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.
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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 across. Each plant can have a spread of about 3.5ft.
Never be tempted to use the hollow stems as blowpipes.
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 contain 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 because 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 weedkiller to kill Giant Hogweed.
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.