We have lived in our home for 3 years and have a problem plant. I won't call it a bush (as we thought) but it is actually stalks that grow to about 8-10 feet tall. The stalks are hollow. When we cut them down more appear in just days. Growth rate is unbelievable. It is also sending runners out 5 ft. What in the world is it? And most importantly how do we get rid of it? Total vegetation killers have not worked.
This aggressive perennial is known as Japanese knotweed (Polygonum cuspidatum). It spreads by seed and rhizome and as you found grows quickly. It is considered a noxious or invasive weed in most states and banned in some. Regular mowing of the plant and all its runners will eventually (in several years) deplete the reserves and kill the weed. Persistence and complete removal of all above ground parts is essential for this method to work. Digging can also help slow the progress of this weed. Bag and dispose of roots and stems to avoid infesting other areas. Any root or rhizome left behind can start a new stand of this noxious weed. Total vegetation killers will work with several applications. Cut plants back to several inches above the ground and apply the product to the cut stem within a half hour. The plant will begin to grow again. Make another application to the leaves. Be sure to treat all the runners. Several rounds of treatment will probably be needed. Whether using traditional or new environmentally friendly products read and follow label directions carefully. All these products are plant killers and can injure or kill your desirable plants as well as the weeds. For pictures and more information see the Purdue Extension Weed Science Bulletin on Japanese knotweed.