Sunlight, sap and strimmer’s rash – how some plants pose a risk to your skin

The recent spell of sunny weather has encouraged lots of us to get active outdoors, enticing walkers and cyclists to explore the countryside and gardeners to tackle weeds, hedges, ditches and lawns.

While the odd nettle sting, or scratch from a thorn is quite a common occurrence amongst those who enjoy outdoor pursuits or gardening, there are other less obvious risks posed by plants, particularly during bright summers.

So before you go rambling, hiking or clearing a patch of over-grown garden, read on…


Certain plants can bring about a distinctive skin reaction called phytophotodermatitis, when the sap (containing compounds that make the skin very sensitive to ultraviolet light), comes into contact with the skin and is then exposed to sunlight.

A red rash develops, often accompanied by blisters on the sun exposed areas, usually 12-24 hours after contact with the plants. As the skin heals, the areas affected may become darker in colour and can take weeks or months to fade.

Strimmer rash

Cases of phytophotodermatitis have included unwitting gardeners, outdoor workers, and the general public. For example, instances of so called ‘strimmer rash’ or ‘weed whacker’ dermatitis have been reported among gardeners following weed cutting on sunny days, where the use of strimming equipment resulted in a spray of sap from the plants being cut.

The Umbelliferae plant family (including cow parsley and giant hogweed, more below) are commonly associated with this type of reaction. The main photosensitising compounds found in these plants are called furocoumarins, which increase the action of ultraviolet A wavelengths of light in particular, on the skin.

What is Giant Hogweed?

Giant hogweed (Heracleum mantegazzianum) is an invasive plant, which is native to the Caucasus Mountains bordering Southern Russia and Georgia. It was introduced to Ireland in the 19th Century and planted in ornamental gardens. However, it escaped from cultivation and is now found throughout the country, frequently found in wet fields, on riverbanks, alongside streams, roadsides and ditches.

A danger to public health

The Department of Agriculture, Food and the Marine, single out Giant hogweed as ‘a significant danger to public health’. The plant’s stems and leaves are covered with fine hairs that contain phototoxic sap, which can cause a severe skin reaction in the presence of sunlight and even blindness (temporary or permanent) following contact with eyes.

In the event that skin comes into contact with Giant hogweed, the affected areas should be covered and protected from sunlight straightaway, then washed quickly and thoroughly with cold water. Medical advice should then be sought. More information here.


Giant hogweed can grow up to 5 or 6 metres in height. It has serrated leaves, a large hollow main stem, and flowers from June to August (flowers are white or occasionally pink). More information here.

Protect your skin and eyes

When gardening or handling plant materials, wear gardening gloves in the first instance but protect your skin further where necessary (e.g. long trousers, sleeves and a visor can protect skin from pieces of plant materials thrown back forcefully when using a strimmer).

Try not to touch your face or eyes when handling outdoor plants.

If you know you have plant material or sap on your skin, wash the area quickly and thoroughly with plenty of running water.

If you need guidance about any of the issues in this post or another skin infection or disorder, contact the ISF Helpline for free assistance and information.