What "winter-thorn" is so sweet?

Context:

A prince am I of ancestry renowned,
Illustrious name my royal sire has found.
When Sirius did in spring its light display,
A child was born, and Tiger marked the day.
When first upon my face my lord's eye glanced,
For me auspicious names he straight advanced,
Denoting that in me Heaven's marks divine
Should with the virtues of the earth combine.
With lavished innate qualities indued,
By art and skill my talents I renewed;
Angelic herbs and sweet selineas too,
And orchids late that by the water grew,
I wove for ornament; till creeping Time,
Like water flowing, stole away my prime.
Magnolias of the glade I plucked at dawn,
At eve beside the stream took winter-thorn.
Without delay the sun and moon sped fast,
In swift succession spring and autumn passed;
The fallen flowers lay scattered on the ground,
The dusk might fall before my dream was found.