Answer: There are several possiblities: 1) cold nighttime temperatures can turn the foliage purple; 2) a nutrient deficiency can turn the leaves purple; 3) feeding by an insect called a midge.
If the problem is weather related, it will outgrow the purple leaves. If it's nutrient deficient, you can feed with a complete fertilizer; 3) insect problems can be treated with an insecticide.
Best wishes with your verbena plants!
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