Hi
Would have been easier to identify seeing it before you cut it - but, it does look like ground elder which flowers at this time of year
does it have white roots, lots of them, close to the surface as well as deeper in the soil?
and before it flowered it had a dense mat of leaves close to the ground?
if so - ground elder, (aegopodium podagraria) No relation to the elder tree as it happens
and a right so and so to get rid of. Various options
but if you want full advice, including site visit and child friendly remedies, sutited to eith er DIY or someone doing it for you removal, I would have to charge (my livlihood!) have a look at
http://plewsgardendesign.co.uk/service/ ... ice-visit/
photo is ground elder in full flower