My only complaint about this summer blooming perennial is its tendency to disappear over the winter. Sometimes it comes back and other times it doesn't. Kind of like our newspaper delivery person. Often it will die back completely from the main plant and re-sprout from a different location along the plant where it rooted. Perhaps the verbena wants to travel? As long as it comes back I don't mind. Wet winter locations aren't very good for it but a well drained area with sun will keep it in good shape for the following spring and summer.
To sustain your verbenas over long periods consider propagating a few extras. Here's what I do. Take a two node or more cutting and drop it in water. Wait a couple days for roots to form then pot it up! Complicated right? Not at all!
All I need to do now is trim the pieces below the root and pot up my new verbena.
Or if you want to make many, many, many more verbenas (and why wouldn't you?) try internodal cuttings with only a single leaf node on the cutting. Leave two small leaves on the cutting and treat it with rooting hormone then stick it in your rooting medium. Very soon you will have new verbenas! Verbenas are extremely easy to root and your success rate should be high. Each fall I prepare a few rooted cuttings to overwinter in pots in the garage so that should one of my verbenas depart for that great garden in the sky I'll have a replacement ready to go.
Verbenas layer very easily so if you go out to examine your plant you may find new roots along its stem. If it does then you can sever the rooted section away from the main plant and move it to a new location.
And if you think verbena sounds easy, you should try propagating coleus! Now go forth and propagate!