There is some controversy over what flower Shakespeare meaning by cuckoo buds/flowers but the best choices appear to be buttercup or cowslip. They are both yellow and may be weedy. Buttercups belong to the genus Ranunculus, a large group of about 600 species in the Ranunculaceae family. Buttercups have five lustrous petals that are cupped, usually yellow, and have a nectary at their base. Cowslips (Primula veris) are in the primrose family (Primulaceae) and are evergreen or semi-evergreen perennials native to temperate Europe and Asia where they grow in open areas including bogs, woodsides, meadows and fields. In spring, a single flowering stem ten carrying ten to thirty dense yellow blooms grows ten to twelve inches above a rosette of leaves. Cowslips are hardy in USDA zones 4-8 and like moist soil and semishade to sun.
In Love’s Labour’s Lost (act v, sc. 2, 904), the song of spring concludes the play presented by the Worthies to the Princess and her ladies;
When Daisies pied and Violets blue
And Lady-smocks all of silver-white
And Cuckoo-buds of yellow hue,
Do paint the meadows with delight.
The cuckoo then, on every tree,
Mocks married men; for thus sings he,
Cuckoo,
Cuckoo, cuckoo:
Cordelia, in King Lear (act iv, sc. 4, 1), describes the condition of her father, King Lear, to the doctor;
He was met even now
As mad as the vex’d sea’
Singing aloud;
Crown’d with rank
Fumiter and Furrow-weeds,
With Burdocks, Hemlock, Nettles, Cuckoo-flowers, Darnel, and all the idle weeds that grow in our sustaining Corn
The buttercup derives its common name from the common but false belief that buttercups give their yellow color to butter. The genus name, Ranunuculus, is the diminutive of the Latin word, rana, meaning frog and refers to the moist habitat of the animal. The common name cowslip may come from the old English word for cow manure and the fact that the plants grows in meadows where cow manure was plentiful. On the other hand, the name may refer to the slippery boggy soil where the plant is also often found. The specific name veris means spring, and refers to the time when the plant blooms.