Dyer's Knotweed or Polygonum Tinctorium is one of the few dye plants that give blue dye. It has a longstanding historical use in Japan as their source of blue dye. It is an interesting plant with segmented stems, apparently part of the Buckwheat family. It is a long season annual, which unfortunately blooms when the light levels drop, which of course makes it well into autumn here with frosts, which it doesn't tolerate. I've grown it several times and had blooms but no seed. It is not as strong as true Indigo ( indigo suffruticosa) but stronger than Woad ( istatis tinctoria), however it grows well in our climate, despite the issue with harvesting seed.
Yesterday I harvested 532 grams of Dyer's Knotweed leaves. All but one of my glass dyeing jars were playing hide and seek yesterday so I used my smallest dye pot to contain them. In order to cook them down, you need to cover them in hot water and then use a bain marie or double boiler system to cook them down. I've done this in as little as 2 hours at 160 F , as per Buchanan's A Dyer's Garden, but a lower temperature over a longer period of time works just as well. Yesterday the water was kept at about 100F for 4 hours and then up to 160 for an hour and there was plenty of colour in my dye pot.
After removing the pot from the double boiler, I strained the liquor into a pail and squeezed the remaining liquid from the leaves. The little pot was crammed full of fresh leaves to start and these few little squished leaf balls were all that was left after straining and squeezing!
I added baking soda to bring up the alkalinity of the liquid to PH 9. Yes, I used litmus paper to test to make sure as it does make a difference in the final results. I used my little stick blender, reserved for crafty stuff only, to aerate it. Every thing I've read suggests that Dyer's Knotweed should foam up the way that Woad does, but I've never had that happen. Instead, the water changes from murkey brown blue to a green blue with a little bit of scum on the top when you let it sit.
Then you add a bit of Spectralite or Thiox. I used Rit colour remover as that is what I had handy and it works like Spectralite does. Half an hour later, it was a yellowy green and ready to go. I did this outside yesterday and it was quite cool, so the vat did lose viability more quickly than normal. However this was a small price to pay for a lovely late afternoon/ evening outside as there was a nice breeze, no mosquitoes and a break from the heat and humidity. I ended up dyeing masses of fleece and rovings as I'd forgotten that all important bit of needing stuff to dye! I'll weigh it when it is dry as I just kept grabbing wads of it until the vat started losing colour. :)