In response to the many gifts of beautiful flowers that the speaker of the poem has received from her beloved, which all act as symbols or tokens of the love that he has for her, she gives a gift in return, which likewise symbolises her deep love and affection for her beloved. The advice that the speaker therefore gives to her beloved is to accept this gift, and to keep them so that the original purpose and message is still clear to him:
Take back these thoughts which here unfolded too,
And which on warm and cold days I withdrew
From my heart's ground.
It is important to realise that the author uses the extended metaphor of the flowers to describe her poetry, her "thoughts" which she gives to her beloved. Just like the flowers she has been given, she has "withdrawn" her verse from her "heart's ground" throughout the year. This explains the final two lines of this powerful sonnet and the advice contained therein:
Instruct thine eyes to keep their colours true,
And tell thy soul, their roots are left in mine.
Thus the speaker urges her beloved to keep the vitality and the "colour" of her poetry "true" and to remember that her verse is a product of the love she has for him in her soul.