Centaurea cyanus
- Photograph, systematic position, morphology, biology, distribution, ecology and economic significance of the Cornflower.
- http://www.agroatlas.spb.ru/weeds/Centaurea_cyanus_en.htm

A Close-up View of Brown Knapweed
- Photographic study by Brian Johnston of Centaurea jacea, with many fine images of the flowers, and microscopic images of the inflorescence and pollen grains.
- http://www.microscopy-uk.org.uk/mag/artmar06/bj-knapweed.html

Centaurea nigra
- Several fine photographs of Knapweed and some identifying features.
- http://www.plant-identification.co.uk/skye/compositae/centaurea-nigra.htm

Centaurea calcitrapa
- Red Star-Thistle. Detailed plant classification, national distribution maps, status as invasive plant and noxious weed. From the USDA PLANTS data base.
- http://plants.usda.gov/java/profile?symbol=CECA2

Purple Starthistle Identification
- Identifying photographs for Centaurea calcitrapa. From King County, Washington, Department of Natural Resources.
- http://dnr.metrokc.gov/wlr/lands/Weeds/star.htm

Centaurea tauromenitana
- Image of plant, close-up of flower head.
- http://www.dipbot.unict.it/orto/0440-1.html