Identify a Weed - Identifying Weeds


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Ragwort - Weed KillerRagwort:

Species
Senecio jacobaea ssp. jacobaea (the typical plant), Senecio jacobaea ssp. dunensis (an atypical subspecies, which produces no ray florets)

Family
Asteraceae

Common Names
Ragwort, Benweed, Staggerwort, St James Wort. Numerous other common names recorded.

Introductory History
Common Ragwort, Senecio jacobaea, is out of place in the rogues gallery of exotic invasive plants. It is entirely native here, and whilst it is an injurious weed, and an opportunistic colonizer, it is not, in the true sense, an invasive plant in these islands.

Ragwort has devoted friends and bitter enemies; about as polarized as the lobbies advocating culling of badgers, to control bovine tuberculosis, and the badger groups who deny that badgers have any more than the most tenuous connection with the disease.

As a true native wild flower, Ragwort has had plenty of time to gather folk names, about fifty of them, many emphasizing that its unpopularity is not new. Try these, and you will get the idea! "Bowlocks, Devildrums, Dog-stalk, Stinking Nanny, Stinking-Davies, Mare-Fart." It also once held occult associations. Fairies, especially those in the Scottish highlands and islands and in Ireland were said to fly on Ragwort sticks. The island fairies, indeed, solved the problem of crossing the straits between the islands by this means! For fairies, Ragwort was also used for shelter in case of rain. As peoples concerns moved away from fairies and on to witches in the 16th and 17th centuries, they too were reported to use this singular method of locomotion. In contrast to all this unholiness, another folk name is James' weed, for St James, whose day is 25th July, when Ragwort is in full flower. The name St James' weed persists in many parts of Europe.

Ragwort has also gathered a large community of insect species; which can be divided into those 30 which wholly depend on it, those 52 which use it as a substantial part of their food, and those 117 for whom it is a major nectar source. Exotic invasive species, however attractive, should be eradicated. Ragwort, which is native and an important part of our fragile tapestry of flora and fauna, should be controlled wherever it poses a threat to livestock or where it appears to be becoming dominant, but eradication is not a desirable option.

Other Ragworts commonly found in Britain are Hoary Ragwort, Senecio erucifolius, which has greyish down and more deeply divided leaves; the bushier Marsh Ragwort, Senecio aquaticus ,which favours damp meadows, and the Oxford Ragwort, Senecio squalidus.

Oxford Ragwort originates from the volcanic cinder slopes of Mount Vesuvius and Mount Etna, and was grown in the Oxford Botanic gardens in the 17th century. It was first noted outside on college walls in 1794, and has since spread to most parts of the UK during the 20th century, mainly spreading along railway lines to urban wasteland.

There are many other common, summer-flowering, yellow wildflowers growing alongside Ragwort, and inexperienced horse owners particularly tend to panic at the sight of any yellow daisy-type flower. I have been asked to examine Hawksbeards, Sow thistles, Bristly Oxtongue and Tansy in this regard. It is very important to correctly identify the plant before attempting control, however diligent and laudable the motivation.

The Ragworts contain toxic compounds known as Pyrrolizidine alkaloids, of which 16 have been identified.* Some of these are known to inhibit cell division, which quality is being researched in the development of anti-cancer drugs.

Ragwort is undoubtedly toxic to horses and cattle, especially so to horses. Sheep seem to show a remarkable tolerance to prolonged ingestion, and may even gain benefit from a vermifugal quality in the plant.

In cattle and horses, the alkaloids act by cumulative damage to liver DNA, leading to death of cells. It is the damage which is cumulative, not that the alkaloids themselves accumulate in the liver.

Repeated ingestion causes repeated asymptomatic damage until a total threshold is passed and symptoms show. At this stage, it is usually too late. The lethal dose may be ingested all at once, over a few months, or over 10 years! It is this irreversible cumulative effect that makes Ragwort so dangerous. The lethal dose for horses is stated to be between 3-7% of body weight, though some researchers claim 20%. Such discrepancies are probably because of variations in gut flora, which can help break down the toxins, from case to case, the amount of water in the ragwort, or its stage of growth. It is not difficult to see how a 500kg horse could easily ingest the equivalent of 15kg of weed over a season or two, especially when dried and palatable in hay. Arguments over how toxic Ragwort is to horses, and the number killed each year in the UK, also seem to me to express a polarization of opinion, rather than interest in scientific truth or simple commonsense.

