how to recognise and kill Giant Hogweed.

These are the effects of coming into contact with Giant Hogweed
Giant Hogweed effect on skin

 What is Giant Hogweed

Giant hogweed, is an invasive weed that can grow up to fifteen feet  tall and can out-compete native and cultivated garden plants. 

It is also dangerous to human health as its sap contains toxic chemicals which react with light when in contact with human skin, causing blistering within 48 hours and can prevent the skin from protecting itself from sunlight, leading to very bad sunburn and possible scarring.

If you do come in to contact with giant hog-weed sap cover the affected area of skin immediately from sunlight and wash the skin with cold water as soon as possible. If contact is with the eyes or the skin blisters get medical advice.

Description of the Giant Hogweed plant.

  It is a tall (up to 15 feet)  cow parsley-like plant, the stems are green with dark-red or purple blotches and are hollow. The flowers are white in colour with a pinkish tinge forming flat-topped clusters facing upwards and growing as large as 2ft acros
Giant Hogweed plant

   It is a tall plant and can reach up to 15 feet. It is a cow parsley-like plant, the stems are green with dark-red or purple blotches and are hollow. The flowers are white in colour with a pinkish tinge forming flat-topped clusters facing upwards and growing as large as 2ft across. Each plant can have a spread of about 3.5ft.

Never be tempted to use the hollow stems as blowpipes. 


How does Giant Hogweed spread into our gardens and countryside?

Giant Hogweed seeds are dispersed over short distances naturally, by wind and also in soil stuck to shoes and machinery. You can see how walking through infested areas can easily carry the seeds back home and into your garden and neat borders. 

The seeds can be spread over much longer distances by rivers and streams. 

The seeds germinate easily and remain viable for up to 15 years.


Giant Hogweed is a biennial, which means it flowers and sets seed in the second year after germination. Biennial plants normally die off after flowering and setting seed, but the giant hogweed species sometimes still flowers in subsequent years.


What's the effect on our gardens and countryside.

The plant's very large leaves results in it shading out less vigorous native and garden plants in its immediate vicinity. In the garden this results in the loss of most of the colour and vegetables and in the countryside a decrease in the biodiversity of the surrounding area. As a consequence of out-competing native riverside plants. Banks of rivers and canals can be left bare in the winter and susceptible to erosion.

What's the effect on our health.

Giant hogweed sap contains a chemical which, in the presence of sunlight, causes a nasty and potentially dangerous skin reaction in almost everyone who comes into contact with it. Resulting in burning, itching and blistering. The lesions are slow to heal and any scarring can persist for at least six years. The reaction can occur by accidentally brushing past leaves and is especially acute in children.

How to stop Giant Hogweed spreading into your garden.

Good housekeeping by thoroughly cleaning shoes and boots after walking through areas infested with Giant Hogweed helps. However, as most seeds are airborne pulling out the seedling as soon as you spot them is the best method of defence.

If it is growing in a garden or a site nearby, then control is going to be difficult and ongoing as re-colonisation by airborne seeding is going to be a problem.

You could try working in partnership with neighbours and local landowners to solve the problem, by at the very least removing the flower heads before they go to seed.


The best way to kill Giant Hogweed

This is the method I use to kill Giant Hogweed.

To kill Giant Hogweed that is growing in your own garden, treat with a Glyphosate based weed-killer. It is the most effective method of killing the weed I have found. It contains an active ingredient called glyphosate that is systemic in action. This means that it is absorbed through the leaves of the weed and travels down through the weed to kill the root, whereas ordinary "contact" weedkillers only kill the green bits leaving the root alive to start all over again.


Even using this systematic type of weed killer to kill Giant Hogweed can take several years to eradicate the weed completely. As airborne seeds from plants growing nearby will be blown into your garden.

Working with neighbours to treat all areas where it is growing will speed things up by reducing the chances of seeds being blown into your garden and germinating.


Spraying or spot treating with Glyphosate based weed killers is most effective when the weed is actively growing. Usually from March, and because the soil beneath an established patch of Giant Hogweed will contain a large number of seeds that will continue to produce new plants. You will need to continue checking for new plants until the seed bank is exhausted. 

How to apply weed-killer to kill Giant Hogweed.

  • It's systemic in action so leaves should be sprayed or spot treated in the growing season.
  • Don't wait until the flowers have turned to seed.
  • It can be applied to the leaves using a watering can, painted on with a paintbrush or sprayed.
  • Choose a dry couple of days, if it rains the same day, you will need to re-apply it.
  • Very tall plants can be cut down to reach the tops. (burn or rot the cutoff bits)
  • Read the instructions for dilution rates.
 I use the undiluted product and dilute it myself.


Glyphosate based products are sold under a number of brand names I use the concentrated (360g/l) product here and dilute it myself, It is a lot cheaper and more effective. 

Always check Strength actually is 360g/l when buying concentrated Glyphosate weed-killer from other sources.