Weed Identification
Weed identification (ID) on a farm can be a tedious task. But weeds must be properly identified prior to weed control activities. Two weed species may look very similar at a given growth stage but may differ greatly in terms of the life cycle, modes of reproduction, crop-weed competition, and responses to control tactics. Resources listed below can help you identify weeds.
Tools:
Web-based apps:

University of Missouri
Weed Identification Key

Virginia Tech
Weed Identification Guide
Smartphone apps (iOS or Android)
Photo Galleries (by region):
Northeast

Rutgers University
New Jersey Weed Gallery

University of Massachusetts Amherst
Weed Herbarium

Penn State University
Weed ID For Pennsylvania
Midwest

University of Missouri
Weed ID

University of Minnesota
Weed Identification

The Ohio State University
Ohio Perennial and Biennial Weed Guide
South

Louisiana State University
Weed Identification

University of Florida
Weed Identification Guide

Auburn University
Weed Identification
West

University of California IPM
Weed Photo Gallery

New Mexico State University
Weed Database

University of Wyoming
Weed ID Guide
Guides (hard copy):
Weeds of the Midwestern United States and Central Canada

Weeds of the Midwestern United States and Central Canada edited by Charles T. Bryson & Michael S. DeFelice
SBN: 978-0820335063
Browse
Weeds of the Wests

Weeds of the West by Tom D. Whitson, Larry C. Burrill, Steven A. Dewey, David W. Cudney, and B.E. Nelson
ISBN: 978-0756711825
Browse
Other Resources:
IPM Pocket Guide for Weed ID in Field Crops

Michigan State University ($16)
IPM Pocket Guide for Weed ID in Field Crops
Weed ID Field Guide

Iowa State University ($10)
Weed ID Field Guid
For more information on weed identification and characteristics click the button below:
The following video shows tips on identifying summer annual grasses commonly found in corn, soybean, and cotton fields:
The following video shows tips on identifying grassy weeds at the end of the season, by their seedheads: