Consult an almanac or a weather service to see when the last frost usually occurs in your area. If you’re unsure about the dates, plan to deadhead early in the autumn. Deadheading is the process of removing dead flowers, growth, or seed heads. This will help your plant grow new flowers next year. As the flowers begin to droop, fade, or die, cut them off at the base of the flower. You can also twist the stem and pinch the flowers or seed heads off. Throw them in a compost pile or in the trash. Seed heads are the top of the flower that contains the seeds after the petals have died or fallen off. Removing the seed heads keeps your verbena from spreading its seeds. If you want verbena to spread across your garden, don't remove the seed heads.  If you allow the verbena to propagate naturally, you won’t be able to control the spread of verbena, but the new seedlings may be hardier and more drought resistant than verbena grown from clippings.  Some people prefer to leave seed heads in winter because it helps liven up the winter garden. If you prefer this look, remove any seedlings when you cut back the plant in spring. While deadheading is useful in autumn, avoid doing any heavier trimming than that. This will help the verbena survive the winter. Save any additional pruning for early spring of the next year. Once you have finished deadheading, add a layer of mulch around the base of the plant. You can use a mulch that contains wood shavings, leaf mold, or compost. This will help protect the verbena in the winter.

Summary:
Aim to deadhead the plants about 4-6 weeks before the last frost. Cut dead or drooping flowers off at the base. Remove seed heads unless you want the verbena to propagate naturally. Avoid heavy pruning in autumn to help the plant survive in winter. Add mulch around the plant to protect it during winter.