Augusta asked in a comment if solar dyeing like this would work with other blossoms as well. I don't know but think it should work with some.
Let's test a random sample of flowers I collected near the house: daisies, bluebells, dandelions, horse chestnut blossoms and kerria (aka Japanese yellow rose).
I included everything in the brew: petals, bits of stalk and centres. They are soaking in water with a few grains of alum and a piece of silk.
Of course this is not the only way to extract potential colour. Other methods (steamed contact print, hapazome, ice flowers...) could give different results and be more suitable for some flowers.
Let's see what happens in a week or so!