Can ragwort really toxify tree-planting projects?

CAN a native plant bearing pretty yellow flowers but also containing toxic chemicals in its cells and tissues poison a well-intentioned tree-planting and reforestation project? That’s a claim made against ragwort by some neighbours of the Heart of England Forest (HOEF) in Warwickshire who claim the weed is out of control on HOEF land, according to a report in the Stratford-upon-Avon Herald last November.

Tree planting is clearly at the heart of this project. The HOEF website states: “We’re planting tomorrow’s great native woodland, one tree at a time, a new broadleaf forest across the heart of the country that’s for everyone to enjoy.” But, according to media reports, not everyone is enjoying the experience. In fact, a number of HOEF’s neighbours told the Stratford-upon-Avon Herald how ragwort was invading their land via wind-dispersed and disseminated seed. HOEF defended the situation with regard to ragwort and claimed it had taken appropriate action. However, there is an article on the HOEF website which extols ragwort’s virtues and paints it as an unfairly maligned plant.

Solid-wall tree shelters offer little protection against weed competition effects of ragwort and other tall and vigorously growing plants.

The Heart of England Forest covers around 7,000 acres of planted woodland, principally in south Warwickshire but with some in Worcestershire and stretching from the Forest of Arden south to the edge of the Vale of Evesham. It is managed by a charitable trust carrying the same name, which is dedicated to reversing a history of woodland decline in England.

RAGWORT IS A POISONOUS PLANT

Most people are familiar with ragwort, even if they don’t know the plant by name. Indeed, you can’t really miss it, especially in late summer, when large areas of Britain’s more open landscape are covered with swathes of the plant’s bright-yellow daisy-like flowers. This is despite long-standing legislation covering ragwort which is classed as an injurious weed under the Weeds Act 1959, and for good reasons.

Like 3 per cent of the world’s flowering plants, ragwort contains potentially poisonous chemicals called pyrrolizidine alkaloids (PAs). These naturally occurring chemicals are essentially harmless while still inside the living plant, but once ingested and acted on by enzymes in the animal gut are chemically converted into toxic secondary metabolites.

Ragwort is visited by a wealth of insects including dipterans – flies (centre) and coleopterans – beetles (lower left).

Most vertebrate animals are susceptible to ragwort poisoning to a greater or lesser extent. Horses and other equines are at greatest risk both in the amounts required to cause veterinary problems and the severity of symptoms and damage caused. Equines and bovines; porcines and poultry; sheep, goats and deer are at decreasing risk from ragwort poisoning in that order. Testimony to the effect of ingested ragwort on livestock is the plant’s alternative common name of ‘staggerwort’ which tells you a lot about what this weed can do to animal health and well-being.

One saving grace is the unpleasant smell emitted by growing ragwort and the reason for an alternative name of ‘stinking willie’. The unpleasant aroma and taste of fresh, green ragwort will usually deter its ingestion by livestock. The biggest danger comes when a grass sward containing ragwort is cut and left to dry. The poisonous chemicals remain intact, but the dried foliage loses its unpleasant smell and taste. It is when livestock are put out to feed on cut-and-dried vegetation which includes dried ragwort or are fed hay containing dried ragwort, toxicity problems will occur.

Ragwort and rosebay willowherb compete with planted trees.

Horses are the most sensitive and suffer a range of affects including fatal liver damage. Hundreds of horses are still thought to die from ragwort poisoning every year, despite the 1959 Weeds Act and the complementary Ragwort Control Act 2003, although the latter makes no provision for control orders, despite indications from its name.

RAGWORT AND ITS CONTROL

Under the 1959 Weeds Act, a landowner or occupier may be ordered to control the spread of ragwort, while the 2003 Act simply allows for creation of a code of practice for ragwort control. In short, there is no compulsion on landowners to remove ragwort from their land, only to prevent its spread, and here is the rub.

Mechanisms of seed liberation and wind dispersal in ragwort, similar to that of the taxonomically related dandelion and other members of the plant family Asteraceae (Compositae), provide highly effective wind-led spread which is difficult to manage. Once the seed is set and the pappuses (parachutes) are airborne there is essentially no way of stopping ragwort spread. A single ragwort plant can produce up to 200,000 seeds which are dispersed by wind up to 70 metres from the mother plant.

Yellow-and-black banded larvae of the cinnabar moth for which ragwort is the food plant are well camouflaged when they feed on the flower heads.

However, none of this totally explains why crucially important tree-planting and woodland-creation programmes may become infested with ragwort and thereby get caught up in livestock poisoning problems caused by this highly invasive and injurious weed.

The underlying reason is that tree planting sites with bare soil and/or thin turf offer ideal germination sites for ragwort seed. Owners, managers or guardians of tree-planting sites cannot prevent arrival of the windborne pappus-assisted seed and subsequent germination on site. However, they can and must for their own and their neighbours’ benefit stop subsequent ragwort plant establishment and growth, preferably in spring when plants are in the so-called ‘rosette’ stage, and crucially before they start to flower in summer.

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The rapidity with which ragwort exploits bare soil can be seen on some older golf courses which are sited close to railways for historical reasons of access. Ragwort cannot colonise closely knit and tight sports turf but once divot damage occurs on fairways and tees, ragwort soon appears, having spread via seed from railway embankments.

Handheld pesticide applicators custom-designed to treat individual rosette weed growths with measured amounts of approved herbicides have been developed over the years. However, it is important to be clued up on weed identification so you can knock out the ragwort while leaving other plant species alone, and thereby apply an absolute minimum of herbicide to the ground.

Cinnabar moth larva feeding on a ragwort leaf amongst the grass (image: Dr Roderick Robinson).