Annual deaths are reported from 10 to 6500 (!) depending on which evidence is employed. Such variation in opinion simply is clearly nonsense. It is also to some extent irrelevant. Death from liver destruction is an appalling end for any animal and correct identification, management and eradication of Ragwort must be practised, by horse keepers and stockmen, to avoid it. Proper paddock management is also important, as overgrazing and lack of good pasture care gives rise to the thin swards that the weed can easily colonize.

Current legislation probably has the situation about right. Ragwort is cited under the 1959 Weeds Act, together with Creeping Thistle, Spear Thistle, Curled Dock and Broad-Leaved Dock. This Act does not make it illegal for these plants to be allowed to grow, but does empower the Secretary of State to serve notice requiring a landowner to stop these weeds spreading on or to agricultural land. He is further empowered to inspect the land to confirm that the required work has been done, and to employ contractors to do the job if not, recovering costs through the courts. The Ragwort Control Act of 2003/4 amends the Weeds Act, by empowering the Secretary of State to oversee the development of a code of practice for the management and control of Ragwort by landowners. The enforcement powers remain the same as in the Weeds Act, with the addition that breaches of the code of practice, or otherwise, are admissible evidence in enforcement proceedings.

The code of practice states:
"common ragwort and other ragwort species are native to the British Isles and are therefore an inherent part of our flora and fauna, along with the invertebrate and other wildlife they support. The Code does not propose the eradication of common ragwort but promotes a strategic approach to control the spread of common ragwort where it poses a threat to the health and welfare of grazing animals and the production of feed or forage."

Ragwort is native to the UK and much of Europe, from Scandinavia to the Mediterranean. It has been long introduced into the more northern regions of the USA, and has become invasive in some parts. Problems are also experienced on Australian and New Zealand rangelands, and it is listed as a noxious weed in several Australian states. The Australian legislation enforces eradication, which is what many people assume the Ragwort Act does, but does not.

Many strategies including pulling, cutting, digging, grazing by sheep, and applying traditional and novel herbicides have been employed against Ragwort, as have biological controls by herbivorous insects.

* acetylerucifoline, (Z)-erucifoline, 21-hydrxyintegerrimme, integerrimine, jacoline, jaconine, jacobins, jacozine, riddelline, retrorsine, seneciveernine, senecionine, senecyphlline, soartioidine, usaramine and (E)-erucifoline.

Description
Ragwort is a hardy, herbaceous plant, growing to up to l.5metres in height. The leaves and stems have a reeking, chemical odour, though the flowers have a honeyed fragrance. The plants are usually biennial, but sometimes taking 3 years for the rosette to develop to sufficient size to produce a flowering stalk. It also can become a short lived-perennial when grazed or mown. Stems, erect, branching with height, sub-glabrous to floccose. Leaves, on the lower part and bases of the stems, lyrate-pinnatifid, petiolate, usually withering by the time the flowers open, middle and upper stem leaves once to twice pinnatifid, semi-amplexicaul, with undersides sparsely floccose. Flowers, in capitula, to 2cm, usually with yellow ray florets; arranged in dense corymbs. Phyllaries, sub-glabrous, ovate and acute. Seeds from the inner floral disc bearing pappus for wind-dispersal, marginal seeds without pappus, heavier and with a thicker seed-coat.

Biology
Seeds germinate mainly in the autumn in which they are shed, and the plant grows as a rosette of leaves from an eventually spreading rootstock, which may develop adventitious offshoots around the parent plant. The rosette builds sufficient size and energy to produce the flowering stalk in its second growing season or sometimes later. Winter chill is required to initiate the development of the flowering stage. Damage, such as pulling that does not remove the whole rootstock, encourages the plant to produce adventitious shoots and to perennat, eventually producing multiple flowering stalks. Seeds can then germinate in the space left by the pulled main crown.

The flowering and seed- producing season can be extremely prolonged, from mid June until November. A large plant can produce over 2000 flowerheads in a lifetime, each producing 70 or more seeds. It is recorded that some plants produce as much as 200,000 seeds. Seed produced is of two types. The central seeds with their parachute of pappus can be distributed by wind up to 70 metres or so from the parent plant. The heavier seeds from the edge of the disc are designed to remain in situ until shaken free, and can often germinate in the gap in the sward produced by the death of the parent plant. Seeds that do not germinate in the autumn of their shedding can persist in the top 4cm of soil for 4-6years, but seeds buried to a greater depth than this, can survive for at least 16 years.

