Common ragweed, found throughout the United States and Canada, only develops and produces seeds in geographic areas where the amount of daylight is 14.5 hours or less. In more northern areas where the amount of daylight may not drop to below 14.5 hours until late August or early September, ragweed will not be able to produce flowers and seeds before the first frost.

Common Ragweed," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995 Microsoft Corporation. All rights reserved.

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