I’ve never found ground ivy to be anywhere near as bitter as the books say, but it could be the wild stuff I use tastes different from British or cultivated varieties.
The fresh, new growth at the tips of red, black, Sitka, or Norway spruce in spring. If it’s too bitter to eat raw, it’s too far along.
This is a rare example of an American, colonial-era brewing “herb” still in regular use; spruce beer even has a Wikipedia entry. There’s also a good, short article on the Craft Beer & Brewing website. It describes the flavor as “a delightful combination of pine, citrus, woodsy, green, and even wine grape or red berry.”