Late July -mid September maybe earlier in Canada or mountains.

Generally right before the goldenrod starts to bloom.

Ragweed flowering is controlled by daylength. In Southern Ontario it starts to bloom about August 15. Start taking Claritin now so you will be ready!

Those allergic to it will have problems not when it pollinates but when it producing pollen, ie when it flowers. Do not confuse ragweed with goldenrod. The Goldenrods bloom in August and September, about the same time as ragweed. Because ragweed is a small plant with tiny non showy flowers and goldenrod is taller with lovely yellow flowers, goldenrod is often blamed for allergies.