What Happens if You Trim Zinnias Back?

Pinching

• Pinching is a trimming technique used on zinnias and other annual flowers such as salvias and petunias. It involves cutting back leggy stems to promote dormant stem buds to sprout and create a plant with more stems and leaves. The method is called "pinching" because snapping or pinching of the stems is most easily done by pinching together fingernails on the stem, rather than using scissors or hand pruners. Zinnias respond well to pinching, especially when they are about 6 inches tall. Pinching the stem 1/4 to 1/2 inch above a pair of lower leaves causes new stem buds to develop from the base of the leaves.

Deadheading

• Deadheading involves removal of faded, deteriorating flowers. When dead zinnia flowers are removed, zinnia plants' hormonal composition changes so that the plant focuses on producing more flowers rather than on stopping flowering and developing seeds in old flowers. Deadhead zinnias by cutting the flower stem 1/4 to 1/2 inch above a lower pair of leaves. Side buds grow to create new stem tips that bear new flowers a few weeks later.

Rejuvenation Issues

• As long as a zinnia is healthy -- not stressed by drought or disease -- harsher trimming or cutting back of the plant is possible. Not all zinnia plants respond equally well to being cut back to rejuvenate. Trim zinnia plants back by no more than 50 percent of their size. Removing too many stems may expose old, yellowing leaves or bare branch bases that do not have healthy leaves to photosynthesize sunlight and fuel regrowth. New growth rises from dormant buds along the stems, just above the base of leaves.

Health Issues

• Zinnias often succumb to fungal diseases, especially powdery mildew. Mildew is especially problematic on zinnias growing in overly wet soil and zinnias that have wet foliage from lots of rain or watering from above them. A humid summer with little wind also can encourage mildew on zinnias, which manifests as a whitish fuzz or film over the leaves. Instead of trimming back fungus-infected zinnias, pull up the diseased plants to remove the fungus from the garden bed. Increase air flower around remaining plants, and do not irrigate the plants in a way that gets their leaves wet, especially at night.

Growing Insight

• Numerous mildew-resistant cultivars of zinnias exist and are better suited than other varieties to grow in regions that have humid summers or heavy, slower draining garden soil. Instead of solely relying on pinching, deadheading and rejuvenation trimming of zinnias for a summer-long display, sow zinnia seeds in successive planting every two weeks. This strategy provides numerous plants that come into peak flower in flushes across the growing season, making reliance on rejuvenating plants for flowers less of a priority.