This is an alien plant to the Canary Islands and is normally native in Africa and Europe. It is an arable weed in the United States where it has proved to be a persistent and difficult plant to eradicate. It isn't large growing only up to 40 cm (this one was barely 20cm tall) but it exhibits allelopathy which means it secretes molecules which inhibit then growth of near neighbours - not a useful thing to have growing amongst crops.

It doesn't produce nuts as the English name suggests but has a roundish (hence rotundus) tuber which can be mistaken for a nut.

Waste ground near Buenavista, Tenerife 14th March 2009

Added 25th March 2009, March 21st 2010, updated 15th July 2014