Answer: This might be a seedling of butterfly bush (Buddleia davidii) or it might be a seedling of blue mist shrub (Caryopteris) or it might be a Vitex. It could also be a Perovskia. These would all be grown as "cut back shrubs" meaning they are cut back short in the spring to remove winter damage and then regrow and bloom on the new growth. But with so many flowers, I wonder if it might be a perennial catmint (Nepeta)-- there are some such as "Six Hills Giant" that can grow that large and they really do seem to have thousands of flowers. I hope this helps.
Q&A Library Searching Tips