JUNE – early
Nobody can say with absolute certainty which specific plant is the “dear little shamrock” and perhaps it was a name given to several plants. The most widely accepted candidates are the species Trifolium dubium (Irish: Seamair bhuí), with yellow flowers or the white-flowered Trifolium repens (White Clover; Irish: Seamair bhán). The yellow-flowering one, a native plant to Ireland and the European mainland, goes by a number of common names in English: Lesser Trefoil, Suckling Clover, Little Hop Clover and Lesser Hop Trefoil.
Right now, the yellow is flowering. Once established, this plant thrives on lawns that are regularly mowed and it has little competition for light or alimentation (though it may not grow as lushly as in damper places) and there it establishes colonies, shamrock patches among the mown grass. Now, in early June, the lawn is dotted with patches of yellow flowers, to be visited by insect pollinators, soon to produce seeds. The plant, like the rest of its family, produces pods but in this case, the pods are tiny and contain only a single seed. Pods protect the development of seeds until they are ready to shed (or in some cases, like the gorse or furze, to explode).
People who like their lawns smooth and well-tended may resent clover patches since they tend not to wear as well as the mixed grasses with which lawns are seeded. Nevertheless, the plant is benefiting the soil and indeed the nearby grasses. It belongs to the clovers, belonging in turn to a very large group of plants in as different in appearance from one another as peas and beans on the one hand and furze (also known as gorse) on the other (but many bearing fruit pods). They are the legume group, plants that concentrate nitrogen in nodules around their roots, making many of them good crops with which to precede plants that require a lot of nitrogen, such as the cabbage family or cereals.
Amateur botanist and zoologist Nathaniel Colgan (1851-1919) once asked people from around Ireland to send him specimens of what they believed to be an Irish shamrock and identified the five most common plant species, of which the two most common were the yellow (flowering) clover followed by the white. A hundred years later, Dr Charles Nelson repeated the experiment in 1988 and found that yellow clover was still the most commonly chosen. According to Wikipedia, yellow clover is also the species cultivated for sale in Ireland on Saint Patrick’s Day and is the one nominated by the Department of Agriculture as the “official” shamrock of Ireland.
Once flowering is over, probably in August, one can dig up a small section and transplant to flower box or pot in order to harvest sprigs of it for St. Patrick’s day on March 17th (a tradition that is nothing as old as people might think).
But nobody planted the yellow in the lawn – it got there by its own natural methods, possibly by wind or in animal excreta. Unlike the lawn on which it has set up its colonies, which was seeded on raked earth or, more likely, laid in grass turf rolls, it is in fact a part of wildlife in the city.