Weed Identification UK
By Jennifer Holmes

Weed Identification UK

Identifying invasive plants

The UK has some amazing native plants, but those we need to worry about are the ones that originated from outside the UK and since being introduced here have become a threat and a nuisance. It may come as no surprise to learn that it’s their invasive growth pattern and characteristics that makes a lot of these non-native species easier to identify. Clumping, ground coverage and smothering are all habitat traits that indicate the plant is taking over, or out-competing other species. A great and well known example of this being invasive Bamboo.

To help with plant identification this article provides a quick overview of the core identifiers, using some of the most prevalent invasive plants in the UK as examples of key characteristics to look for.

Visual FEATURES

Plants change seasonally and we use the different parts of the plant as they change to help identify it. Seeds, shoots, leaves, flowers, stems, roots are all visual identifiers.

With so many plants to choose from, we’ve chosen Himalayan balsam as a good example here.

The seed pods are the most defining characteristic of balsam. Known as ‘policeman’s helmets’ due to their shape, they explode on contact casting hundreds of seeds into the air. The seed pods appear after flowering, and the flowers are another defining part of the plant, however often several different plants can have extremely similar flowers, so it’s best to look at all elements of the plant to get a bigger picture.

Balsam flowers look similar Antirrhinum (or Snapdragon), but they grow predominantly from the top of the stalk whereas Antirrhinum flowers grow along the stem. Often the flowering season can be a good indicator, with balsam the flowering period is quite lengthy – between July and October, whereas with other species the flowering season can be spring only, or for shorter periods.

Shape, texture and colour of stems and leaves are also good features to study. Himalayan balsam stems look a little like Bamboo but whilst they are mostly green or a slight reddish colour and growing up to 10ft in height – Bamboo stems tend to be more yellowish and with more defined nodules, so stems and leaves are features that really define plant species very well. Leaf shape, structure and growth pattern are key pointers. Balsam leaves have serrated edges, are elongated and grow along the upper section of the stem, lying horizontally in whorls.

Roots or rhizomic root systems if you’re able to see them are a good indicator of the plant type. Himalayan balsam roots are quite distinctive being red at the base and stringy roots visible when the plant is pulled up.


UNIQUE SmellĀ 

Whilst most flowering plants have an aroma, for instance Winter heliotrope smells of vanilla or cherry, there are some invasive species have a completely distinctive smell and this is quite a good alert system. A great example of this being American Skunk cabbage which smells like its namesake a skunk – otherwise rancid like rotting flesh! You’re more likely to smell this plant before you see it.

A more fragrant example is Common ragwort which has a rather pleasant aroma – it smells of pollen. Often overtaking vast swathes of land its smell can be quite powerful.


Previous season

When identifying plants outside of their flowering season we have to be more creative and this is where looking for evidence of the previous season’s growth comes in. Whilst a lot of plant species either completely die back leaving no visible above ground growth or collapse to the ground, some simply change in appearance. For example, Giant hogweed remains upright with its summer stems turning brown in winter and the distinctive creamy white flower heads turning brittle.


PREFERRED Habitat

The habitat (or ecosystem) a plant thrives is often a distinctive identifier. Depending on its makeup and growth pattern certain plants will only grow in a defined habitat, be that a rocky cliff, woodland or near water. For instance Hemlock water dropwort requires a wet or damp environment to thrive, as seen below (top image) these juvenile plants happily grow in or beside water, you won’t see it in a meadow for instance.

Whilst we do recommend handling Hemlock water dropwort as it is extremely toxic, colour is a defining feature as cut the stem exude a staining yellow liquid. And this brings us onto the next identifier – colour.


DEFINING Colour

Colour is probably the most used identifier. Even green-only plants can be identified by their particular shade of green. For instance, ‘Japanese knotweed green’ is well known among surveyors. Invasive plants such as Ragwort is easily identified in early spring by it’s rosette formation of leaves which then develop into tall stems with bright yellow flowers. Aside from colour it can be identified by the presence of its biological predator the Cinnbar moth caterpillar.


Striking Characteristics

Stripes, variegated colouring, patterns, size (scale) or structure are unique identifiers that only exist in certain plant species. Often with invasive non-native plants these characteristics have evolved due to surviving or adapting to their native climate or habitat. These plants are often strikingly different to our native plants. For this characteristic we’ll look at Field horsetail (Equisetum arvense) as it changes quite dramatically through the year. In early spring its fertile stems looks like a tall mushroom, then as the growth season progresses it looks like mini xmas tree.

As mentioned above scale is often a good identifier, and none better than some of the smallest and largest invasive plant species. A good example being Giant rhubarb (or Gunnera tinctoria), which originated from Chile, this large herbacious hardy perennial is easily recognised by its enormous serrated leaves, unusual flower heads and clumping formation. It is, however unrelated to rhubarb, as the two plants belong to different orders, but they can look similar from a distance and both are edible, though most people would choose rhubarb over Gunnera. Capable of reaching 6ft span and 10ft tall.


Ask the experts

The best way to identify a plant is to ask our team to look at some pictures of the suspect plant. Simply upload pictures online to us and we’ll get back to you with the results.

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