United Kingdom 
Some years ago a neighbour gave us a herb to use for sweetening rhubarb pies. She told us it was Sweet Cicely. I have just been researching herbs on the internet and i have learnt that Sweet Cicely has white umbelliferous flowers. The plant we were given has yellow flowers. Does anyone have any ideas what thuis plant might be? Could it be poisonous?
4 Aug, 2010
Answers
Although Fennel and Lovage spring to mind as large, yellow flowered herbs.
4 Aug, 2010
Yes. more info needed really. I though this one was going to be about Stevia but yellow flowers? Not that then.
Liquorice (Glycyrrhiza glabra) has forms with greenish-yellow flowers and the root has been used as a sweetener. I wonder if it could be that?
5 Aug, 2010
Assuming that your plant is also an Umbellifer then Angelica archangelica (the culinary Angelica) might be another possibility. This is actually one of the traditional partners of rhubarb too, although, as with Liquorice, the flowers could only really be described as really greenish-yellow at best.
5 Aug, 2010
Fennel maybe? It is used in some recipes with rhubarb. See if this picture looks what you remember
http://en.wikipedia.org/wiki/File:Fennel_flower_heads.jpg
8 Aug, 2010
Hi Guest,
Could you provide us with a little more information about the plant that you do have - all you have told us is that it has yellow flowers, which I'm sure you will appreciate doesn't narrow the field of possibilities much!
4 Aug, 2010