If you are planning an old-fashioned picnic...

If you are planning an old-fashioned picnic or a romantic stroll in the countryside this weekend, study these photographs carefully.

Poison oak can turn an outing into an itching, blistery nightmare that lasts one to three weeks.

One of California’s most widespread shrubs, Toxicodendron diversilobum produces a severe skin inflammation similar to poison ivy on the slightest body contact.

The shrub bears small greenish flowers that bloom in the spring and produce smooth white or brown berry-like fruit that are a quarter of an inch in diameter.

Each three-lobed leaf is 1 to 3 inches long and has a bumpy surface. New leaves are reddish, then turn glossy green, only to become reddish again in the fall.

In open spaces or in filtered sun, poison oak grows as a dense, leafy shrub. But it grows best in shaded, moist areas below 5,000 feet.

The deciduous plant can grow as an erect shrub up to eight feet tall or intertwine other shrubbery. Poison oak can also sprawl over rocky or grassy hillsides or up tall trees, attaching itself vine-like by aerial roots similar to those of ivy.

The plant can be difficult to recognize in the winter when it has lost its leaves. Unsuspecting hikers often acquire a rash when picking its leafless twigs for kindling. Sitting around a campfire, backcountry travelers can then ingest the poison when oil particles are inhaled with smoke and ash.

After they are treated, the plant’s branches are widely used in weaving baskets.