In between Oxydendrum and Polygonum which one has  about 220 species?
Oxydendrum Sourwood or sorrel tree, Oxydendrum arboreum, is the sole species in the genus Oxydendrum, in the family Ericaceae. It is native to eastern North America, from southern Pennsylvania south to northwest Florida and west to southern Illinois; it is most common in the lower chain of the Appalachian Mountains. The tree is frequently seen as a component of oak-heath forests.
Polygonum tenue "Polygonum tenue", the slender knotweed or pleat-leaf knotweed, is a North American species of plants in the buckwheat family. it is widespread across south-central Canada (Ontario) and the eastern and central United States from Maine to Georgia, west as far as Minnesota, South Dakota, Nebraska, and Texas (though apparently now extirpated from Maine and New Hampshire).