Youngia is a genus native in Asia. It belong toAsteraceae family (Compositae family), in Cichorieae tribe.
Description on "Flora of China":
http://www.efloras.org/florata...
USDA plants mentions two species naturalized in North America
http://plants.usda.gov/java/Cl...
Youngia japonica (Oriental False Hawksbeard) and Youngia thunbergiana (Tall False Hawksbeard).
I don't know why Youngia thunbergiana does not appear in "Flora of China".

Y.thunbergiana was considered synonym for Y. japonica subsp. elstonii, but recent studies have shown that it is a distinct species. ("Youngia thunbergiana (Crepidinae, Cichorieae, Asteraceae), a Species Overlooked in the North American Flora" - Lowell E. Urbatsch, John F. Pruski, and Kurt M. Neubig. See, abstract of article, here: http://www.bioone.org/doi/abs/... on BioOne Online Journals)

I don't know what are the differences between these two species. I think it would be interesting to see comparative pictures.
USDA distribution for Y. japonica:
http://plants.usda.gov/core/pr...
and for Y.thunbergiana:
http://plants.usda.gov/core/pr...
And of course, if possible, photos and for other species of genus Youngia.

Youngia japonica February 02, 2014, Victoria, Tx.

Few pictures of different species of Youngia on "Chinese Virtual Herbarium"
http://www.cvh.org.cn/db/data_...
http://www.cvh.org.cn/db/data_...
http://www.cvh.org.cn/db/data_...
For convenience I wrote all 3 page links, containing photos of Youngia species.
On first link above, scroll down, at the bottom are shown photos of Youngia pseudosenecio. According to article quoted above, Youngia pseudosenecio is synonym of Youngia thunbergiana.

I do not know if it's helpful for those who may have I do not know if it's helpful for those who may have Youngia thunbergiana in their area, but I noticed in the third picture, which show flowers and leaf (Image 23 of 25 on page 18), pattern "like sharp spine" on tip of leaf undulations, longer and more sharp, comparing to what I so on leaf of Youngia japonica. But it's possible that I am wrong, so this is just an idea, I'm not sure if this is a distinction between species or not.

[b]Youngia japonica[b] - Oriental False Hawksbeard

Native to eastern and southeastern Asia, the plant has become naturalized around the world in tropical and subtropical zones, so today it has a pantropical distribution and it is considered a cosmopolitan weed.
Description on eFloras.org:
http://www.efloras.org/florata...
http://www.efloras.org/florata...
Other useful links:
http://www.freshfromflorida.co...
http://www.southeasternflora.c...
http://ocmga.org/uploads/WOTMJ...
And two interesting blog pages:
http://myfloridabackyard.blogs...
http://rufino-osorio.blogspot....
It seems that the plant is expanding.
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Habit (Photos taken on February 02, 2014 - Victoria, Tx)

Plants have appeared spontaneously in the backyard in an poor lawns areas. It seems to prefer disturbed areas with sun in the morning and more shadow on afternoon. Frequent alongside or close to house wall. Probably for shadow and humidity.
Note the variability in the overall appearance of inflorescence between the first and the last picture above. Last has a very compact inflorescence.

Note: I try to ID the plants accurately. But anytime an mistake is possible. If so, the correction is more than welcome.

Thank you Justin, you are always so thorough

Thank you Josephine.
Every time when I trying to identify plants, I try to read as much I can, about them. In books or on the net. This it is useful to improve my English.
When I take photos to one plant, I try to get as much details. Unfortunately I don't always succeed as I wish.

Few more details taken on various plants (February 02, 2014 - Victoria, Tx ).

@Justin,
Thank you for the photos; they are excellent. If we have a 'Photography Forum' I would vote you to be in charge.
Many of us could use a little help to learn to achieve good photos of our plants.