Introduction To Dandelions

By Bruce Homer-Smith

Dandelion Flower Heads

Dandelions are a tribe in the Aster Family. Aster blooms are organized in flower heads which hold many florets.

With dandelions, a flower head contains florets that radiate out from a central base, so they’re called rays. You might suppose that each ray is a petal but it’s actually a complete floret, containing not just fused petals but reproductive parts at its base.

Other Asters may also have ray florets, but dandelion flower heads are distinct in a couple of ways:

· Other Aster flower heads always have central disc florets in addition to possible ray florets. Disc florets are held in the center of the flower head and have no obvious petals. Dandelions never have central disc florets.

· Also, a dandelion ray floret has 5 fused petals, called a ligule. Other aster rays have only 3 fused petals.

Dandelion flower heads have no disc florets.

Common Dandelion – Keir Morse

Other aster flower
heads have disc florets.

Diffuse Daisy – Ron Vanderhoff.

This is not a dandelion.

Here’s how dandelion heads and rays are organized.

Dandelion head in cross section

An individual ray floret

Images © Donald C. Drife, michigannatureguy.com/blog

The flower head has a receptacle at its base that sits at the top of a stem. Each ray floret connects to a spot on the receptacle. An involucre of leaf-like phyllaries, often green, wraps around the flower head, providing protection to the head, especially when it is young. These phyllaries are often distinctive, making a good ID characteristic.

Each ray floret has an ovary at its base that sits on the receptacle. When pollinated, the ovary develops into a fruit containing a single seed, a thin covering, and pappus that will help the seed float away on the wind when it’s mature.

Several structures grow out of the top of each ovary. 5 fused petals extend outward, creating a visual display that attracts pollinators. Remember that there are many ray florets on the head, so the flower head looks like a many-petaled flower.

Growing up from the ovary is a hollow column of 5 fused stamens covered with male pollen. Inside the column, a female style grows, pushing pollen out as it extends beyond the stamen column. After the pollen is spent, the style opens up a receptive bi-lobed stigma, ready to receive pollen from other flowers of its species.

Dandelion flower heads vary by the number of rays they produce. For instance, chicory and wire lettuce produce only a handful or a couple dozen rays, making them easy to distinguish from heads of over 100 rays.

Pappus – a Dandelion Parachute

Also growing out of the top of each ovary, and outside the base of each floret’s 5 fused petals, is pappus. It’s made up of bristles, and sometimes scales as well. During flowering, pappus parts are small but once the floret is pollinated, pappus grows in species-specific patterns.

Fruit, narrow beak

and bristles

Common Dandelion – Keir Morse

Elongated fruit, scales

and bristles

Silver Puffs – © Neil Kramer

Plumose bristles

Bristly Ox Tongue

When an ovary is pollinated, it becomes an emerging fruit. The fruit begins a complex generation of cells as it starts to develop a new living thing. For instance, the fruit often grows a narrow column at its tip, called a beak. Pappus scales and bristles grow from the top of the fruit, preparing an effective parachute for when the fruit and its seed are mature and ready to fly to a new location.

Almost every dandelion grows pappus, but with different materials and in different patterns. This makes pappus a useful character to check when figuring out what species you have.

· Many times, an upside-down cone of dozens of bristles grows on top of each fruit (see the first picture above). The individual bristles are hair-like, forming a light parachute to catch the wind. Some bristles are plumose, with fuzz along their length, increasing their wind resistance (see the third picture above).

· Another pappus arrangement includes scales. Scales are thin and long but also have width. They create a papery surface, making a sail to catch the wind. They often end in a hair-like bristle (see papery starburst below).

Pappus components combine to create several distinctive looks which I refer to in the key:

Fluffy sphere		Fruits have beaks that end in many bristles. The overall effect is a fluffy sphere, with space on the inside.
Dense sphere		Fruits do not have beaks, so bristles grow closely to them, creating a dense sphere without space on the inside.
Brush shape		Fruits do not have beaks. There are only a handful of fruits, so the overall effect is spaced bristles, rather than a dense sphere.
Papery starburst		Fruits connect to triangular, flat, papery scales that end in a bristle. I call this distinctive look a papery starburst.

Beautiful pappus is a hallmark of dandelions. Use the key to have fun looking at it closely.

Dandelion Leaves

Douglas’ Silverpuff

Hairy Cat’s Ear

Bristly Ox Tongue

White flowered Hawkweed

Dandelion leaves vary a lot, which makes them a great identification tool.

· Most species have basal leaves, either lying along the ground in a rosette, or reaching up like blades of grass.

· Stem leaves tend to get smaller as you go up the stem. Some wrap around the stem.

· Leaves are generally long ovalish shapes or narrower, and often have lobes along their edges. The shape and pattern of these lobes is often useful in identification.

Dandelion Stems

Most dandelion stems have a milky white sap which you can see if a stem or branch is broken. Other asters typically do not.

Several dandelion species have stems that are bare, free of leaves or branches. They support a single flower head at their top, often drooping when in bud and becoming erect in flower.

Other dandelion stems are full of leaves and often branched.

Corrections/Comments: bruce@PlantID.net