I doubt you have much of a concern. Powdery mildew is quite different from other fungal problems in that it is entirely climate/culture related. It also needs living plant material to survive - no living plant parts = no concerns for spores wintering over. That's why inground plants subject to powdery mildew seldom develop it until later in the season when the conditions are right. Make sure you have proper air circulation and sufficient light and you should be fine.

btw, Verbena bonariensis will self seed very easily all on its own.....to the point where it is a borderline invasive in many areas. Just allow the seeds to fall or scatter wherever you want this plant in your garden and you will have it forever!!

Yes they pop up in all kinds of places in my yard! Fortunately they're easy to pull up when they come up where I don't want them. These seedlings will be for someone else. :-)

Deanna