Question
I live in northern Kentucky and have a front sunny garden that I'm very happy with April to September.  In summer there are purple verbena with red geraniums behind and these are backed by California giant zinnias.  In addition, there are various perennials interspersed.  The problem is that I must cut the zinnias down at the end of August because they are too leggy, even though I cut them back drastically all summer long, and they are showing a lot of mildew.  I have twice tried growing later zinnias elsewhere in the yard and transplanting them to the bare spots but they don't attain the size and volume that the early summer zinnias have. The perennials have finished blooming by late July so the verbena and geraniums are too weak in the garden by themselves.  The space is close to the front sidewalk (perpendicular) and is very visible to anyone walking by so I don't think dahlias have a strong enough form.  I need a flower that is taller than geraniums, that can grow up among the zinnias during summer or be transplanted from elsewhere in the yard or from pots in late August and has strong blossoms in September and October.  I don't mind growing plants from seed.  I don't want to replace the summer zinnias because I love them, my whole neighborhood comments about them and people in cars often stop and ask about them.  I do get two and a half months of fabulous blooms.  

Answer
Susan,
You need Ageratum "Blue Horizon" !  This TALL variety of ageratum is not often found in garden centers that only sell the dwarf varieties, but it's easy to grow from seed!  It grows between 20 and 30 inches tall (pinch first bloom before it opens to encourage branching)  and is a great cut flower.  Best of all, it totally comes into its own in Sept and Oct.  Beautiful blue flowers that look GREAT with both geraniums and zinnias.  You can get seed for this plant from Parks Seeds or Thompson and Morgan.

All the best from my garden to yours,
C.L. Fornari
www.gardenlady.com