Rooftops were not a good place for a garden in the psalmist’s day. Grass there would wither quickly, and there was not enough soil for a crop. The psalmist looks upon Zion’s enemies, who were oppressing his people. He saw them as high and mighty, but unable to bless, or be blessed. That is why he likened them to grass on a rooftop.
LORD, make us into people who bless others, not people who are always condemning others.
Psalm 129