August 21,
Blue Hills Reservation, Milton
Ragweed is a long-night plant: it does not go into bloom until
nights become long enough in summer. Those allergic to ragweed pollen have to
take vacations and go away in August. Ragweed is an annual, so in the north,
where nights are not long enough in summer, ragweed cannot
flower and produce seed. This is the major reason for it not to
spread northward.