Earlier in the week I got this email from a reader named Dave. "There's a tree or flower in Philadelphia that I smell every September. I've smelled it for 12-15 years and always wondered what it is. It is an extremely sweet smell that has a pretty good range, as I usually smell it while running, biking, or driving on the streets. Any idea what this mystery smell might be?"

The reader went on to mention the places he had noticed the scent including Kelly Drive, the block bordering Cliveden Park, and an area near Wissahickon Avenue. All these places can be a little overgrown, an in our region the first plants to take over unmanaged areas are vines, including Clematis terniflora, aka Sweet Autumn Clematis or Virgin's Bower, an invasive plant from China.

Sweet Autumn Clematis is aptly named, as it is one of the most powerfully scented of the fall bloomers. It's been flowering for a few weeks now, and maybe you've taken note of the thousands of creamy white star-like blossoms that are covering walls, buildings, and fences around Philadelphia. Or like Dave, maybe you've noticed its scent; a combination of candy, baby powder, and something slightly feral that like other white flowered plants I've written about, becomes stronger in the evening.

The fragrant flowers are wonderful, but the ensuing fuzzy seeds blow all over the place, and they germinate wherever they land. I pull out dozens of young vines every year just in my yard. Within a few years, the vine can grow to more than thirty feet, so vigilance and hard pruning are counseled if you decide to grow this plant intentionally. Cutting it back immediately after it flowers and before it goes to seed may also help to control its promiscuity.

If you've noticed a plant very similar to Sweet Autumn Clematis that isn't fragrant, don't be too quick to yank it. The native Clematis virginiana is similar in most respects, except that it isn't invasive and it has no fragrance.

I sent Dave a photo of Sweet Autumn Clematis, and he's going to look for this plant the next time he gets a whiff of mystery smell. He promised to get back to me to let me know if we've solved the case.