Thistles, knapweeds & allies

Thistles, Knapweeds & allies

Spear Thistle Spotted Knapweed Yellow Thistle Musk Thistle

What are they?

Thistles are readily recognised by most people by their spiny leaves and stems. Along with the closely-related knapweeds and burdocks, they have reddish or purple (though occasionally white or yellowish) flower heads which are made up of many individual flowers; each flower has narrow, strap-like petals, giving the flowers the appearance of small pompoms - some rounded, some flat-topped.

Where are they found?

These are mostly plants of open ground, often growing vigorously in abandoned farm fields, rough ground and road shoulders. Most flower in mid-summer.

Identification

Some species of thistle are very similar in leaf, so it is best to confirm identification when the plant is in flower; in particular, pay attention to the phyllaries on the outside of the flower head.

One technical term that it is useful to know when identifying this group is Phyllary. The phyllaries are the greenish outer part of the compound head of flowers and which can differ quite significantly between two, otherwise very similar, species.

Musk Thistle Carduus nutans

(Nodding Thistle) Introduced from Europe and occasionally found in old fields and disturbed ground. Currently known from three sites, all found in 2011, but probably waiting to be found elsewhere. Flowers June to September. The large, often nodding heads are distinctive.


Habit	Flower head	Leaf	Stem

Spear Thistle Cirsium vulgare

(Bull Thistle) An introduced species from Europe which is the commonest and most widespread thistle in Cape May County. A tall species, growing up to five feet high and often found in colonies - though each plant is rooted separately (see Creeping Thistle). Found in fields, disturbed ground and roadsides. Can be found in flower any time from June to November.


Habit, with wide-spreading crown	Phyllaries with long, spiny tips	Each leaf lobe ends in a long, spiny tip. Note pale veins

Creeping Thistle Cirsium arvense

(Canada Thistle) An invasive alien from Europe, though strangely often known as Canada Thistle in North America. This species is the bane of many an arable farmer as it spreads from fleshy rootstocks to form dense, dominant colonies in disturbed ground. Not common in Cape May County as there is not too much suitable habitat, but can be found in disturbed, grassy areas. Flowers June to October.


Forms spreading colonies	Flowers are a distinctive pale lilac color	Phyllaries small and tightly overlapping	Leaves unlobed, their wavy edges having many short spines

Field Thistle Cirsium discolor

A native thistle of grassy meadows, roadsides and old fields. Flowers August to October. Best told from the introduced Spear Thistle by the combintion of golden (not green) spines on the phyllaries and the felty white undersides to the leaves.


Habit	Flowers pale pinkish purple	Phyllaries have silver stripe and spiny tip	Leaf underside covered in whitish hairs

Yellow Thistle Cirsium horridulum

A plant of wet, often shaded ground in marshes and woodland edge. Distinctive in flower, but when in leaf, can be confused with other thistle species. This is the common thistle species in Cape May Point State Park. Flowers May to July (and occasionally again in fall).


Flower	Flower is surrounded by rosette of long, leafy bracts	Basal rosette of leaves visible long before flowering	Stems downy with white hairs

Lesser Burdock Arctium minus

Introduced from Europe. A plant of marginal habitats such as roadsides, abandoned fields and waste ground. Occasionally in open woodland. Found at several locations in 2011. Flowers July to October. Before flowering, best told from Greater Burdock by the hollow leaf stems.


Flower heads	Basal rosette of leaves visible long before flowering	Leaf stems hollow	Seed heads

Common Cornflower Centaurea cyanus

Introduced from Europe and formerly a weed of waste and cultivated ground. Now an occasional and non-persistent species, usually originating from so-called 'wild flower' mixes that often contain a number of non-native species. Flowers May to October according to sowing time. The wild species is blue, but pink, purple or white forms may also be found. Stems and leaves covered in silky white hairs, giving a silvered look to the plant.


Flower head	Flowers may be other colors	Phyllaries	Leaves and stem silvery

Spotted Knapweed Centaurea stoebe

An increasingly common, alien invasive species from Europe, Spotted Knapweed is plentiful in disturbed ground and even grows abundantly on the poor soils of the Magnasite Plant at Cape May Point due to its tolerance of semi-arid conditions. Flowers June to October.


Habit with open, wiry stems	Flowers purple with paler centers, but may also sometimes be white	Phyllaries with dark brown 'beards'	Leaves deeply cut into narrow segments

New York Ironweed Vernonia noveboracensis

Not uncommon in wet meadows and sunny edges of swamps, sometimes forming large colonies. Flowers August to September.


Habit	Phyllaries dark purple	Leaves willow-like	Seedhead

Safflower Carthamus tinctorius

Introduced from Europe. Occasionally found throughout the USA on waste ground or in marginal and disturbed areas. Probably a short-lived coloniser, originating from birdseed. Flowers June to August.


Habit	Flower head	Leaves silvery

Cape May Plants An Identification Guide