When Paul Thomas was 10 years old, his grandmother told him a secret.

"She taught me how to find a four-leaf clover," said the horticulturist with the University of Georgia Extension Service. "Her techniques on how to collect them have been passed down from generation to generation. It's great fun to do."

In fact, Thomas has two methods. The first one involves a trained eye.

Clovers have a slight white line in the heart of each leaf. Viewed from above, the white lines on the leaves of a three-leaf clover form a triangle. The lines on a four-leaf clover form a square. This technique works best on frequently mowed patches of clover.

"Just stand over a patch of clovers and tell your eyes to look for squares only," he said. "You can scan large areas quickly. After a bit of practice, squares start jumping out at you."

The second method is based on the fact that four-leaf clover stems are more vigorous than three-leaf stems. The stems are thicker so they respond differently to slight bending.

"Brush over an unmowed colony of clover with the flat of your hand," he said. "The three-leaf clovers will swing up slowly while the four-leaf clovers will spring up right behind your hand. Try it. You'll be surprised how well it works."

Four-leaf clovers started popping up around the end of February, with the highest populations in May and June. They'll stay around until the end of July. A few pop up again in October.

"Four-leaf clovers are probably a genetic variation," Thomas said. "But they're also sensitive to stress. Clover thrives in cool, wet conditions. But not in Georgia's summer heat. Four-leaf clovers are commonly found where clover grows best."

Nobody knows what the fourth leaf is for, Thomas said. Maybe it's just for luck.