I changed it to "Kudzu Seed" on ZBlader's and patch's suggestions. Thanks!
I kept the Immaterial status for now. My justification:
1) The immaterial is only on the spawned perms, not the original permanent. You can stop further effects by just destroying the permanent (or, if future cards allow this, remove the second ability).
2) Now that shards can stack, permanent slots will be filled even less quickly. The immaterial status of the "dead" Tree cards would really only hurt rainbow stalls, since they usually rely on a lot of permanents. For other decks, they usually are too fast to be affected by the (once-per-turn) spawning of dead perms, or only rely on a few key permanents (pillars, HGs, and Shards come to mind).
If enough people suggest against it though, I'll change it.