People make a bit of a deal about following the thin thread to the tuber (pignut).
If it breaks, just dig up some earth around where it broke, ie go a little deeper and wider and sift through the earth you pull out. 99% of the time you can still find it if the thread breaks. There's not a lot you can mistake pignut for but still be a little careful to check you've got the right one. They'll have a brown sort of skin that flakes off when you squeeze the pignut or you can simply scrape it. I use the small blade of my SAK for this. The leaves arn't dis-similar to chamomile sort of feathery
http://images.google.co.uk/imgres?i...vnum=10&hl=en&lr=&cr=countryUK|countryGB&sa=G
Check out Yarrow on this site too. I've tried Yarrow tea and it's a nice wild tea. Comes out mid - late summer