Wooden barbecue skewers with string? Chopsticks? Just plain sticks? Maybe chicken wire that the plant can grow through?

You can cut long "switches" or sticks, let them dry, and them stick them in the ground around the plants in a circle. Take twine or some other string, and tie it to the stakes around the circle and zig zag between them, and you have created a great grow-through staking system that will be almost invisible when the plant comes up through it. Do it early before the plants get tall for the most natural look.

Someone I once knew used metal hangers, unknoted and doubled over. They seemed to do the job for small to medium plants.

Thanks! I will try your ideas and see what happens!. I had no idea that scarlet flax would get so tall based on my past experience with them.