I know that buckthorn berries from another species of buckthorn were used to make a yellow wool dye in past centuries, so I decided to see what I could do with the buckthorn berries from our local park, after they'd fermented on the trees all winter.
I collected them while everything was still frozen and stored them in the freezer until I was ready to use them (a few weeks).
Following the general tips in Rita Buchanan's book, I put the berries into a gallon ziploc bag and smashed it gently with a rubber mallet to break up the skins. Then I put the berries in an empty cottage cheese container with just enough water/vinegar mixture to cover them (4 parts water to 1 part vinegar). I left the container on the kitchen counter for a week - I was worried there would be a strong fermented smell, but it never got bad, certainly not as strong as when we're fermenting to make beer or wine!
After one week, I tested the pH using my red cabbage strips; it was definitely in the acidic range, but I can't easily get more specific than that:
Then, it was time to dye! I heated the dyebath to a simmer and added my wool. I let it simmer for about an hour, then left the dyepot to cool for the day. The dyebath wasn't a color that would make my list of favorites, but at least it showed some promise:
Unfortunately, when I pulled the wool out and rinsed it, all I got was a very pale, muddy yellow brown. I'll try overdying it with dandelions or garlic mustard something and see what I can do with it... guess buckthorn berries (Rhamnus cathartica species) aren't going on my list of favorite local dyestuffs! Would have been nice to use that invasive pest for something...