TCG Rulings

• This is a Multi-Trigger Effect ^[sic] that is activated in a chain to the opponent’s Spell or Trap Card.^[1]

• You can activate “Inmato” when another Plant you control is targeted by a Continuous Spell / Trap Card that was previously activated.^[1]

• You cannot activate “Inmato” during the Damage Step.^[1]

OCG Rulings

• Tributing this card is a Cost to activate the effect.^[2]

• When an opponent's Spell/Trap Card targets a Plant-Type monster, you activate this effect as a Quick Effect in Chain to the Spell/Trap Card.^[2]

• You cannot activate this effect during the Damage Step.^[3]

References

1. ↑ ^{1.0 1.1 1.2} Konami Gameplay FAQ: Crimson Crisis -- Card Rulings (version 2.0)
2. ↑ ^{2.0 2.1} Konami FAQ: Effect Monster > Inmato
3. ↑ Konami FAQ: [Damage Step] Can the effect of "Inmato" be activated in the Damage Step?