On the last 10 successful Scavenges, I got:

2 Grass Thread
2 Grass Thread
2 Grass Thread
6 Grass Thread
4 Grass Thread
1 Grass Thread
2 Grass Thread
3 Grass Thread
6 Grass Thread
6 Grass Thread

(Started keeping track after I had already gotten 21 Grass Thread, have 103 Grass Thread atm)