Is poison sumac deciduous

Yes, poison sumac [Toxicodendron vernix] is deciduous. Deciduous plants lose their foliage in the space of less than a year. Such is the case with poison sumac.

But the leaf-dropping doesn't make the plant any less poisonous. The urushiol oil that causes the allergic reaction in many humans is found in all parts of the plants. In fact, that reaction may be brought about by rubbing against something on which the oil dripped as much as 1-1/2 years previously!