mhernandes1204, you're definitely correct on the verses containing lines of iambic pentameter, but take a second look at the first two lines of the refrain, as I think you've missed a stress in your counting:
/ / - / - /
"Green grow the rashes, O"
There are four stresses, but each foot does not follow the spondaic pattern. A spondee is made up of two stressed syllables together. We see that in "Green grow." However, the rest of the line follows an unstressed-stressed pattern, as in, "the RASHes, O." This pair of feet must be iambic, then.
So, the meter for those lines begins with spondaic monometer (two stressed syllables together, one foot), then continues into iambic dimeter (two feet of unstressed-stressed syllables).
Therefore those lines actually consist of three feet of mixed meter, consisting of 1 spondee + 2 iambs each.