Control of ragwort will not only placate concerned neighbours, especially those with horses, but also protect newly planted trees from what is a highly aggressive weed. At maturity ragwort is a tall erect plant reaching 90 cm and bearing large flat-topped clusters of yellow daisy-like flowers from July to October. It doesn’t take much to work out the damage that weed plants of this size and calibre can do to newly planted trees through competition for light, space, water and nutrients. The definition of a weed is a green plant in the wrong place at the wrong time. Ragwort on a railway embankment may well be OK but not when competing full-throttle with recently planted oak or pine trees. There is a time and place for ragwort, but newly established woodland is clearly not one.

I am continually amazed by the number of new tree plantings that I visit which are riddled with tall weeds like ragwort, rosebay willowherb and bracken. When I get home and look at my pictures, I see weeds as tall as the planted trees and often competing with those who planted them. Good for biodiversity, perhaps, but not so good for secure tree establishment and good growth.

A ragwort-free tree planting as it should be.

Anyone who doubts the impact of weed growth, including ragwort, should read the Forestry Commission Handbook 2 – Trees and Weeds: Weed Control for Successful Tree Establishment, authored in 1987 by R.J. Davies. The section covering weed competition for light, water and nutrients, and the physical damage caused by weeds to young trees, is as true today as it was 33 years ago. However, readers should avoid the section on the chemical control of weeds. It highlights the application of herbicides which have long since been banned.

Not everyone hates ragwort. Many extol its value for biodiversity which is what the charitable trust in charge of Heart of England Forest has said on its website. Whether this is justified may well depend on which ragwort you are talking about, and whether the one in the spotlight is native or exotic. There are a number of different species of ragwort in the UK, with two in particular being the most frequent and ubiquitous across the wider environment. They are common ragwort (Jacobaea vulgaris, syn. Senecio jacobaea) and Oxford ragwort (Senecio squalidus).

Most people in the UK are familiar with ragwort, even if they don’t know the plant by name. Sometimes you can’t miss it.

Common ragwort is a British native plant species traditionally found growing on dry open sites. Oxford ragwort is an exotic hybrid species found in rocky and mountainous areas within its native Sicilian distribution. Oxford ragwort is actually a naturally occurring hybrid of two Senecio species (Senecio aethnensis and Senecio chrysanthemifolius) which are native to Mount Etna in Sicily. Senecio squalidusis was introduced to the British Isles in 1690 from Sicily for plant-novelty purposes. It was grown at the Oxford Botanic Garden by the Horti Praefectus Jacob Bobart but eventually escaped into the wider environment.

However, Oxford ragwort’s opportunity for spread and progression to weed status came years later with the railway. The track ballast used to create solid and stable beds for railway tracks had many things in common with the volcanic rock on which Senecio squalidus thrives in its native Mediterranean home. This, together with air draughts from moving trains spreading seed along the track, and the presence of nearby germination sites on the ballast and along the railway embankments, meant Senecio squalidus spread from its Oxford base and across the country in no time at all.

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Common ragwort flowers are highly attractive to many different insects. Ragwort may well be poisonous to vertebrate animals, but when in flower is a fantasia of hoverflies, robber flies, solitary bees, bumblebees and beetles which feed on the nectar and pollen. Indeed, it would be unusual if a wealth of native insects had not co-evolved with a truly native plant which is abundant with nectar and pollen. Wildlife trusts claim 200 different species of insect are recorded on ragwort and it is the flowering plant most frequently visited by butterflies in the UK.

Handheld herbicide applicators are ideal for dispatching ragwort at the rosette stage.

However, perhaps the closest and most cunning relationship between common ragwort and an insect involves the cinnabar moth (Tyria jacobaeae). Caterpillars of this highly attractive red and green moth display alternating yellow and black coloured bands around the larval body. This colouration not only acts as a good camouflage when larvae are amongst the dense yellow flower heads but is a traditional warning sign to many predators including birds to leave well alone. And for a very good reason, because the poisonous chemicals accumulate in the larvae as they feed on ragwort leaves and flowers.

As a landowner it is essentially your choice on whether to destroy ragwort growing on your land by spraying, cutting or pulling up the plants. Despite the legislation contained in two acts there appears to be no legal compulsion. Native common ragwort at least is without doubt a hive of insect activity when in flower, as well as being the main food plant for the larvae of the cinnabar moth. Against this is the continued safety and well-being of yours and your neighbours’ livestock, including horses, which can suffer a painful death from eating ragwort. And if you are in the business of planting trees and reforestation, success of your endeavours and investments may well depend on controlling ragwort and other invasive and aggressive weeds.

Ragwort at the vegetative, rosette stage in early spring.

LET THEM PLANT TREES

There is currently a country-wide aim to plant as many trees as possible but from personal observations and reports, whether at estate or local authority level, this is not always followed up by adequate post-planting tree care and maintenance. Any confirmed report of recently planted woodland being overrun by ragwort would appear to support this contention. That an unacceptably high proportion of woodland in the UK is under-managed or not managed at all has been a widely held view for many years.

Everyone agrees that many millions more trees need planting, especially in England. However, suspicions are that the current priority is getting the maximum number of trees in the ground but with considerably less concern about what happens after.

A recent proposal by the Bennett Institute for Public Policy at Cambridge University is of real interest. The proposal is for the UK Government to set up a National Conservation Corps to help meet its tree-planting target and simultaneously tackle the jobs crisis caused by the pandemic.

The current COVID-19 crisis will cause a truly massive shake-out of jobs. This could be the ideal time to answer calls to get more people and especially youngsters into forestry. The government should pick up and run with the proposal put forward by the Bennett Institute and use it as an opportunity to give people a taste for trees and to provide further education and training in forestry. And not just to plant sufficient trees to meet the government’s 2025 target but to secure the industry for at least as long as the rotation of English oak plantings. Always remember that trees invariably outlive those who plant them.

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