BerniceC:

I understand how many varieties of Oxalis could become a pestic plant in certain locales.  It could not survive the winters such as we have in the northern USA.  It is typically sold in the USA as an ornamental houseplant, especially around St. Patty's Day.  I can imagine though that a field of them could be quit lovely to see, however difficult they are too kill off.  Much like bamboo.
In Maine there are certain varieties of lithrum which are considered weeds, but purchased here in NY by avid gardeners for their beauty and hardiness.  In Hawaii the verbena and impatiens that are naturalized there are pulled out as weeds.