What constitutes plant kin? Coming from the same batch of seeds?

The article left out that information.  Dammit. So, seeds and rhizomes?

Some species of tree can regenerate extensively by issuing new vegetative shoots from their underground rhizomes, after damages caused by disturbance by fire or harvesting. InNorth America, trembling aspen (Populus tremuloides) can regenerate very effectively in this way, and stands dominated by genetically identical "trees" of this species can sometimes occupy an area of several to many hectares (up to 40 ha). These stands may represent the world's largest "individual" organisms, in terms of biomass.Read more: Rhizome - Rhizomes, Species, Plants, and Shoots - JRank Articles http://science.jrank.org/pages/5867/Rhizome.html#ixzz4H0uK2Gaf

Seeds and rhizomes sounds right to me. :)