Pinching off the top part of plants will often cause them to “bush out.” Some people recommend that you pinch off the tops of hardy mums to about 6-7 inches in the early Spring, then every 2-3 weeks until July 4th so that they will be bushy and full by the Fall.
“Pinching off” or removing the new growth of plants causes a plant to become fuller because plant hormones that cause elongation of plant cells (auxins) are located in the tips of growing plants. Once the tips of new growth are removed, lengthening of the plant is no longer stimulated by auxins, then plant hormones such as cytokinins (which direct lateral growth of plants) cause increased width of the plant.
The action of plant hormones also explains why plants bend toward light. Auxins move to the shaded side of the plant stem to stimulate cells on that side of the stem to lengthen more than the plants cells on the lighter side of the stem. This increased length of the stem on one side causes the stem to bend toward the light.
Written by Sue Vincent