By the hot relentless sun
E'en the dew is parched up
From the teasel's jointed cup.
Oh poor birds where must ye fly,
Now your water pots are dry?
He refers here to the paired leaves of the teasel which form a water-holding structure but I have never seen birds making use of these tiny reservoirs. The jointed leaves are apparent on the plant in the foreground of the picture below - a photograph taken earlier in the year.
The teasel, Dipsacus fullonum, gets its name from past use by fullers, who would make a brush from the prickly seed heads and so tease the surface of cloth to produce a nap. It was once regularly used to raise a nap on snooker tables. In most situations teasel heads have been replaced by other devices but for very fine cloth - including, I believe, the uniforms of the Household Cavalry - it is still used.