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Injurious weeds

Taken from the Weeds Act 1957

Injurious weeds can be controlled using a number of chemical and cultural means. Care should be taken to choose the most appropriate method for each site. This applies particularly to sites of special conservation interest where control of the injurious weeds may risk damaging rare or valuable flora and fauna. In these situations expert advice should be sought before any action is taken.

Injurious weed control using herbicides

Instructions for use including operator and environmental protection, the crops or plants on which the product may be used, maximum dose, harvest interval and other details are shown on the product label. Each time a product is used you must READ THE LABEL AND FOLLOW THE INSTRUCTIONS.

Some products are only available to operators who hold a certificate of competence.

Non selective herbicide treatment

Control of injurious weeds can be undertaken using a non-specific herbicide such as glyphosate either as an overall spray or using a height selective applicator or spot treatment.

Selective herbicide treatment

Injurious weeds can be controlled using selective herbicides. Although most products are generally used as an overall spray, some can also be applied through a selective height applicator or as a spot treatment to improve their selectivity.

The following shows the most favoured active ingredients for the control of each injurious weed specified under the Weeds Act 1957. These active ingredients may be available alone or in mixtures with other chemicals and qualified advice should be obtained to determine the most appropriate product especially when mixed populations of weeds occur.

1. Spear thistle (Cirsium vulgare)Spear Thistle

Spear thistle occurs widely on lowland and upland grassland and waste places. The weed competes effectively with crops for water, light and nutrients. It is biennial and only spreads by seed. Mature plants are normally 30-50 cm tall, with flowers from July through to late autumn. Large numbers of seeds are produced which can be blown by wind across farm and field boundaries.


The plants can be cut each year before mid-July to prevent shedding of viable seed. It is also possible to remove them by digging. Long-term control is possible from herbicide treatment; spear thistle is susceptible to clopyralid and moderately susceptible to MCPA herbicides. Where clover is an important constituent of the sward, a mixture of MCPA and MCPB herbicides is more appropriate.

2. Creeping or field thistle (Cirsium arvense)Creeping Thistle

Creeping thistle can quickly dominate vegetation in grassland or waste ground. The weed forms dense patches which suppress crop plants.

Mature plants extend 30-100cm in height, with flowers from July into late autumn each year. The plants produce only a few viable seeds which can be blown by wind. However invasion is more often by spread of the plants’ underground root systems.


Cultivation is not an effective means of control as the number of root pieces which can throw up new shoots is increased. Control on arable land therefore is usually by use of a range of herbicides depending on the field crop grown.

On grassland, cutting at flower stem extension but before opening of the flower buds will prevent seed spread for a particular season.

Repeated cutting at the same growth stage over several years may "wear down" an infestation.

MCPA herbicide applied during the early bud stage will kill the aerial parts of the plant, but repeat treatments the following year may be necessary for complete control. One application of the herbicide clopyralid* is normally sufficient to achieve an acceptable level of control.

3. Broad-leaved dock (Rumex obtusifolius) and
4. Curled dock (Rumex crispus)

Broad-leaved dock thrives in high nitrogen environments, open swards and where there is heavy treading by stock. Curled dock occurs more commonly on arable and waste land.

Both species produce many seeds which can remain viable in soil for decades. Buds on pieces of tap-root broken by soil disturbance or treading will produce new plants. The two species are similar in appearance but leaf shape differs, as reflected in their names. Hybrids are common between the species and this can hinder identification. Flowering for both species is from late June until early autumn with inflorescences reaching over 100cm in height.

ControlBroad leaved dock

Broad-leave dock

MCPB for grass clover reseeds.
Mecoprop or MCPA for grass reseeds without clover.

Asulam for grassland with clover.
Fluroxypyr, 2,4-D, Triclopyr, or Thifensulfuron for grassland without clover.

Curled dockCurled leaved dock

MCPB for grass clover reseeds

Asulam for grassland with clover.
Fluroxypyr, 2,4-D, MCPA, Mecoprop or Triclopyr for grassland without clover.
Mecoprop or MCPA for grass reseeds without clover.

5. Common ragwort (Senecio jacobaea)

This is the only ragwort species specified in the Weeds Act 1957; other species of Senecio are not so widespread as common ragwort. Flowering is from late June onwards to early autumn when the characteristic yellow inflorescences usually extend between 30 to 100cm in height.Ragwort

The weed occurs in neglected grass fields, on uncropped ground and sand dunes. It prefers light soils of low fertility, particularly in over or under-grazed pasture. Common ragwort is biennial when undisturbed but can develop perennial characteristics following cutting or treading.

Poisonous to livestock

Cattle and horses are particularly susceptible to poisoning by common ragwort but sheep are also susceptible. Palatability of the weed increases when plants are conserved in hay or silage or treated with herbicide. An added problem is that livestock cannot easily reject fragments of ragwort in conserved herbage and its poisonous alkaloids are unaffected by the conservation process.


Although short-term action can be undertaken to clear existing plants, re-infestation will be rapid unless overall husbandry is improved, particularly for uncropped ground and grassland.


