Invasive and Harmful Plants

Japanese Knotweed   Giant Hogweed  Himalayan Balsam  Ragwort Hemlock water dropwort Blue green algae  Rhododendron ponticum

Guidance for gardeners

Japanese Knotweed

Japanese Knotweed was introduced in the UK in the 1850's as an ornamental plant and as a fodder plant. It is a very invasive weed though not harmful if consumed. It is generally found along rivers and streams, road verges, in gardens and on waste ground.

Careless disposal of cuttings and soil containing root fragments are thought to be the most common cause of the spreadof Japanese Knotweed.
It is an offence under the Wildlife and Countryside Act to plant or otherwise causethe plant to grow in the wild.

The main problems caused by growth of Japanese Knotweed fall into the following categories:

  • Nature conservation where its aggresive growth smothers all other vegetation.
  • Recreation and landscape where its growth can restrict visibility on roads, cause litter traps, and lead to soil erosion the winter after foliage dies off.
  • Flood defence where growth may impede free flow of water
  • The cost of maintenance of grounds and river banks is greatly increased.
  • Built environment where the strong growing shoots of the plant can push through asphalt damaging road surfaces and penetrating foundations of buildings, land drainage and lifting interlocking blocks.

How to identify Japanese Knotweed:

Stem

close up shot of some Japanese Knotweed

  • Grows to 2-3 metres
  • Green with red/purple speckles
  • Hollow with distinct nodes like bamboo
  • Forms dense clumps
  • Plant dies back in winter but stems persist as upright brown hollow stalks

Leaves

Alternate, green heart shaped leaves 120mm long

Flowers

Delicate creamy sprays August to October

Hazard to Health

No part of the plant is poisonous

Action to take if Japanese Knotweed is found

On land that you own:

Regular cutting or pulling will, after a number of years, eventually exhaust the rhizome and kill the plant. Cut material must not be removed from site and cannot be composted.
Herbicide control

Dense stands of Japanese Knotweed can be treated with a glyphosate-based herbicide, . If the Japanese Knotweed is sparsely distributed, spot-treat or use 2, 4-D amine, which is specific to broadleaved plants and will not harm the grasses. It may take two or three years to completely kill the entire plant. Most effective control can be achieved if Japanese Knotweed is cut or sprayed in early summer, and then sprayed again in late summer, just before the plant dies back in winter. SEPA must be informed about the use of herbicides and advice should be sought from them on restrictions for herbicide use near water.

On other peoples land:

contact the land owner directly.

Giant Hogweed

How to identify Giant Hogweed

StemsGiant Hogweed (image by GerardM at nl.wikipedia)

  • Start growing March/April reaching up to 5 metres tall
  • Green with dark purple spots or blotches
  • Furrowed or ribbed with sparse spiky hairs
  • Hollow
  • Up to 100mm across

Leaves

  • Emerge dark green in a rosette in the spring
  • Lower leaves up to 2.5metres long in summer
  • Leaf lobes deeply cut and spiked at the ends

Flowers

  • White
  • Appear June to September
  • Up to 500mm across forming one large umbrella like flower head

Hazard to health

Sap is an irritant which can cause severe burns on skin

Action to take if Giant Hogweed is found

Chemical control spraying can only be carried out during the growing season (March to August), when there is green, leafy material present. In order to be effective, spraying must be carried out before the plant fowers and sets seed, otherwise there will be thousands of additional seeds on the ground ready to grow at some point in the future.
Never use a strimmer or chipper on Giant Hogweed material as this can cause sap to become airborne, resulting in injury.

Himalayan Balsam

How to identify Himalayan Balsam

StemsHimalayan Balsam

  • Pinky red colour
  • Up to 3 metres tall
  • Hollow and jointed
  • Sappy and brittle

Leaves

  • Spear shaped
  • Shiny and dark green with a dark red midrib
  • Up to 150mm long
  • Arranged opposite on stems or in whorls of three

Flowers

  • Purplish pink to pale pinks
  • Slipper shaped on long stalks
  • Appear June-October

Hazard to Health

No part of plant is poisonous

Action if Himalayan Balsam is Found

Chemical Control: Can use glyphosate or 2, 4-D amine. Need to be used while plant is actively growing in early spring for best effect.

Cutting/mowing/strimming: Cut at ground level using a scythe, before the flowering stage in June. Do not cut earlier as this promotes greater seed production in any plants that regrow. Cutting should be repeated annually until no more growth occurs.
Pulling: Shallow rooted plants can be pulled up very easily and disposed of by burning or composting, unless seeds are present.

Grazing: Grazing by cattle and sheep is effective from April throughout the growing season. It should be continued until no new growth occurs.

Ragwort

Ragwort is classified as a native species in the new Atlas of British and Irish Flora. It is one of five injurious weeds covered by the provision of the Weeds Act 1959. Ragwort is poisonous to horses, ponies, donkeys and other livestock and causes liver damage that has fatal consequences. Animals die a slow and very painful death from Ragwort poisoning.

Under the Weeds Act, Scottish Ministers may serve a notice on the occupier of land on which injurious weeds are growing, requiring the occupier to take action to prevent the spread of these weeds.

