So, I've referenced the SRD and other RPG Stack Exchange questions on this, and can't find a definitive answer.
The SRD entry for Damage Reduction speaks only of weapons (using the word weapon 32 times, plus a number of weapon-exclusive mentions as well), saying that spells, supernatural abilities, etc. ignore DR.
In light of this, how does damage reduction work for a Wall of Thorns cast on/around creatures with DR 10/cold iron and good? Our group is divided and the DM remains uncertain of how to treat my actions.
The thorns deal slashing damage per the spell, but this is neither a weapon nor natural attack — it comes from magical (mundane-fire proof, materialized out of thin air, etc.) thorns.