Mixed Colour Strawflower Seeds Heirloom Flower. The old fashioned flowers are exceptionally bright and colourful in the garden, range from 5-7.5 cm (2-3″) across and still make one of the prettiest dried flowers going. Upright, branching plants can reach up to 90 cm (3′) in height. When harvesting for dried flowers, cut stems before the flowers fully open and hang upside down to dry in a shaded well ventilated site.