Cutting and stem removal at the early flowering stage reduces seed production but does not destroy the plant. Cut plants left lying in the field are a serious risk to grazing animals and may still set seed. These should be removed and burned.

Pulling and digging:

Pulling or digging can also prevent seed spread but may not give long-term control. Plants should be removed and burned.


No single herbicide treatment will completely eliminate a ragwort infestation due to successive germinations of the weed. Treatment with selective herbicides can be made to the plant rosettes usually late spring and in the autumn before frost damages the foliage. The most effective material for overall spraying is 2,4-D* but this will damage clover and a number of other plant species.

6. Wild oats (Avena fatua) and (Avena Ludoviciana Duriev)

Wild oat infestations are now a serious problem in many fields on the Isle of Man.

The wild oat germinates in both autumn and spring, although the majority of seeds germinate in the spring. The seeds can remain viable in the soil for long periods, up to 10 years in undisturbed land and 4 to 5 years in cultivated soils. Severe infestations are normally associated with continuous cereal growing but can be serious in rotation with peas and beans. In winter wheat yields can be reduced by as much as 1tonne/Ha and in spring cereals 0.5 - 0.6 tonne/Ha. Wild oat is spread in many ways but contaminated seed corn, straw and combines, together with pigeons, rooks and game birds constitute the major agencies.


Good management will help reduce infestations, sow certified seed, if using home-saved seed have it tested, in fields with severe infestations take out of cereals for five or more years, inspect all cereal fields every two weeks after crops have headed, thoroughly rogue any wild oat plants, place in a bag and burn, clean out combine after harvesting an infested field to prevent spread to other fields and do not spread bedding straw from infected crops on land for cropping.

If a field is known to be badly infested use a recommended herbicide. There are many available and choice should depend on crop, severity of infestation and timing of applications.

7.Giant hogweed (Hereacleum mantegazzianum)

Giant hogweed can be recognised by its:Giant hogweed

Height - 3 to 4m (9 to 12 feet) when fully grown - some may grow even taller.

Stem - Hollow with dark reddish purple spots, and strong enough to support the plant’s huge flowering head.

Leaves - About 1m (3 feet) wide, rough, bristly.

Flowers - Many small white flowers making up a large flowerhead, 50cm (18 inches) across.

Giant hogweed was introduced to the UK by the Victorians as a magnificent garden plant. However the plant has now escaped from its original sites and has spread rapidly, especially alongside rivers and burns.

Giant hogweed prefers growing beside water although it can also be found on roadsides, railway embankments, waste land and even gardens.

Giant hogweed produces a toxic sap which is released when the hairs on the stem and leaves are touched. When this sap gets onto skin and the skin is then exposed to sunlight, painful, watery blisters are formed. These can take a long time to heal and may need hospital treatment.

The skin may remain sensitive to light for several years after the initial contact.

Children must be warned to keep away from giant hogweed - not to use the stems as blowpipes or telescopes. Blisters around the mouth and eyes can be very painful and dangerous.

Once established giant hogweed can spread very rapidly and is difficult to control. Along river banks and roadsides, giant hogweed can take over and eliminate wildflowers and grasses. Favourite walks and play areas can be invaded and may become inaccessible.

Giant hogweed is a perennial. It will grow and produce leaves for 3 or 4 years before flowering. Once the plant has flowered it sheds around 5,000 seeds, spreading further afield before it dies.


Giant hogweed can be controlled but it requires time, effort and money. There is no single easy method of control but below are some of the options available.

Before starting to attack a single colony of plants on your own land some thought should be given to where the plants have come from, whether they might re-invade from nearby colonies, your responsibilities as a landowner and the risk that it poses to you or others.

  • Identify the source of the seed. Plants upstream or nearby produce seeds to re-invade a cleared site.
  • If several neighbours are involved, control should be a co-operative effort.
  • Control is only effective if you are committed to it. Efforts must be maintained for 3 or 4 years to be properly effective. There will still be seed in the soil and seedlings will appear for several years after the main plant has been killed.
  • The site should be inspected for several years to make sure new plants have not started to appear.
  • Whatever you do, do not touch it without wearing gloves!

Depending on the site and the number of plants, one of the following methods will be suitable for your site:

Remember, April and May are the best months to control giant hogweed - so start to plan early.


A useful and effective method if there are only a few plants to deal with.

Use a spade in the early spring and cut the plant below soil level while it is still small. (You may need to cut it again during the growing season).

Wear rubber gloves and boots and avoid contact with the plants.


If it is later in the year and you decide to cut down the plants remember that the root will still be active and will re-grow next year. Cutting down is only a temporary solution but it will remove the hazard for the time being.

Try to cut down the plant before the seed head develops. If this is not possible, collect all the seed heads in a bag and burn. Do not bury or put out in normal refuse collection - this will just spread the seeds to a new site.

Cutting the plant, and allowing some re-growth to occur then spraying the re-growth with herbicide will weaken the plant considerably. It may still require treatment the following year.

Try not to work near the plants in full sunlight - choose a dull day. The plant seems to produce more sap on sunny days increasing the danger of contamination of the skin.


The most effective herbicide for the control of giant hogweed is glyphosate

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