(in German) Baumführer Bad Honnef, Stadtinformation Bad Honnef / punkt-und-pixel Grafik, p. 1 Retrieved on 27 September 2012. No name, no license
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Katie Hogan (2007-06-27). The Tulip Tree Liriodendron Tulipifera(in en). herrinhs.org. Retrieved on 2013-03-17. No name, no license, backlink to Commons
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Тюльпановое дерево(in ru). liveinternet.ru (2011-09-18). Retrieved on 2013-03-17. No name, no license