Where does “*barku” come from?

*barku (Old Saxon) comes from Proto-Germanic barkuz, from Proto-Indo-European bʰerHǵós — birch.

Ancestry of “*barku”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	barkuz	bark of a tree
2	Proto-Indo-European	bʰerHǵós	birch