By: Esther Hegt and Dr. Pieter B. Pelser ( University of Canterbury- Biological Sciences, Christchurch, New Zealand)

On the internet (1,2,3,4) and in other media (5,6) people express their concern that Common ragwort is becoming more common in the Netherlands and that this species will soon be present in more and more areas where it was previously absent. In addition to this, there is a call for fighting Common ragwort not only in horse fields and their direct vicinity, but also in areas that are more remote, because the wind dispersed seeds can travel large distances and can therefore still end up in areas where horses graze. In this context, it is often mentioned that a Common ragwort plant can produce up to 200.000 seeds and that these can be dispersed many miles away from the mother plant. In order to improve our understanding of the factors that contribute to the dispersal of Common ragwort seeds, we studied the literature to answer the following questions:
•          How are the seeds of Common ragwort dispersed?
•          Is wind dispersal as important as is assumed?
•          Does every seed become a adult plant?

Seed dispersal

Common ragwort generally produces between several hundreds and about 200.000 seeds (7,8,9). Large plants usually contain more flowers than small plants and therefore generate more seeds. The flowers of Common ragwort are placed in flower heads. At first sight, these flower heads resemble individual flowers, but if you look more closely, you can see that a flower head is actually composed of dozens of flowers that each produce a single one-seeded fruit (7). Just like the fruits of Dandelion, ragwort fruits carry pappus. This is clearly visible in the pictures. The pappus is composed of about 60 hair-like structures of each about 6 mm long (8.9). These structures carry many short appendages. The function of the pappus is to aid in the dispersal of the seeds to places where they can germinate and grow into adult plants. This can happen in many different ways. The pappus, for instance, makes it possible that the seeds can easily be dispersed by wind. In addition to this, the appendages of the pappus enable the seeds to stick to the fur or feathers of animals (8.9). Also humans can act as dispersal agents. Seeds can be dispersed by the turbulance generated by passing trains and cars and sand used for building infrastructure and housing is usually brought in from other locations and may contain ragwort seeds.

The efficiency of wind dispersale

Scientific research into the efficiency of wind dispersal of ragwort seeds shows that most seeds land close to the mother plant. Only 0.5% of all seeds that a plant produces travel more than 25 meters (8.9). Although each of the up to 200.000 seeds has a chance to be dispersed far away from the mother plant, only a few seeds get that far in reality. The majority doesn’t get much further than several meters away (8,9). It is, however, important to realize that it only takes a single seed to be dispersed over a large distance to enable this species to reach a new area.

The faith of a seed

We know that Common ragwort can produce many seeds and that a small fraction of these seeds can travel large distances . But if all of these seeds would grow into adult plants, the Netherlands would already be covered with a dense layer of ragwort (7). In controlled conditions, such as in a greenhouse with an optimal humidity and temperature, about 80% of the seeds will germinate (7). In nature, however, circumstances are never as perfect as in a greenhouse (7). Chances are small that a seed will land on a spot that has conditions favorable for germination. It may, for instance, be too dry or too wet, or there may not be enough light (7). Even if a seed germinates, it is still the question whether it will live long enough to develop into an adult plant (7). Seedlings have to compete with other plants for light and nutrients and only the strongest plants will survive. A part of the seeds will get a second chance if they fail to germinate in the first growth season (10). If these seeds are covered by soil, they may become dormant. The seeds in this seedbank can retain the potential to germinate for up to ten years and perhaps even for decades (10). When soil disturbance causes these seeds to surface, this will induce germination (10).

Conclusion

Common ragwort plants can produce many seeds, but most seeds remain close to the mother plant. Even if a seed is dispersed into a new area, there’s still only a small chance that it will grow into an adult plant. The statement that a plant can produce 200.000 seeds that can each travel many miles and give Common ragwort the potential to spread at an extremely fast rate, should therefore be nuanced.

References:

(1)   Ragwort facts info
(2)   Socialistiche Partij Drenthe
(3)   Kruiskruid.nl
(4)   Kruiskruid forum ( this forum is closed)
(5)   Algemeen Dagblad
(6)   Nieuwsblad Belgie
(7)   Van der Meijden, E. 1974. Zebrarupsen en Jacobskruiskruid. In:Cron Michielsen, N. (ed.),Meijendel. Duin-water-leven: 95-108. W. van Hoeve BV, Den Haag/Baarn
(8)   Harper, J. L. & W. A. Wood. 1957. Senecio jacobaea L. The Journal of Ecology 45: 617-637
(9)   Poole, A. L.  & D. Cairns. 1940. Botanical aspects of Ragwort (Senecio jacobaea L.) control.     Department of Scientific and Industrial Research Bulletin 82: 2-61.
(10)  Weeda E.J., R. Westra, CH. Westra & T. Westra. 1987. De Nederlandse oecologische flora, wilde       planten en hun relaties