Leaves of Three, Let It...Bloom?

Growing up, I was never allergic to poison ivy, so when I started gardening as an adult, I never bothered to learn what it looked like.  After an intense weeding session several years ago, however, all of that changed.  I was apparently no longer immune to poison ivy - and I was bound and determined to never suffer such a bout of itchy miserableness again!  I quickly learned to spot the infamous 'leaves of three'!

This area is known for having copious amounts of poison ivy (as many fellow gardeners told me after moving up here to the Northeast), so I was dismayed but not surprised when many 'leaves of three' popped up this spring in the shady area under my back deck.

Every few weeks I went into the backyard carefully armed and protected with gloves and plastic bags and removed all the offensive seedlings.  My kids were not allowed to play there under the deck, and when we had to stain the wooden deck supports this summer, Mr. Red House and I wore protective boots and dreamed about our future patio that would smother all of this poison ivy.

Then the other day I noticed a very curious thing.  I spotted a poison ivy plant that I had missed in my weeding, and it was... blooming?

All those plants under my deck weren't poison ivy at all!  Those impostors were actually Bidens frondosa - I had been most carefully pulling up a harmless native wildflower.

Bidens frondosa

An annual native to North America, Bidens frondosa is also known under such descriptive names as Devil's Beggarticks, Common Beggar-ticks, Devil's Pitchfork, Sticktights, Bur Marigold, and Pitchfork Weed.  I somehow get the impression people are not very fond of it...

This Bidens has been introduced into other parts of the world, such as Europe, Asia, and New Zealand, and has proved to be a noxious weed there, as it grows so readily.  The seeds of this Beggar-tick are much like all the other similarly nicknamed plants - they cling onto animal fur or clothing or anything that brushes up against them and can be a pain to get off.  (That, and the fact that the seeds are shaped vaguely like a pitchfork, is probably what earned it the nicknames 'Devil's Beggarticks' and 'Devil's Pitchfork'.)  As you can see, this is one of the non-showy Bidens, as it doesn't have outer yellow petals like some other Bidens have.  Thus the flowers are not really noticeable or that pretty (unless you are a pollinator, anyway).

Bidens frondosa prefers moist soil and will grow easily in light shade or in full sun.  It has compound leaves, usually with either 3 or 5 leaflets.  Thus it can sometimes be mistaken by certain gardeners for poison ivy...

So how can you tell the difference between this wildflower and poison ivy?  Well, apparently the leaves of poison ivy alternate along the stem, while the leaves of Bidens frondosa are opposite each other.

Aha! Leaves are directly opposite each other = not poison ivy!

Well, I am thankful that our backyard is not as infested with poison ivy as I thought.  I do wonder about this plant in the front yard though...

It looks rather suspicious...

What do you guys think?