Ragwort is thought to suppress neighbouring vegetation by secreting allelopathic chemicals. It also shades out vegetation under the ground-hugging rosettes, so perpetuating the bare ground conditions following death, which subsequent seedlings can exploit.

The symptoms and veterinary aspects of Ragwort poisoning are outside the scope of this article, but suffice to say that they are extremely severe and distressing for the affected animal. Cattle and horses avoid fresh Ragwort unless little other forage is available. Once eaten, however, there is some evidence that a kind of addiction can develop, which will lead to disastrous consequences. Palatability increases dramatically as the plant dries, but toxicity is not reduced. For this reason most cases of poisoning seem to arise from the ingestion of contaminated hay or silage. Topped pasture containing the weed is also a danger, as the dried stems can be eaten. Plants killed by herbicides and subsequently drying are also eaten.

Scope
Ragwort does not tolerate cultivation, so is not a weed of tilled arable land. It also does not prosper on acid, peaty soils. These apart, it is, however, an aggressive colonizer of any bare soil on which seeds may alight. Thus it is much found on set-aside agricultural land, where there is no established sward following cultivation; on pasture land especially if over grazed, poached by the feet of livestock, or otherwise poorly maintained. Neglected and under grazed pastureland conversely loses sward density and admits Ragwort. It grows widely on wasteland, and has spread along the motorway network where embankment plantings and establishing swards give it opportunity.

Control Methods
Despite the well-known toxic effects on cattle and horses, sheep have a very considerable tolerance to Ragwort and are willing to eat it in the fresh state. As toxic levels in sheep seem to be at a consumption of about 200-300% of bodyweight, winter to spring grazing of lightly infested pasture can greatly weaken the rosettes and much reduce seeding. Grazing later in the season can encourage the development of multiple crowns and encourage perennation. Pulling or the use of purpose-designed forks is often advocated. My experience is that pulling is virtually useless, on heavy soils in the flowering season, even with single crowned plants, as some roots always remain in the soil. Only 1cm of root can develop into a new plant. Pulling multiple crowned plants, though removing this year's seeding potential, always leaves subsidiary crowns intact, and provides a seed bed for the following year's flowering. Use of special forks on single rosettes in winter and spring can be effective, but is unlikely to be permanently successful where plants have multiple crowns or when the soil is dry.

Herbivorous insects native to the UK have considerable control potential, and are being introduced in the USA and Australasia, where invasive problems are being experienced.

These are the Cinnabar moth, Tyria jacobaeae, and the Ragwort Flea-beetle, Longtiarsus jacobaeae. The moth is a strikingly beautiful insect in carmine and black, with orange and black striped caterpillars, looking like bizarre liquorice allsorts. These grubs eat first the flower buds, then the leaves, and finally strip the green cambium from the stems. Though much reducing seed production, plant densities are rarely greatly reduced; plants responding by entering the perennial multi crowned mode. The tiny flea beetle, whose larvae tunnel the roots and crowns and adults eat the leaves, attack much earlier in the season, and can destroy plants, resulting in reduced densities. The long season of attack, combining use of both these agents, has resulted in reports of populations declining by 99.5% at some sites.

Chemical controls are also much employed. A novel herbicide, based on 22.9% of Citronella oil (derived from tropical grasses of the genus Cymbopogon) in water, has developed a good reputation for efficacy, whether applied to flower heads to prevent seeding, or to kill crowns during the winter and spring.

A research project was undertaken at the Centre for Evidence-based Conservation at the University of Birmingham during 2004, which was to examine and report on the research undertaken to date to determine which of the available conventional herbicides produced population reductions when applied to common ragwort species.

Unsprayed plots were used as controls. In the broadest terms, the two herbicides producing the highest levels of mortality and population density reduction, over a one year period, were 2,4-D amine and M.C.P.A.. In the case of Senecio aquaticus, the Marsh Ragwort, Asulam was shown to be most effective (Marsh Ragwort is not an aquatic or riparian species, but mainly grows in damp meadows; therefore the restrictions of use for Asulam near water should not apply). Further research on aspects such as application method (spot-spraying, rope-wick or weed-wipe) needs to be undertaken. As with the Citronella based products, conventional herbicides can be applied to flowering plants to prevent seeding and to rosettes in winter to spring, when high levels of success can be expected. back to top

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