Well, I know that this response is coming 2 months after the post and likely isn't needed, but I have my own last rose of summer sitting on my desk so I feel compelled to respond.
I'll not leave thee, thou lone one!
To pine on the stem;
Since the lovely are sleeping,
Go, sleep thou with them.~I won't leave you alone on the stem to mourn the death of the other roses. Since they have gone, I will help you to join them~
Thus kindly I scatter,
Thy leaves o'er the bed,
Where thy mates of the garden
Lie scentless and dead.
~With kindness I will scatter your leaves (more likely the petals) over the rose bed or flower bed where the other roses have fallen.~
So soon may I follow,
When friendships decay,
From Love's shining circle
The gems drop away.
~Here the poet begins to speak about himself. The gems dropping away from Love's circle refers to family, friends and loved ones dieing before him. He may soon be like the rose, the last one left of family and friends, waiting to die himself.
When true hearts lie withered
And fond ones are flown,
Oh! who would inhabit,
This bleak world alone?
~"True hearts" and "fond ones" are again the people the poet loves and is fond of. "Lie withered" and "are flown" describes their passing and having died. You could read this as When loved ones lie dead and friends are gone.




