Oscar Sunderland, 15, was left with a 10cm leg burns and needing hospital treatment after coming into contact with giant hogweed.
The Kettlethorpe High School pupil could be left scarred and unable to expose his leg to sunlight for up to seven years.
Oscar was kayaking with dad Mark, 47, when he walked through weeds by the river Calder in between Horbury Lagoon and Thornes Lock.
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Mum Kim Sunderland, 41, said: “The next day he had a pink link down his shin. On Tuesday it had blistered and got a lot bigger.”
It got no better after Oscar’s GP prescribed antibiotics and he ended up in A&E.
But it was only after the family did their own research that they realised it was caused by giant hogweed.
Oscar now visits the Regional Burns Centre unit at Pinderfields Hospitals twice a week and has to dress the wound daily.
The family learned that hogweed sap burns the skin when it reacts with sunlight.
Mrs Sunderland: “It needs to be washed off straight away.
“If you don’t know what it is and then go in the sunlight that brings it out.”
Doctors said Oscar may need to keep the burnt area out of the sunlight for around seven years.
Mrs Sunderland said: “It destroys the natural UV protection. It’s crazy we have something so dangerous growing in this country but know nothing about it.”
Previous safety warnings have been issued about giant hogweed sap which warn it can lead to blindness if it gets into the eyes.
Mrs Sunderland said: “It’s got quite spiky leaves and the trunk is thick and hairy. It’s about two-and-half to three metres tall. The toxic sap comes off the leaves.
“My son’s 15. If he’d been four years old and smaller it could have been much more serious.”
Hogweed advice on the NHS Choices website said: “If you touch a giant hogweed, cover the affected area, and wash it with soap and water.
“If you feel unwell after contact with giant hogweed, speak to your doctor.”