Here are the results from the solar dying experiments I did last week. I left these in a jar for about 5 days, then rinsed them out and dried them.
Mercerised cotton thread soaked in (from left): strawberry, raspberry and loganberry tea infusion, forsythia flowers, and dandelion flowers.
I was pleasantly surprised at how vivid the yellow turned out to be from the forsythia pot, the colour looked fairly weak when it was soaking.
The first two pieces of fabric on the left don't seem to have absorbed any of the dye at all, and the other piece of polycotton has been dyed a very faint yellow hue, although it just looks dirty here. And again, the linen soaked in forsythia came out in a fairly strong shade of yellow. I wish they were still flowering so I could gather them up and try and obtain a stronger shade of yellow, but sadly, all the bushes around here seem to have shed most of their flowers.
But I have got four other pots on the windowsill right now, and I will post results when I rinse and dry out the swatches.