DANDELIONS WEED OF THE WEEK Biology and classification


Taraxacum is a large genus of flowering plants in the Family Asteraceae. They are native to Eurasia and two species are found worldwide. They have very small flowers (florets) collected together into a composite flower head. Some species produce seeds without the need for fertilisation apomictic these include the common dandelion Taraxacum officiale.

It is possible that many species such as dandelions originally came from landslides moraines beaches, dunes or similar habitats. It may be that the post glacial period was when many of the micro species of dandelion became differentiated especially as their production of seeds exhibits features such as apomixes that are often met with in artic or sub artic situations.

Dandelions are unaffected by day length and can flower at any time of the year.

Dandelions have been found growing at heights up to 2700 feet above sea level. Seedlings have been raised from bird excreta and therefore the seed can pass through the digestive tract of a bird unharmed.

 All dandelions have a basal rosette of leaves. Each year leaves are produced above those of the previous season, but the root undergoes a periodic longitudinal contraction often amounting to between 20 to 30% of its length which consequently ensures the leaf rosette remains at ground level.

Incidentally it is this feature that enables the dandelion to suppress other herbage in the short turf around. Dandelions have a milky juice that exudes on wounding and, like blood, congeals to form a protective scab. They also show a capacity to regenerate from small portions, even the lowest of the fleshy tap root. The leaf form of common dandelion is extremely variable. In the seedling and juvenile stage the leaves may be devoid of any lobbing and scarcely toothed. As the plant becomes older the successive leaves develop more and more lobes with the typically back curved segments. However, if the rosette of an adult plant be removed, the leaves of the new shoots revert to the juvenile form.

 

From the centre of this rosette arise the flower heads; these are borne upon hollow leafless tubular stems and each consists of numerous strap shaped yellow to orange coloured flowers enclosed by a number of protective green bracts. The flower heads close at night, when the protective bracts completely ensheathe the flowers; they open in response to light stimulus and high temperatures, a reaction that takes an hour to complete.  A rosette may produce several flower stems at a time. After flowering, the flower head dries for approximately 24-48 hours. The dried petals and stamens drop off, the sepals recurve and the parachute opens. Common dandelion produces seed heads that contain up to 400 or more achenes, but the average is about 180. Germination which is often over 90% can occur soon after being shed but these seedlings do not flower until the following season. The reproductive capacity of the common dandelion in one season is usually about 2,000.

 

Red veined dandelion Taraxacum spectibilia is a low short growing perennial, the leaves are often dark spotted with a reddish stalk and midrib. Flower heads are 20-35mm the outer florets are sometimes reddish or purplish beneath. The achene body narrows gradually at the apex where it passes into the beak; And sepal like bracts broad and often closely pressed but not recurved. It flowers from April to August in moist and wet places often in mountains.

 Taraxacum palustria has narrower leaves than the others which are very finely toothed or untoothed. It is neither spotted nor reddish. The sepal like bracts are broad and closely pressed with a pale broad margin. In  Taraxacum  paludosum the fruit passes abruptly into the beak It flowers between April and June and can be found in stream sides, marshes and fens.

 The other two dandelions have in the adult state, leaves with normally quite pronounced lobes, often of unequal size and the larger curved towards the base of the leaf-blade. In both the outer bracts are usually curved backwards. Taraxacum vulgare  is a low short perennial with flower heads 30-35mm outer florets sometimes  grey violet beneath sometimes reddish or reddish purple never red. Sepal like bracts are broader than lesser dandelion but narrower than red veined dandelion. Erect or recurved. It flowers all year, but especially in April to June. The colour can be variable but not purplish red.  Its habitat consists of grassy and waste places. It is common throughout Europe.

Our common dandelion Taraxacum officiale which flourishes on a diversity of soil types especially the heavier chalk ones, has the familiar golden flower heads that are often 2 inches across. On the drier types of soil, especially the sandy ones, another dandelion is sometimes common; this has smaller flower heads, often less that an inch across and the colour is pale yellow. From strains of the common dandelion with small flower heads this small dandelion (Taraxacum laevigatum) can be distinguished by the excrescence on the outer face of the tip of the inner bracts, which give these a split appearance. Lesser dandelion Taraxacum erythrosperma has smaller flowers 15-25mm sepal like bracts narrowly apressed, the tips of the inner bracts appearing forked. It flowers throughout Europe between April and June in dry grassy and bare places especially on lime.

 

References

Salisbury E Weeds and Aliens Collins London 191-193(1961)

Fitter R Fitter A Blamey M the wildflowers of Britain and Northern Europe Collins  254 (1978)

Phillips  EP the weeds of South Africa Union department of agriculture (1938)

 

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2 Comments

  1. argylesock said,

    16/01/2013 at 8:31 am

    Reblogged this on Science on the Land and commented:
    argylesock says… You might choose to look at other posts about dandelions on changinglifestylesblog. I picked this one to reblog because it’s about the science. At some point, I might start a ‘Weed of the Month’ series on my own blog.

  2. 16/01/2013 at 5:30 pm

    I was told as a boy that dandelions were evil and had to be purged from the lawn. I wish I had read this first.


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