Thistles, burdocks, knapweeds and allies
The family Asteraceae is one of our largest plant families and is extremely diverse in the appearance of its species, though they are generally recognisable by their heads of small flowers, clustered together to create the appearance of a single, larger flower. Most people have an idea of what constitutes a thistle - a plant with a 'shock' of pink to purple flowers atop a spiny plant. But there are other species that are quite closely related and could be confused with thistles. This page should help you to identify a plant by narrowing down this group into smaller subgroups. So often, these plants all get lumped under the heading of 'thistles', but there are many species, some of which are infuriatingly similar, so care must be taken in identifying them accurately. For ease, the descriptions here talk about 'flowers' but in this family, what appears to be a single flower is actually a cluster of several to many flowers - often refered to as florets - with the inner florets often being petalless, while those around the outside bear the petals - the whole appearing to form a single flower. As you work through the choices on this page, the structure of these compound flowers will be important, most notably the outer bracts that surround the compound head. These bracts are known as phyllaries and details of their size and shape can really help with identification. You should also take note of the leaves, as there are plants with thistle-like flowers that have very different leaves. As this is such a large group, this page will help narrow your search, once you have arrived here from the flower colour pages.
The main purpose of this page is to attempt to break this family up into bite-sized chunks, so that the pages you are led to do not get too big by containing a large number of species. At worst, if you follow a link from here and do not see your plant, return to this page and try another link. Note that some species may come in a variety of flower colours; in such cases, the species is shown on more than one page, so you should still find it.
At this stage, be sure to read the texts below carefully, as they contain identification information that may not be visible in the photographs; clicking to the next stage will help you get to the exact species. It is important with this family to not only look at flower colour but also to look at detail of the green outer part of the flower head, which is made up of overlapping segments called phyllaries. It is also helpful to note details of both basal and stem leaves.
Use the photos below to narrow your search to a group, then click on the picture to go to the next stage.
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True Thistles These plants have the classic appearance of a thistle, with pinkish to purple flowers topping a plant with spiny stems and leaves. The phyllaries are typically spine-pointed. The flowerheads may be solitary and relatively large, or smaller and carried in clusters. |
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Burdocks Although closely resembling thistles in their flowers, these plants have large, almost rhubarb-like leaves. Plants have spiny tips to their phyllaries but are otherwise spineless. They often grow tall, to two metres in height, with the flowers clustered towards the ends of the branches. |
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Knapweeds & Allies This is one of the larger groups of thistle-like plants and contains a wide diversity of plants with flowers in a variety of colours. The thistle-like, pink or purple species sometimes have spines on their phyllaries but are otherwise usually spineless, with simple or compound, spineless leaves. |
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Globe-thistles The overall appearance of these plants is rather thistle-like, but their compound leaves are spineless and usually whitish underneath. The flowers are rather bristly-looking, but the bristles are soft and the flowers are carried in tight heads that form dense balls at the top of the plant. |