ug raha hai dar-o-deewar se sabzah Ghalib
ham bayabaan mein hein aur ghar mein bahar aai hai.
उग रहा है दर ओ दीवार से सब्ज़ा ग़ालिब
हम बयाबान में हैं और घर में बाहार आयी है.
Greenery is growing out of the doors and walls 'Ghalib'
I am in wilderness and spring has arrived at my house.
What wonderful play of words. Difficult to describe, but I guess you can feel the irony and sarcasm in the verse. Ghalib says while he wanders in wilderness having left his home, weeds in form of grass grow all over his house. This he sarcastically compares to the beauty of spring. There is a resonance of another Ghalib's sher here "Ghar mein tha kya tera gam jise garat karta..".
Looking at this sher another way, "wilderness" is metaphorical in that it is Ghalib's state of mind, not the description of his physical location. There is a delicious irony here for Ghalib is in wilderness while his house is green as in spring!