Ragwort seeds are wind blown. Many people therefore assume that the seeds readily colonise new sites. This is not so. Research has shown that it is not a common event. Certainly given the comparatively low toxicity of ragwort,(See How toxic is Ragwort) there is no need to worry about seeds colonising pastures giving rise to problems before they are spotted.
The important subject of the number of seeds per plant, which is often highly exaggerated in claims on internet pages and in the press, is dealt with in a different article ragwort seed productionOne of the problems is that if ragwort is pulled up out of the ground small pieces of roots remain. These roots can then regrow into new plants. In fact they look just like seedlings. People frequently pull ragwort and then blame seeds blowing from elsewhere even if they are too far away to have blown there. (1)
Several studies have been carried out into ragwort seed dispersal.
An important study was carried out in New Zealand(2), where because it is not a native plant it has few natural controls. The study calculated the behaviour of around fifty seven million seeds. The following are a series of direct quotations from this comprehensive and detailed study. Explanatory comments, derived from the rest of the paper have been inserted in Square brackets.
"The amount of seed recovered from the trays in the central area [Essentially at the base of the plants] indicated that approximately 33,900,000 seeds were deposited at the base of the plants. This is approximately 60 per cent. of the total seed produced. The ground in many places within the central enclosure was thickly covered with seed. A considerable quantity of seed was retained within the seed-heads. This was estimated to be about 22,800,000 seeds. ""Thus, 56,700,000 seeds were accounted for from the central area out of the estimated 57,000,000 produced. A close estimation of the seed deposited within a circle having a radius of 40 yards from the central area has been made. This figure reveals that just over 218,000 seeds fell within the area of the circle. As the number of seeds in the trays has dropped to almost zero at 40 yards, it is doubtful if more than a few seeds were carried very much farther "
" From the figures obtained in this experiment it is obvious that only a very small amount of seed is wind borne in this area. The amount carried any distance from the infestation must also be very small. The drop in the number of the seeds from the 10 yards trays to the 40 yards trays precludes the possibility of a great deal of seed being deposited beyond this distance."
Another study was carried out by scientists at Oregon State University and published by The Ecological Society of America in their respected scientific journal Ecology. It showed that, when tested in a variety of conditions, 31% of the seeds traveled only 1 metre, 89% of them 5 metres or less and none were collected more than 14 metres from the source. The study involved studying the dispersal of over fifty three thousand individual seeds.(3)
There can be no doubt that ragwort can disperse over distance but the claims made about the number of seeds that can be set. and their dispersal power must be set in context. Whilst ragwort seeds obviously can occasionally disperse over distances the overwhelming majority of them do not do so. It is clear therefore that Ragwort growing on waste ground does not pose a significant risk to livestock. The studies show that it cannot colonise fast enough to build a population capable of causing harm without becoming obvious. In addition there are other factors such as the ability of the seeds to germinate.
Another study exists but it does not give as detailed information and wasn't specifically designed to study ragwort alone(4). It appears to have used a large source of seed that wasn't a point source so is more difficult to establist the figures with accuracy. They detected seeds at 72.5 metres but the majority were deposited within 12 metres.
It is important to remember when talking about seed dispersal that the conditions for the seed when it lands are an important consideration. It is also important to remember that a plant will on average only produce one offspring.
In the context of the information often presented in the UK media Ragwort is often presented as a plant that is invasive and takes over land. This is rarely the case the ecological data clearly show that it rarely does this and where it does so it is the result of over grazing not the plants colonising ability which is actually not that great in the UK. Indeed the term "Invasive weed" which is often applied to ragwort is more properly applied to plants that are foreign to an ecology. Ragwort is, as is documented elsewhere on this website, a native plant that is part of the natural native ecology of the British Isles.
The simple and fundamental truth of the matter is that ragwort seeds while capable of dispersal over great distances only do so under very rare and unusual circumstances. Consequently ragwort growing on roadside verges where it is of great ecological value to biodiversity does not usually need to be controlled.
References
(1) Harper JL, Wood WA (1957) Senecio jacobaea L. J Ecol 45:617-637
(2) Poole, A. and D. Cairns. 1940. Botanical aspects of ragwort (Senecio jacobaea L.) control. Bulletin of the New Zealand Department of Science and Industrial Research 82: 1-66.
(3) McEvoy P.B. and Cox C. S. Wind Dispersal Distances in Dimorphic Achenes of Ragwort, Senecio jacobaea
Ecology 68(6) 1987 pp 206-2015
(4)Jones N E & Naylor R E L (1992).Significance of the seed rain from set-aside. BCPC Monograph No. 50 Set-Aside, 91-96.