When forty winters shall besiege thy brow,
And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gaz'd on now,
Will be a totter'd weed of small worth held:
Then being asked, where all thy beauty lies,
Where all the treasure of thy lusty days;
To say, within thine own deep sunken eyes,
This follows the  metaphor  of “beauty’s field”.  The “weed” grows in the field, but in Shakespeare’s time “weeds” meant clothes.  So in this context the  metaphorical  ‘tattered garment’ will be all that is left of the youth’s beauty. 
  The  alliterative  ‘w’s and long vowels in 'weed’ and ‘worth’ add emphasis.  The slow pace of the line suggests weariness and ageing.
Sonnet 2