Adding food coloring to the water of plants is a staple of elementary school science fairs and one of a number of tricks used by commercial florists to add unusual colors to arrangements of cut flowers. Generally, the coloring added is non-toxic and does not harm the plant. Depending on the color added and the type of plant, the coloring may change flower, leaf or sometimes fruit color slightly or more dramatically. Some food colorings are also derived from plants.
White flowers, especially carnations and Queen Anne's lace, take readily to non-toxic dye. Rich colors like red and blue work the best. Dye is added to water in which cut flower stems have been placed. The plants draw water up through the stems to replace moisture lost through the tiny pores in the leaves. The colored water eventually reaches the flowers, changing their color. When dyed flowers are left in plain water for a long period of time, some of the dye may flow back out into the water, tinting it.
Celery and Food Coloring
In science classes, stalks of celery are sometimes placed in containers of water and food coloring to allow students to study how water moves through vascular plants. Students observe which parts of the stem carry the colored water up or down and record their observations over a specific time period. Experimentation involving exposure of the water/dye-filled celery stalk to varying amounts of light demonstrates how light affects the rate of water movement.
Plants That Produce Food Coloring or Dyes
Some vegetables, flowers, roots and herbs have long been used to color foods. Beet juice produces a red color. Carrot juice was once used to add golden color to butter. Red cabbage can be used to produce the color blue. Annatto seed pulp has been used in the past to color dairy products golden orange. Green can be obtained from spinach. Though some plant-based food colorings are relatively tasteless, others may add some amount of flavor to foods.
Food Color and Plant Variables
When dying celery stalks, the stems turn the color of the dye used, but not all plants respond to dyes in the same way. Plants with deeply colored stems or leaves may show little change. The plant being dyed contains its own specific chemical compounds and these, combined with the food coloring, can affect the final color of the affected stems and leaves
Plants in Space
A variation on the celery stalk experiment was performed by students working in a NASA educational program. Study participants measured the amount of colored water taken up by plants in a normal gravity environment and compared those findings to the amount of colored water taken up by plants in a microgravity environment on a NASA plane. The students found little difference between the results in the two environments. According to the teacher in charge, it is possible that the minimal difference in results may have been due to the short duration of the microgravity test.