The basics of training and pruning plants

Training and pruning have little or nothing to do with leafy greens and herbs, since these crops are short-term and plant architecture does not require any modification. However, when we turn our attention to vining greenhouse crops, training and pruning is an important aspect of production. Vining crops can grow stems that, by the end of the crop, can be several to many meters long or tall.

For vining crops, the training systems we select and use can impact productivity and canopy maintenance. Different training methods are used for different crops or for different production seasons. In addition to proper training, plants are also frequently pruned to maintain predictable growth and development. Much of the pruning techniques relate to “source and sink” plant physiology. Simply put, sources produce and sinks consume. If one part of a plant is growing vigorously, some other part may not be. Sometimes problems can arise because sources are stronger than their respective sinks.

Training and pruning is essential to maintain the productivity and quality of vining crops including tomato, pepper, cucumber, and eggplant. This article aims to introduce training and pruning techniques frequently used in hydroponic food crop production.

Training

During the production of vining food crops in the greenhouse, we are almost always training plants to improve different aspects of production such as yield, quality or ease of maintenance. Regardless of the crop, training begins shortly after transplanting. The first step in training is to begin trellising stems. After plants are transplanted into systems, the stems are trained to grow upright by placing clips below leaf petioles and attaching them to a string hanging from overhead metal support wires. As the stem grows, additional clips are used to keep providing support for the increasingly tall canopy.

Once stems have begun to be trellised, different canopy training methods are used depending on the crop or growth conditions. Tomatoes are grown using a training method called layering or leaning and lowering (Fig. 1). Since stems cannot grow taller than the support wire, the growing point is lowered by releasing excess support string, and the stems are laid or leaned down parallel with the trellis support wire.

Unlike tomatoes, pepper and eggplant stems are simply trained upwards. These stems are usually not lowered because they are not as strong as tomatoes and can be damaged by this process. These crops are generally grown up to the height of the support wire then terminated.

There are several different canopy training methods that may be used for cucumber. The simplest method is the drape method, where individual stems are trained up to the support wire, then simply draped over the wire and continue growing back down towards the base of the plant. The umbrella method involves pruning the main stem once it reaches the trellis, allowing two stems to develop that are then draped over the trellis to grow. While this is like bifurcating, this process may be repeated up to two more times. Finally, a v-cordon may be used to improve light interception, especially during low-light periods of the year. Although the wire for trellising is generally right above the base of the plant so stems grow straight up, support wires can be moved out and away from the base of the plant. As the shoot develops, it is trained and grows upward. However, when cucumbers are grown in double rows, the v-cordon allows more light to penetrate lower canopies because the stems in the double row are growing up and away from each other. Once shoots reach the support wire, they are simply draped over and allowed to grow straight down.

Pruning

One of the primary responsibilities in any vine crop production system is “sucker” removal. Suckers are the shoots that develop from beds located where the leaf petiole meets the main stem. While there are a few instances where select, axillary shoots are allowed or encouraged to develop, they are generally unwanted. Side shoots, if left to grow and develop, result in excessive vegetative growth and reduced marketable fruit production.

Suckers are removed by hand, with or without tools. The first step is to correctly identify suckers, because you don’t want to accidentally remove the growing point instead. While you may want to remove suckers as soon as you see them, be sure they are large enough to remove. If they are too small you may damage the stem trying to remove them. Alternatively, do not let them get excessively large, as energy will not only have been “wasted” on sucker growth, but a larger wound to heal is left after removal. If the right size, around 1 to 1.5 inches, suckers can often be removed by grabbing them with your thumb and index finger and giving them a slight roll or twist. For some crops, such as peppers, suckers may not be completely removed. Rather, they may be pruned back to a few leaves to leave photosynthetic area on the plant. When using pruners or other tools for sucker removal, sanitize them between plants to minimize the spread of diseases. Sanitizing tools to minimize pathogens is a general best management practice and applies to any use of pruners for any of the activities discussed in this article.

In addition to unwanted side shoots, leaves are often removed from stems to maintain productivity and fruit uniformity. Crop canopies are also reduced by removing leaves. There are two different types of leaf removal that are common: 1) lower-leaf removal, and 2) canopy thinning. Lower-leaf removal is done to reduce the overall vegetative load of a crop (Fig. 1). The lower leaves can be removed because they are often receiving the lowest amounts of light and, therefore, are not always as productive as leaves elsewhere on the canopy. Additionally, as is seen with tomato, removing lower leaves promotes free air circulation around ripening fruits. Canopy thinning involves removing leaves from the body of the canopy, not only the oldest or lower leaves (Fig. 2). During the summer, more leaves are removed than in the winter. While this may seem counterintuitive, if there is too much photosynthetic area (leaves) on a plant, it can diminish yields and fruit quality.

Fruit thinning is performed to increase the uniformity and quality of produce. The amount and degree of fruit thinning crops require vary with species, cultivar, and fruit product specifications. By thinning fruit, fewer total fruits are produced, but the uniformity and quality of remaining fruits is better. For example, it is common to reduce a cluster of tomatoes with six or seven fruits to four or five fruits to get uniform fruit size and ripening. Similarly, although some long cucumbers can produce more than one fruit per leaf axil at a time, only one should be allowed to develop to ensure full development (Fig. 3).

The take-home message

Vining food crops take space and time. To create and maintain productive plants, consistent and continual maintenance is required. Training and pruning plants takes time and effort, but the end results can be worth it. Just like any other production practice, it will require some trialing to learn how to best manage your crop for your specific growing environment.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu