Coreopsis seeds will germinate quite easily. Here they are in my garden mid-September. If you have watched the video, you will recognise this flower patch. I planted these out rather late this year, but they still grew right on until the beginning of December and have only just been removed to the compost heap, where I notice they are still trying hard to carry on flowering. This picture shows the plants just opening in flower. They are about 2ft tall, maybe taller in places, and often hit by the wind where we live, (watch my short video clip on the Kidney Beans entry), but that doesn't hinder them flowering. I stake them and run (hand-spun) 'garden twine' around the plants to hold them up so they don't smother neighbouring plants.
I harvested every third day, usually when the blooms are a little past their best, and used fresh or dried for later use. Here are the blooms spread out on my dehydrator, but I have equal success spreading them out on an old screen up in the garage loft where it gets very warm.
Next morning, heated gently, barely a simmer, for about 40 mins. I tend to always leave the dye pot after simmering, to sit half a day or better still, overnight. That way the rest of the colour can just gently run out and you know you are getting it all, and it helps the fix I think.
My recipe? Well, I used here 3 litres of fresh blooms to 8oz wool. Recipes I have say anything from 4 times weight of dried blooms to 1 of wool (ie. 1lb dried blooms to 4oz wool ...that's an awful lot of flowers!!). I tend not to use that quantity.
The 8oz batch on the right was in the first dye pot, and the 8oz on the left from the waste dye from the first batch. These were dyed with about 3 litres of blooms in approx. 4 gallons of water. There was still a lot of dye value left in the dye pot after the second batch, so I dyed a third batch of wool cloth that also came out fairly well.
By the way, the Coreopsis stalks and leaves make a good dye bath also. I had far more than I could use, but I did make a good orange-brown dye bath from some of them.