Plants that are slightly overgrown or need some shaping may benefit from being manually pinched. Pinching is often used to increase branching, shape plants and reduce plant height. Pinching removes the apical dominance of the shoot which prevents branching. Apical dominance results from the production of auxin, a natural plant hormone by the terminal growing point and young leaves. Removing the terminal growing point and young leaves (pinching), removes the source of auxin and allows dormant buds below the pinch to grow.
When and how a plant is pinched is important in relation to branching and time of flowering. Pinching plants too soon after planting can delay plant establishment, and pinching too late can result in poor branching from hardened, woody stems.
A soft pinch is the process of removing the terminal growing point and one or two uppermost leaves just above a node or pair of buds using your thumb and forefinger, a knife, scissors or clippers. Growers often make one or more soft pinches to hanging basket plants to control their overall size and shape, to increase flower number and to create full, thick growth. Pinching is often used to shape plants in mixed planters containing plants with different growth rates.
Growers may use a hard pinch when plants are overgrown, beyond using a soft pinch. A hard pinch is done by removing the terminal growing point and two to four leaves. A hard pinch delays flowering more than a soft pinch and may also result in undesirable branching from the plant if not enough nodes are left on the stems. A hard pinch will delay flowering or re-flowering usually by two or more weeks, depending on the plant and weather.
Some growers will make subsequent pinches at three- to four-week intervals as needed, depending on the plant species and growing conditions. Additional pinches result in fuller growth and add to the quality, but also add significantly to total production time. Plants can be pruned and shaped at any time to reduce stretch and to improve aesthetics.
There are times when plants may need to be cut back (removing one-half or more of a plant) to reduce its size. Some species respond well to being cut back by producing abundant new growth, but others may respond with poor branching from hardened woody stems.
If the response to cutting back a particular species is unknown, test it on a few plants to determine their ability to recover. Cutting back should be reserved as a last measure.
To prevent transmitting diseases such as Tobacco mosaic virus (TMV), always work in blocks when pinching plants and clean tools and hands between blocks of plants.
Tina Smith, UMass Extension
with input from Douglas Cox, Stockbridge School of Agriculture
Preventing and Rescuing Overgrown Plants, UMass Extension