Dyeing with Weld Plants

A week ago I stood a tray of weld seeds in full sun on the greenhouse shelf because I'd decided long hours of daylight would germinate the seeds fast. No sign of life today, but no surprise because I found the compost dry as a Ryvita. My companion, Elinor Gotland, called from her deckchair on the lawn.
"Were you right about a bit of sunshine getting your weld seeds started then, Beaut?"
A full ten minutes watering the greenhouse had left me gasping in the humidity. I staggered out and veered across the lawn, attempting to dodge the question. 
"What a dramatic change this heat is from all the cool weather we've had." I reached the dye garden and stood there dripping sweat and trying to look nonchalant. "Rain then sun has really suited the weld plants, just look how many new flowers have grown." 

Since its main flowering spike was cut two weeks ago, my biggest weld plant has sprouted over a dozen lateral flower spikes. Quite an impressive effort.

That first main spike weighed 125g and has gone on to dye an even more impressive 250g wool yarn. Every batch of plant dye turns out a little differently, but since this one went particularly well, it seems a good point to record my current method.

I have found the strongest dye comes from chopping the plant material into large chunks and leaving it to ferment in cold water for at least three days, preferably a week. The water becomes faintly cloudy, slight frothy and properly stinky. Simmered for an hour, the dye bath looks only pale yellow and will test acidic at about pH 4 if you have indicator paper. 

Adding enough dissolved soda ash to bring the pH up to neutral 7 will turn a weld dye bath deeper yellow and I think leaving the plant material in the pot while dyeing also adds to the strength of colour. Starting with 125g weld, I first added two 50g skeins of wool yarn mordanted with 10% alum, simmered them for an hour and left them to cool overnight. Next day, they were a deep golden yellow, more like the colour from Dyer's Chamomile than the acid yellow I usually get from weld. I heated one skein with some dissolved iron to sadden the yellow to green and repeated the whole process with another two skeins, which went a more typically lime yellow. A last 50g skein was simmered soaked for a few days while I was away from home and even that turned primrose yellow.

"I think the first flower spikes give the strongest dye, Elinor. As they've given me plenty of dyed yarn, do you think I should do some contact printing with this second lot or just cut the spikes and hang them up to dry?"
"Best you let those flowers set seed, Beaut. Somehow I suspect you need to sow another weld seed tray."