Grief made the young Spring wild, and she threw down Her kindling buds, as if she Autumn were, Or they dead leaves; since her delight is flown, For whom should she have wak'd the sullen year? To Phoebus was not Hyacinth so dear Nor to himself Narcissus, as to both Thou, Adonais: wan they stand and sere Amid the faint companions of their youth, With dew all turn'd to tears; odour, to sighing ruth.
More personification is on tap: Spring is "wild" with grief, and it makes her throw her flower buds down, as if she was Autumn, who kills the buds.
Why does she do this? Because her "delight is flown," Adonais is gone. Spring doesn't think it's worth starting a new year if Adonais isn't around to enjoy it. These lines may also be a reference to February, when Keats died. Spring was just around the corner.
Phoebus, also known as Apollo, was the god of many things, including poetry. The speaker says that Apollo's lovers, Hiyacinth and Narcissus, weren't as dear to Apollo as Adonais was. In fact, despite their famed beauty, they are "sere" (withered up) compared to the youth.
Our speaker describes the other deities as "faint" beside Adonais, too. Their dew and smells turn to tears and grief ("ruth").
Again, Shelley is still using personified nature to mourn Keats. By throwing in a veiled reference to the month that Keats died, he reminds readers that this isn't just a poem about mythological beings.