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Giant Hogweed Update

posted 19 Jun 2015, 13:13 by Communications TEAM   [ updated 11 Jul 2015, 06:55 ]
We've written about Giant Hogweed before back in 2013 when both Dunblane & Bridge of Allan Community Councils along with Forth Fisheries & Rail track, undertook spraying of hogweed in the area. Over the last two years there has been real success and the numbers of giant hogweed have reduced greatly. The treatments will be ongoing though and Bridge of Allan Community Council have just contributed funds for the purchase of another batch of Roundup.

To sum up work completed to date:

  • 40l Roundup used so far - 25l from Forth Fisheries; 10l from BoA CC; 5l from UCP Ltd. (Roy tried to persuade Stirling Council to help fund chemicals supply - rejected on H&S grounds!)

  • both banks of Allan Water sprayed from start of Darn Walk to rail bridge in Cornton (see photo)  Big Darn Walk colony revisited.

  • Small park opposite Blairforkie Drive sprayed - areas in unmown area here that Council do not touch; risk of public contact with Hogweed sap.

  • UCP factory grounds - this is large and sprawling area with substantial hogweed infestation. Incl. downstream RH bank to Blue Bridge.

  • Haws Park surrounds (Roy Baker also made valuable contribution here). Again, danger to public of contact with plant.

  • Mill of Keir area - large Hogweed colony that adds to infestation risk along Allan Water.

  • alongside Lecropt road to Scottish Meat plant - Hogweed colony that, as far as we are aware, has never been tackled.

  • edges of Graham's field from UCP boundary to Cornton rail bridge.

  • extensive colony near Dunblane sewage works down to Allan Water bank (behind M&S store) again, source of downstream re-infestation..

All in all, there is confidence that a good dent has been made in the spread of the highly invasive Giant Hogweed plant along the lower Allan Water and I hope that the results are evident in Bridge of Allan. The challenge now is to keep this momentum going.

There are also plans to spray some Japanese Knotweed colonies that have appeared along the riverbank from the A9 bridge down to Blue Bridge in late Aug/ Sep. So far, these do not appear to have spread to vulnerable buildings but need to be watched.

We also thought it would be useful to show some images and share some information about Giant Hogweed to help folks identify the plant. Most of the text below is from the RHS website.

What is giant hogweed?

Although an impressive sight when fully grown, giant hogweed is invasive and potentially harmful. Chemicals in the sap can cause photodermatitis or photosensitivity, where the skin becomes very sensitive to sunlight and may suffer blistering, pigmentation and long-lasting scars.


Giant hogweed (Heracleum mantegazzianum), is a tall, cow parsley-like plant with thick bristly stems that are often purple-blotched.

The flowers are white and held in umbels, (flat-topped clusters), with all the flowers in the umbel facing upwards. The flower heads can be as large as 60cm (2ft) across.

It can reach a height of 3.5m (11.5ft) and has a spread of about 1m (3.5ft).

Giant hogweed is usually biennial, forming a rosette of jagged, lobed leaves in the first year before sending up a flower spike in the second and then setting seed. True biennials only live for two years, dying after flowering, but giant hogweed does not always behave as a true biennial and may flower in subsequent years.

How to Control Giant Hogweed

Plants that are undesirable, out-compete desired plants, or simply invade half the garden are classed as weeds and require control.

First, consider whether this can be done using non-chemical means such as digging out or suppressing with mulch. Where these methods are not feasible, chemical controls may need to be used.

Choose a weedkiller that is appropriate for purpose by reading the label carefully before buying or using. Those of low persistence such as contact weedkillers or glyphosate may suffice. Residual weedkillers persist in the soil for several weeks or months and can move deeper or sideways in the soil, leading to possible damage of underlying roots. Particular care must be taken when using residual weedkillers.

When controlling giant hogweed always wear gloves, cover your arms and legs, and ideally wear a face mask when working on or near it. Cut plant debris, contaminated clothing and tools are potentially hazardous too. Wash any skin that comes in contact with the plant immediately.

Non-chemical controls

Consider if non-chemical controls are an option;

On a garden scale, appropriate measures include pulling up young plants by hand when the soil is moist. Do this in May when the giant hogweed has reached a reasonable height, but before it has produced its flowering spike. For larger plants it might be necessary to loosen the roots with a fork first.

Never let hogweed set seed, but allow the flower spike to form and then remove it before the flowers fade. At this stage, the plant is less likely to survive trimming than earlier in the year.

Protect yourself from any skin contact with the sap, especially your face, when cutting stems, and carry out control measures in overcast weather avoiding sunny periods. Wash off any sap as soon as possible.

Larger scale areas are probably best left to the professionals, who should wear full protective clothing, especially if they are using a strimmer. As strimmers send sap and fragments flying face protection is essential.

Chemical controls


Where there are many plants, try applying a tough weedkiller containing glyphosate (e.g. Scotts Roundup Ultra 3000, Scotts Tumbleweed, Bayer Tough Rootkill, Bayer Garden Super Strength or Doff Maxi Strength Glyphosate Weedkiller). Ideally, spray the young foliage in May. Plants should be re-treated in August or September, if necessary.

Alternatively, cut back flowering plants and then spray any young foliage that re-grows in August and September. Mature plants are likely to need more than one treatment to kill them. Remember that glyphosate damages any plants it touches, so cover up ornamental plants with polythene or cardboard boxes before spraying.

Triclopyr (residual weedkiller)

Applying Vitax SBK Brushwood Killer (based on triclopyr) to the hollow cut stems after cutting back may be effective. Triclopyr is a residual weedkiller that does not harm long grass.