Plains coreopsis--a beautiful flower, especially when they fill the whole meadow with their sunny yellow blossoms.
But Coreopsis tinctoria (as you might guess from its Latin binomial) isn't just pretty to look at when it's blooming. It's a great dye plant, and has often been used to create a strong yellow-orange color on wool that has been pre-treated ("mordanted") with an acidic or alkaline substance to "fix" the color. I've used alum, which is easy to find and use and not as caustic as some of the other mordants.
Picking the flowers is fun--you'll need about a quarter pound of fresh flowers, half that of dried. (I often dry mine, to do the dyeing later.)
Cover the blossoms (dried or fresh) with water and soak overnight in an enamel or stainless steel pot, then simmer for an hour. Take it off the heat and let it cool.
Meanwhile, soak your pre-treated wool yarn or fiber (you'll need about the same amount, by weight, as the flowers) . When the dye is cool, strain out the flowers through a couple of layers of muslin (you can compost the flowers or put them through another dyebath, for a lighter color). Squeeze the water out of the yarn and immerse in the dye. Bring to a simmer (please don't boil). Gently agitate the fiber every now and then (you don't have to stir, just push it around). After an hour, turn off the heat and let cool. Take out the fiber, rinse well, then wash in a mild soap and hang to dry. You can reuse the the dyestuff until the color is exhausted.
Here's an example of coreopsis-dyed alpaca, from hookedanddyed.com. Isn't it beautiful? You can see more here, and read about solar-dyeing.
Reading note: The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way.--Heraclitus