"Come, thick night, And pall thee in the dunnest smoke of hell, That my keen knife see not the wound it makes, Nor heaven peep through the blanket of the dark, To cry “Hold, hold!” (1.5.48)
Growth / Nature
"Or so much as it needs, To dew the sovereign flower and drown the weeds" (5.2.30)
The imagery of darkness can be seen all around in this play including this very powerful scene of Lady Macbeth calling upon the evil spirits to unsex her. Ultimately meaning to deprive her of gender's qualities and asking to leave her with only the evil traits. This shows the darkness and evil being that she is.
Lennox is explaining that they will give as much blood as they need to water the royal flowers, meaning to make Malcolm the King, and drown the weeds, to get rid of Macbeth. The imagery of growth and nature is used as a metaphor to show Macbeth is a quickly growing, nasty weed that needs to be killed.