Explain Iago's "parable of the garden" (lines 314-327)

 Gardens and various types of foliage seem to be Iago's go to for parable stories and metaphor.   

Our bodies are our gardens, to the which
our wills are gardeners: so that if we will plant
nettles, or sow lettuce […] either to have it sterile
with idleness, or manured with industry, why,
the power and corrigible authority of this lies in our wills.

"Honest Iago" is all about planting the seeds of doubt in his victims and carefully "watering" and cultivating that doubt to grow until it becomes a flowering case of paranoia and vindictiveness.