The Control of Ragwort can be achieved in one of three ways:

  • Cutting, which reduces seed production but does not kill the plant. Cut plants left lying are a serious risk to grazing animals and may still set seed. Plants should be removed and burned.
  • Pulling and digging, which will prevent seed spread but may not give long long term control. Plants should be removed and burned.
  • Herbicides, no single treatment with herbicide will completely eliminate ragwort infestation due to continued germination of seed. Treatment of plant rosettes in late spring and early autumn proves the most effective.

How to identify Ragwort

Stemragwort

  • Upright, tough and often tinged with red near base
    Brighter green and branched above the middle

Leaves

Deeply dissected with irregular jagged edged lobes

Flowers

Large flat topped head of flowers June-October

Hazard to Health

Plant poisonous to mammals particularly horses

Action If Found

If on land that you own:

  • Cutting, which reduces seed production but does not kill the plant. Cut plants left lying are a serious risk to grazing animals and may still set seed. Plants should be removed and burned.
    Pulling and digging, which will prevent seed spread but may not give long long term control.
  • Plants should be removed and burned.
  • Herbicides, no single treatment with herbicide will completely eliminate ragwort infestation due to continued germination of seed. Treatment of plant rosettes in late spring and early autumn proves the most effective.
  • Do not compost

Hemlock water dropwort

Hemlock water dropwortIn recent years there have been some incidents of domestic and farmed animals dying as a result of consuming hemlock. Hemlock water dropwort resembles a number of non toxic plants but its roots or tubers are highly toxic to animals and may become exposed following floods and high rainfall periods.

 

Rhododendrom Ponticum

Rhododendron ponticum was first introduced from south-west Spain in 1763. It often grows in sensitive habitats, such as heath, peatlands, broad-leaved woodland, roadside verges and dunes, where dense growth can considerably alter the structure of the habitat.

Rhododendron forms extensive dense thickets which cast a very deep shade, leading to loss of woodland ground flora, including epiphytic bryophytes and lichens, modifying the fauna and preventing tree regeneration. Dispersal can occur either by vegetative layering or seed from established plants.

It is an offence under the Wildlife and Countryside Act 1981 and the Wildlife & Natural Environment (Scotland) Act 2011 to plant or otherwise cause the plant to grow in the wild.

The main problems caused by growth of Rhododendron ponticum are:

  • It is poisonous to both humans and animals. Consumption of honey produced from Rhododendron flowers can result in 'Mad Honey Disease', or 'Honey Intoxication'. It results in relatively short-lived intestinal and cardiac problems and is rarely fatal.
  • Reduction in biodiversity where its aggressive growth smothers all other vegetation and in relation to soil, a toxic layer of humus prevents natural regeneration of native plants
  • Recreation and landscape where its growth can restrict visibility on roads, compromise access, cause litter traps, and lead to soil erosion
  • Restoration of habitats invaded and dominated by Rhododendron ponticum are difficult but not unsurmountable to restore, this is due to the toxicity of the humus layer, this can be overcome by removal of the layer which will allow for artificial re-generation.
  • There is some evidence for allelopathic interactions (the production of adverse effects on other species) between Rhododendron and other plants. This may include the inhibition of germination, or establishment of the seedlings of competing species.
  • In general, people enjoy the abundance of the seasonal colour of Rhododendron ponticum; this tends to overshadow their lack of understanding that important habitats and species are being lost.

How to identify Rhododendron ponticum

Rhododendron ponticumHeight: Grows to up to approximately 3 meters- forms large dense rounded clumps

Leaves: 6-12cm in length, oblong in shape, leathery in texture, dark green at the top of the plant and paler below 

Flowers:  conspicuous and numerous in May/ June; flowers form rounded heads, clusters of terminal racemes with 5 parts and the corolla are bell or funnel shaped, dull violet purple in colour.

Hazard to Health

The plant is poisonous to humans and animals.

Action to take if Rhododendron ponticum is found on land that you own:

Physical control:

1.    Lever and Mulch: leaflet available on this technique: https://secure.fera.defra.gov.uk/nonnativespecies/downloadDocument.cfm?i... (pdf download)

2.    Cutting and burning followed by chemical stump treatment with glyphosate

Chemical Control:

1. Stem injection

The process is to drill holes in each stem and squirt glyphosate via a nozzle sprayer into the reservoirs created by mechanical drill -about 2ml/ hole. This has been shown to kill the plant in situ. However, the dead plant will remain for many years unless physically cut or mulched. It is particularly useful application near water ways and in most weather conditions.

2. Herbicide control

Foliar Spray using glyphosate- recommended on stump re-growth and smaller plants- the latter will need to be physical removed once controlled. Spraying must be carried out in dry and windless conditions in order to achieve maximum effect and to avoid drift to other species. It should be noted that there are restrictions for herbicide use near water, the Scottish Environment Protection Agency (SEPA) must be informed and advice should be sought from them.

Note that in order to eradicate any Invasive Non-Native Species, one control type application will be insufficient, therefore it is best to factor in a period of checking and control until no further growth is observed.

On other people’s land:

Contact the land owner directly.

Useful link: http://www.nonnativespecies.org/index.cfm?sectionid=47