marred and with stint of petals, (line 2)
If something's marred, that means it's been messed up in some way. It's damaged goods. And if it's "stint of petals," that means it's probably missing more than a few. Why does the poem start out by describing all of the rose's defects? Well, it doesn't want us to be under any illusions. This isn't any ordinary love poem.
meagre flower, thin, sparse of leaf, (lines 3-4)
Notice the strange syntax in these lines? How often do you say that something is "sparse of leaf"? (Or, for that matter, "stint of petals.") Such contorted phrases hearken back to mythic poems, when heroes were stout of heart and maidens were fair of face.
a wet rose single on a stem – (lines 6-7)
These are the roses that you buy on Valentine's Day – the perfect, dew-covered blossoms that cover cheesy cards. Are they lovely? Sure. But are they interesting? That's the real question.