Hello Joylynn,
If you could make something useful out of giant ragweed, that would be absolutely AMAZING! As for me, I could never do it as I am so terribly allergic to the stuff that actually working in it might put me at risk for anaphylactic shock--and that is no exaggeration. But it is a long, fibrous plant and it seemingly grows absolutely everywhere, despite my attempts to control it by mowing. So if this plant which has always been a noxious weed to me can become a useful, practical resource to you, then you have accomplished something that I have never done and likely never will. As for just exactly how good a twine it will make, I have no idea, but by all means, please do find out and let us all know.
Great work,
Eric