Or even *{wutlh bartIq} an “underground branch”? 

The only plant parts I know of are:

Qechjem’a’         trunk (of tree)

bartIq                   branch (of tree)

por                        leaf (of plant)

naH                       fruit, vegetable

vIychorgh            juice, sap of a plant

--
Voragh

From: mayqel qunen'oS

I marrion-webstered "root", and I found:

"the part of a plant that grows underground, gets water from the ground, and holds the plant in place"

So perhaps one could say:

{wutlh Sor 'ay'mey}

the underground parts of the tree