O thou with dewy locks, who lookest down
Through the clear windows of the morning, turn
Thine angel eyes upon our western isle,
Which in full choir hails thy approach, O Spring!
The hills tell one another, and the listening
Valleys hear; all our longing eyes are turn'd
Up to thy bright pavilions: issue forth
And let thy holy feet visit our clime!
Come o'er the eastern hills, and let our winds
Kiss thy perfumèd garments; let us taste
Thy morn and evening breath; scatter thy pearls
Upon our lovesick land that mourns for thee.
O deck her forth with thy fair fingers; pour
Thy soft kisses on her bosom; and put
Thy golden crown upon her languish'd head,
Have you ever walked in a field of flowers and winds come by and you can smell the fragrance of the flowers? That’s what Blake is describing in this stanza. Especially with the use of pearls. 
 Pearls could either mean dew drops, or rain drops but for this stanza they could mean small white flowers like this. 
 
 It especially clicks when one knows that a lot of white flowers are the most fragrant. The flowers are her garments at pearls that the wind blows and scatters. 
 The imagery here of the personification of morning coming “o'er easter hills” comes from poetic convention: 
 But, look, the morn, in russet mantle clad, Walks o'er the dew of yon high eastward hill:“ ( Hamlet  1.1) 
 Blake uses this convention as imagery for a spring morning.
To Spring