Hello Jon,

We do not recommend adding anything to the spray that the label does not specifically allow. You may end up altering the formulation's effectiveness and the soap (detergent) itself may cause irreversible foliage damage on the arborvitae. All we can suggest is that you spray one arborvitae at a time so it's less likely the spray will dry before you move on to the next. Given your quantity of plants to treat, for the future, hiring a professional pesticide applicator may be more cost-effective and less labor-intensive. They can still use organic pesticides if you wish, or they could use a root-applied systemic instead, and since arborvitae are not flowering plants, that should have little to no impact on pollinators or beneficial insects.

Miri