O thou, my lovely boy, who in thy power
Dost hold Time's fickle glass, his sickle hour;
Who hast by waning grown, and therein show'st
Thy lovers withering, as thy sweet self grow'st.
If Nature, sovereign mistress over wrack,
As thou goest onwards still will pluck thee back,
She keeps thee to this purpose, that her skill
May time disgrace and wretched minutes kill.
Here, “Nature” is  personified  as a powerful woman. This means that Nature has the power of destruction (wrack) as well as creation. This idea is mentioned several times in the sonnet sequence, notably in  Sonnet 20  where Nature “creates” the Fair Youth.
Sonnet 126