow fastly, they reproduce easily and abundantely, they are able to survive in varied ecological conditions. Some of them can produce a large number of seeds, dispersed by wind, rivers, animals, sometimes over long distances.
Some others can regenerate from shoots or roots (asexual reproduction or vegetative reproduction). They can extend from rhizomes (underground stems which can create new shoots) or from stems that root easily when they touch the ground. With the japanese knotweed (Fallopia japonica), for example, a small rhizome fragment of a few grammes can regererate the whole plant ! Lots of invasive plants are able also to sprout after cutting, meaning they create new shoots when the main stem is cut. Such plants are specially difficult to control when established in nature. Besides these natural dispersal capacities, man also contribute to their extension (see introduction pathways).
Plants with diversified of reproduction modes...
The giant hogweed (Heracleum mantegazzianum) mainly reproduces by seeds. One single individual can produce 7000 to 12000 seeds which fall within a distance of 4 meters from the mother plant.
Photo : J. Bakker
The alien shrub Rosa rugosa, the rugose rose (or Japanese rose), is an alien shrub invasive in coastal dunes. Through vegetative reproduction by rhizomes, populations can spread from a few clones to a more or less contiguous area of 3.5 ha in less than 50 years.
Photo : Kolmann
Landscapes components like waterways, roadways or railways function as effective dispersal corridors for spreading invasive plants. Plant (seeds, fragments of stems, of roots, etc.) can be easily dispersed by vehicules or water. Urban areas, parks and botanical garden sometimes function as “source” zones. Invasive plants used as ornemental for green areas or roadways management can escape into surrouding nature. Seeds dispersal by birds can reach a few hundreds meters. Wind dispersal is generally limited to a few tens of meters, but can reach several hundreds meters in case of storms. Rivers can also transport seeds or plant fragments several kilometers away from the plantation site.
Urban rivers as dispersal corridors for wind-dispersed invasive tree species
A recent stuty carried out in the city of Berlin (Germany) has shown that seeds of invasive tree species planted along rivers may be transported over long distances (several kilometers) by water current. The study focused on tree species like boxelder (Acer negundo) and tree of heaven (Ailanthus altissima). Once fallen in water, floating samaras could be disseminated far away downstream and colonise natural floodplains outside cities.
On the left: the tree of heaven (Ailanthus altissima) is frequenlty planted in green areas (photo: M. Halford).
On the right: when escaping, the plant is also found in natural habitats, specially on drier soils (photo : Y. Dumas)