Your sentence looks like it needs a less technical word than some of the other answers. I would go with this:
I actually have some young seedlings of this species. It will be interesting to compare them to their older cousins.
of course cousins refers to close family members (but not as close as brothers and sisters) so here it is used as a metaphor. But it is an extremely common metaphor.
The disadvantage is that it is vague and only tells us that the other plants are closely related, not that they of the same species. But you already told us that they are of the same species in the previous sentence. Nature documentaries frequently use the word cousin in this (rather literary) way to refer to organisms of the same species, the same genus, or even the same phylum when it is clear from the context what is meant.