How to identify invasive plants

The seven plants, one by one

Japanese knotweed. The RHS describes reddish-purple shoots from pink buds in spring that become hollow, bamboo-like stems with reddish-purple speckling and conspicuous nodes. Leaves are spade-shaped with flattish bases and sit in a zig-zag along the stem. In winter, the stems die back to dry brown canes. Underground, the rhizomes look like tree roots, but are orange under the dark brown skin. GOV.UK notes that a new plant can grow from a piece of rhizome as small as 1 cm. More detail on the Japanese knotweed page.

Giant hogweed. Ontario's giant hogweed page gives the scale: up to 5.5 metres in ideal conditions and a flower head almost a metre wide. The stems carry reddish-purple flecks and stiff hairs filled with sap. Its look-alikes are smaller: cow parsnip, angelica, valerian, lovage and Queen Anne's lace. The plant flowers once, then dies, and seeds stay viable in the soil for up to 15 years. The sap causes severe burns when skin is then exposed to sunlight. See the full giant hogweed profile before you go near one.

Himalayan balsam. A tall annual of riverbanks. The RHS page lists hollow stems with reddish tints about 2 m tall, lance-shaped leaves with finely serrated edges and scented pink to purple flowers from June to November. The seed pods explode when touched and can throw seed up to 7 m. The roots rarely go deeper than 15 cm, so the plant pulls up easily, and in autumn it leaves straw-coloured dead stems. See Himalayan balsam.

Purple loosestrife. Ontario's Invading Species Awareness Program describes purple loosestrife as woody, square stems, many from one rhizome, with spikes of pink-purple flowers and smooth-edged leaves set opposite or in whorls. Each plant can make up to 2.7 million seeds a year. Its look-alikes include fireweed, blue vervain, blazing stars and two native loosestrifes, so check the square stem and the five to seven petals. More on the purple loosestrife page.

Wild parsnip. In its first year it is only a low rosette; in the second it flowers on a tall stalk and dies. The Ontario wild parsnip page describes a single smooth green stem, mitten-shaped toothed leaflets in pairs and yellowish-green umbrella flower heads. Yellow flowers set it apart from the white heads of giant hogweed, cow parsnip and Queen Anne's lace. The sap can make skin react to sunlight, causing burns, rashes or blisters. See wild parsnip.

English ivy in North America. NC State Extension's English ivy profile describes two stages: a climbing stage with thick, glossy leaves on non-flowering stems that cling with small roots, and a shrubby adult stage that flowers in early fall and sets blue-black berries. It climbs trees and can kill them, and on the ground it smothers seedlings. Look-alikes include climbing fig and other ivies; see the English ivy page for leaf tests.

Common ragweed. A summer annual, usually 1 to 3 feet tall, with deeply lobed, fern-like leaves and inconspicuous flowers in spike-like clusters at the top. The male flowers release heavy pollen from late summer to fall. The common ragweed page covers giant ragweed and mugwort, which are easy to confuse with it.

What the agencies ask you to do

England and Wales. GOV.UK's knotweed guidance says the landowner, not the Environment Agency, is responsible for knotweed. You must stop it spreading off your property; you don't legally have to remove it unless it's causing a nuisance, but you can be prosecuted for causing it to spread into the wild. It advises against treating knotweed yourself without the right skills. Giant hogweed and Himalayan balsam are among 36 listed species of special concern: plants already on your land are not an offence, but you must not grow, sell or transport them, or intentionally let them spread. To record a sighting, the NNSS points to iRecord and asks for a photograph to help with identification. Scotland, Wales and Northern Ireland have their own guidance, linked from the GOV.UK page.

Canada. Reporting runs through provincial programs. In Ontario, the Invading Species Awareness Program asks you to call the Invading Species Hotline at 1-800-563-7711 or report on EDDMapS. Ontario recommends hiring a professional to remove giant hogweed, and says that small patches of wild parsnip (fewer than 100 plants) can be dug out with full protective clothing. The Invasive Species Council of British Columbia asks you to report a plant that looks "out of place" through its Report Invasives app or website form, and to include photos. For purple loosestrife, Ontario's advice is to bag the plants, seal the bags and put them in the garbage, not the compost.

United States. Invasive plant reporting is run by each state. New York's Department of Environmental Conservation is a good model: if you think you've found giant hogweed, photograph the entire plant, stem, leaves and flowers or seed from a safe distance. Then note how many plants there are, how big the patch is and where it is, with GPS coordinates if you can. Report it by email, on the DEC Invasive Plant Information Line (845-256-3111), through iMapInvasives or via your local PRISM. Other states have similar lines through their natural resources department or extension service.

If you decide to use a herbicide, follow the label and local rules. GOV.UK adds that in England, anyone spraying knotweed near water may need Environment Agency agreement, and that protected sites need Natural England's permission.

How to photograph a suspected invasive plant

Every agency above asks for the same evidence, and it's the same set of photos that makes an identification reliable:

1. The whole plant and its setting. Stand back far enough to show the height and the patch, with something for scale.
2. The stem, close up. Speckles, blotches, hairs, hollow or square: for knotweed, hogweed, balsam and loosestrife, the stem is the single most useful photo.
3. A leaf, top and underside, and how leaves attach. Zig-zag, opposite, whorled or in pairs of leaflets.
4. Flowers or seed heads, if there are any. Fill the frame.
5. The location. Keep location on in your camera, or note the road, landmark or GPS coordinates.

For giant hogweed and wild parsnip, use the zoom and stay out of the patch. NY DEC warns that the reaction can begin as soon as 15 minutes after sap touches skin. Its advice after contact: wash the area thoroughly with soap and water as soon as possible, keep it covered and out of sunlight for 48 hours, and see a doctor if you have a reaction. The photography guide covers light and focus in more detail.

Plant Identifier names a plant from one photo and gives its scientific name, which you can compare with the chart above before you report anything. The app shows no confidence score and no list of alternative species, so check the stem and flower tests yourself, and let the agency confirm. If the answer surprises you, the follow-up chat under the result lets you ask how that plant differs from the native look-alike you suspected. For weeds in a lawn rather than a riverbank, see how to identify weeds in your lawn.