Purchase a few white carnations. With your group of children (any age works), measure water into clear vases or containers. Then add food coloring to each container. With the younger children I did this with the primary colors (red, yellow and blue) and older children we mixed the primaries to create secondary colours (green, orange, purple). Place one carnation in each container of colored water. Monitor each day to see what happens to the petals of the carnations. I made a chart with the group of what they predicted may happen. This ties in with labeling the parts of a plant and the function of those parts.
I teach 3-4 year olds and have seen the 2’s and 4-5’s teachers do this as well. We put petals, stems, leaves, and full flowers in our sensory table when talking about themes including flowers. Many flower shops or grocery stores that have old flowers that are no longer good enough for sold arrangements will give flowers for this use. We talk about the different colors, shapes, veins, temperature, and textures. We rip them apart and even include magnifiers for the children to inspect them. You’ll need to beware of thorns on roses and make sure you don’t get toxic flowers–especially if you have young children that still like to put things in their mouths. (Afterwards, these flower parts can be used for a flower collage.)