The first time I posted about willow immersion dye for fabric, I showed how I use fresh willow. That's fine for me, because I can always pop out back and cut some fresh willow (even if it doesn't have leaves on it). But people who don't grow their own or have access to wild willow can still use willow to dye fabric.
This is dried basketry willow cut into short lengths. Like fresh willow, I cover the dyestuff with cold water and let it sit for a day.
The dried willow will float (even more than fresh willow), so needs to be weighted down to keep it mostly under water. It's also going to absorb a lot of that water, so you may need to add more.
After a day or more of soaking, bring the dyepot to a simmer over medium heat and let it brew for an hour or so. Then let the dyestuff cool, fish out the willow, and pour the liquid into plastic jugs for storage.
But don't throw that willow away. Put it back in the pot, cover it with water, and repeat the process as many times as it seems you're still extracting color. You can also put the willow bits into bundles.
As soon as I have a chance to get instructions written up, I'll add dried willow for dyeing fabric to my Etsy shop. But if you happen to know a willow basketmaker, you might just offer to sweep up after them and get yourself all the dyestuff you need!