Compared to modern laminate armor — very weak
Plant fiber hasn't been used since, possibly, the Japanese samurai as an armor. It is very weak compared to metal and laminate counterparts.
Mixing metal shavings into the mix doesn't solve your problem. In fact, it probably makes it worse. Metal is strong because the bond between metal molecules is strong. Thus, an uninterrupted and uncontaminated sheet of steel is incredibly strong. By cutting it up you're robbing the metal of everything that makes it strong. (This is why modern armor is laminated — you can't mix things together that don't want bond together and expect it to work.)
Consequently, when anything impacts your plant-metal hybrid, the metal shavings would have the tendency to acts as additional shrapnel to help tear the plant fibers apart.
However, DO NOT LET THIS STOP YOU! I love the idea and think it sounds great!
If all science fiction were 100% science based then most of it wouldn't exist and the rest of it would be boring. Most science fiction is dreaming about what you could do, about what might be, if we just understood the issue better or if history had taken a different course. I'd run with this idea because it provides a story justification for why this particular group is very tough to deal with and provides unique and interesting weaknesses, like it takes longer to build weapons due to waiting for the organic component to grow.
Indeed, stories always need a balance of this type: harder to destroy on the battlefield vs. longer to manufacture replacements. That's what gives rise to unique tactics and the political and economical pressures that lead to the victory-crisis-redemption roller coaster that makes stories successful.
And if I want to be honest with myself...
My front yard is graced with a tree that I've been told is called "Tree of Heaven." It's anything but a heavenly tree! It grows shoots everywhere and I have yet to cut a branch off it that doesn't dull my chainsaw blade in seconds.
So the idea of using plant fiber as part of the armor-building process works for me.