Stringing, or trellising, is a practice in greenhouse tomato cultivation, especially for indeterminate varieties. These plants continuously produce flowers and fruit along the main stem, potentially reaching lengths of 30 to 40 feet. The string system provides the necessary vertical support, training the plant upward toward an overhead structure. This method is the standardized approach used by commercial growers to manage the vertical growth of these productive tomato plants.
The Necessity of Vertical Training in a Greenhouse
Training tomato plants vertically responds to the confined, humid greenhouse environment. This technique maximizes limited floor space by directing growth upward instead of allowing it to sprawl outward. The vertical orientation improves air circulation around the foliage, which is beneficial where high humidity favors fungal diseases.
Growing upward also ensures the vine receives consistent light exposure, supporting better photosynthesis and fruit development. Furthermore, the single-stem orientation simplifies routine maintenance like pruning and makes harvesting easier. By concentrating the plant’s energy on a single leader, this method increases overall productivity and fruit quality.
Setting Up the String Support System
Setting up the string system involves securing a horizontal anchor wire or cable, typically 8 to 10 feet above the plants, running the length of the row. This overhead wire must be taut and robust to support the weight of mature vines, which can exert a downward pull of 10 to 12 pounds per plant. Poly twine or biodegradable sisal twine is dropped from this anchor point for each plant.
The string must be long enough to reach the base of the plant, plus extra length for future lowering. At the bottom, the twine is secured to the young plant using a loose knot around the stem or by attaching it to a clip, stake, or ground anchor. The knot or clip must not be tight against the stem, as this would girdle the plant as the main stem widens.
Daily Management: Winding and Pruning
Once the string is set, daily or weekly management guides the plant’s growth and maintains the single-stem structure. Winding involves gently wrapping the main stem around the vertical string as the plant grows upward. This wrapping must always be done in the same direction—either clockwise or counterclockwise—to prevent the string from unwinding. The string carries the weight of the vine and its developing fruit trusses.
Simultaneously, growers must perform regular pruning. This involves removing the small side shoots known as “suckers” that emerge from the leaf axils (where a leaf joins the main stem). Removing these suckers directs the plant’s energy exclusively into the main stem and fruit production, which is the foundational principle of the single-leader system.
Advanced Technique: Managing Excessive Height
Indeterminate tomato plants continue to grow as long as conditions allow, often exceeding the greenhouse height. When the growing tip nears the overhead wire, the “lower and lean” technique extends the plant’s productive life. This process involves releasing the string from its top anchor and letting out a measured amount of twine.
The lower section of the main stem is then carefully lowered and leaned horizontally along the ground or a low support, creating a coiled base. Before lowering, old leaves and spent fruit trusses are removed from the section of the stem that will be laid down. This action brings the growing tip back down to a manageable working height, allowing the plant to continue upward growth and fruit production without the need for ladders.