The answer is:
And big deep trenches in thy beauty's field,
and
Will be totter'd weed of small worth held.
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The deep trenches is a beauty's field is saying that an open field of life and nature soon became trenches from man for war. Therefore is aged.
Will be totter'd weed of small worth held means that the weeds always were alive on the field but now they died because they got old.
Hope this helped you on the question and in the future!