Five children suffered severe burns and one was scarred for life after brushing a toxic plant found growing in two separate parks.

Annie Challinor, seven, was left with a permanent scar after her arm blistered when it touched a hogweed plant during a family walk in Clifton Country Park, Salford, Greater Manchester.

And four teenage boys suffered chemical burns after coming into contact with it while playing in Moses Gate Country Park, Bolton.

The youngsters' families are warning others to be on the lookout for the Triffid-like weed.

But hogweed is just one of many harmful plants to avoid on days out.

Giant hogweed is just one of what are known as ‘non-native invasive plants’, so called because they were brought to this country by humans and they threaten habitats and other species around them.

They’re not all poisonous, but it’s a criminal offence to plant or cause to grow these pesky plants.

“Invasive species in domestic gardens aren’t generally a problem because gardeners don’t allow them to become invasive and tend to dig them out before they become a problem,” says Guy Barter, head of the Royal Horticultural Society’s advisory service.

“It’s when they overrun non-garden areas like riverbanks and waste ground that they become a problem.”

For example, ferns that proliferate in ponds monopolise oxygen and block out the light, causing other sub-aquatic species and even fish to die.

Non-native invasives are also very difficult and expensive to remove. It’s estimated that removal of the species from the London Olympics site cost £70 million.

6 suspect species

But unless you accidentally suffer agonising burns, how do you know if an invasive plant is in your garden? Guy runs down the six most common non-native invasive plants and why they’re so risky. Follow his advice on the RHS website if you find any of the following species on your property.

Giant hogweed

A close relative of cow parsley, giant hogweed can grow more than 10 feet tall, topped with white umbrella-shaped flowers.

The sap of giant hogweed can cause severe burns in bright sunlight, so you should always wear gloves and adequate clothing if attempting to get rid of it using weedkiller or non-chemical methods.

Japanese knotweed

Japanese knotweed is not damaging to health but it’s very hard to eradicate. Characterised by strong, bamboo-like stems, it damages the environment, growing vigorously on river banks, and has been a problem in the construction industry because once in soil on vacant sites, it can spread quickly.

It is not illegal to grow Japanese knotweed in your garden, but you must not allow it to spread into a neighbour’s garden or non-garden areas – if you do you could be on the receiving end of an Asbo.

Himalayan balsam

This grows in wet places such as riverbanks and can be a problem in damp gardens. It spreads via exploding seed pots which can cast the seeds two metres away.

Once established, if you remove Himalayan balsam from riverbanks you cause erosion and destruction because there is nothing left to take its place. It looks like a giant busy Lizzie, growing between four and five feet and producing pinky-purple flowers.

Rhododendron ponticum

This is another purple-flowering non-native which has invaded areas of the New Forest and Welsh mountains. It is an evergreen shrub that spreads anywhere where there is acid soil and promotes a disease called ramorum (sudden oak death).

You do find rhododendron ponticum growing in people’s gardens and it spreads by seeds – in a wet year plants may emerge within a distance of 200m. The bush will shade native flora and fauna, which can mean the death of ferns and other low-growing species.

Parrot’s feather

This is an invasive aquatic species which used to be sold as a pond plant, but was made illegal in 2014.

It spreads quickly in ponds, lakes and canals, swamping native vegetation. It is mainly found in Southern England but is spreading.

Cotoneaster

Many of us have this spiny plant in our gardens, using it as hedging, but it is quite invasive in certain areas.

People who have rural gardens should steer clear because it can end up in great swathes in downlands, the seeds being carried by birds and spread over wide areas.