Fig. 1. Tomato crop stand where the lower leaves have been removed and temporarily left on the ground
Photo courtesy of Celina Gómez

Growing high-wire tomatoes in the greenhouse is a labor-intensive practice recommended only to those who enjoy working in a dynamic growing environment. Without careful attention to detail, everything that can go wrong will go wrong! However, understanding key production concepts can help you produce high-quality, vine-ripened fruit that will make your business thrive.

1. Purchase transplants from the experts.

Typically, grafted transplants are purchased from specialized growers who provide flower-induced plant material that will enter production shortly after transplanting in the greenhouse. Buying transplants will allow you as a grower to concentrate fully on the intricate practices of exploiting this heat-loving crop.

2. Manipulate the environment.

One of the most important aspects of producing indeterminate tomatoes in the greenhouse is to understand that manipulating the environment can be to your advantage. Promoting vegetative growth will enhance root and shoot development that will be helpful in the initial establishment of your crop. On the other hand, promoting reproductive growth will stimulate flower development and thus, fruit production, which is what ultimately drives the success of your business. Maintaining a balance between the two is the challenge, and that is exactly what you, as a tomato grower, want to achieve. Remember that while you want your plants to be productive, they need their “engine” (i.e., roots to take up water and nutrients and leaves to photosynthesize) to produce fruit.

You know your plant is too vegetative if it’s producing thick stems, curled, thick, and dark leaves, or elongated flower clusters with pale petals and large sepals that are not setting fruit. If you see any of this, it is time to take action! Some common environmental techniques to steer plants towards reproductive growth are: increasing the difference between the day and night temperature, quickly transitioning from day to night cooling, raising ambient temperature, reducing relative humidity, limiting irrigation frequency, and if applicable, increasing your CO2 supplementation. Practices contrary to these will help steer your crop towards vegetative growth. To help you remember this, visualize excessive vegetative growth as foliage plants that evolved in the “jungle” (i.e., under high humidity, high temperature, experiencing small differences between day and night temperature, and exposed to frequent rain).

Fig. 2. Suckers growing out of the axillary buds in a tomato plant
Photo courtesy of Celina Gómez

3. Favor conditions that allow for effective pollination.

Understanding the effect of your growing environment is also crucial for good pollination. Optimum conditions to ensure successful pollination are 50 to 90 degrees Fahrenheit, 50 to 80 percent relative humidity, and bright, sunny days. If it’s too hot, pollen grains can dry and will not germinate. If it’s too cold, development of the pollen tube will be slow and the pollen grain may run out of energy before reaching the ovary. High humidity may cause the pollen to become too sticky to release properly from the anthers, or it may lead to clumping of the pollen grains, resulting in the physiological disorder known as cat facing, caused by uneven pollination and fertilization. Low relative humidity may desiccate the pollen grains, or it may cause the stigma surface to become too dry for the pollen grains to stick. Lastly, overcast days can slow the development and germination of the pollen grains and result in poor fruit set. Pollen is normally shed from morning to afternoon. Thus, you want pollination to occur sometime during the middle of the day. Bumblebees are excellent pollinators and can provide huge labor-savings compared to manual pollination. Commercial hives can be purchased from many reputable bio control companies.

4. Properly prune the indeterminate tomato canopy.

Knowing how to properly prune an indeterminate tomato canopy is important to divert the plant’s energy into growing fruit. New suckers (a.k.a. side shoots) growing as axillary buds are removed once the apical meristem shows visible signs of a newly formed flower cluster, never before! This is to prevent removal of the growing tip before securing a flower cluster. Keep in mind that the apical meristem can be easily confused with a sucker (it happens to the best of us!) and accidental “de-topping” can occur. In that case, the “undesirable” axillary bud on the top of your canopy can save the day and become your new main stem. Sucker removal can be done by simply grasping a sucker with the tip of your fingers and bending it sharply downwards to give a clean break from the branch. Constantly removing suckers will help you maintain a non-bushy productive crop stand.

5. Prune leaves from the lower canopy.

Older leaves below the upcoming cluster-to-be-harvested are typically removed to allow for air movement into the lower canopy, a strategy that can help speed the maturation process and will help reduce humidity levels that can be inducive to disease. Lower-leaf removal facilitates ease of harvesting and spraying and helps control insect pests that like to lay eggs in the lower-leaf canopy. In addition, removing older leaves allows your tomato clusters to capitalize from the energy produced by the plant. As a rule of thumb, a tomato plant will add three leaves per week. Therefore, three leaves should be removed per week.

Fig. 3. Leaves growing out of a fruit cluster, a sign of an excessively vegetative plant
Photo courtesy of Celina Gómez

6. Thin the body of the canopy.

In addition to lower-leaf pruning, canopy thinning, a more general leaf pruning from the body of the canopy, is sometimes recommended to help balance vegetative and reproductive growth. However, take into account that the number of leaves left on your plant will depend on the time of year and the variety grown: high light = more leaves to shade the fruit; dry climate = more leaves to increase humidity; wet, cool winters = less leaves to reduce humidity and/or disease pressure; vegetative varieties = less leaves; reproductive varieties = more leaves. Leaves are either cut with a sanitized tool or popped off manually at the abscission zone. Be careful not to tear off leaves, as this can lead to a slow-healing wound that can become an entry point for disease. Extra tip: it is best to remove leaves in the morning when plants are turgid.

7. Prune tomato clusters.

Cluster pruning is another practice to consider when targeting a consistent harvest. Tomato clusters with large number of fruit are not desirable because they will indirectly reduce individual fruit size and may result in fewer marketable fruit set by the newly developed clusters. Recommended fruit load varies with environmental conditions, production practice, and variety. Thus, it is best to follow breeder recommendations for a particular scenario. As a general overview, higher light availability will allow you to maintain more fruit. The opposite is true for light-limited environments. Keep in mind that the vigorous root system of a grafted plant may allow a crop to support more fruit per cluster relative to ungrafted plants. Also, remember that excessive vegetative growth will sometimes lead to suckers growing out of the fruit clusters (and sometimes from leaves!). If that’s the case, remove the suckers and adjust your environment to steer the plant in the right direction.

Understanding how to make proper use of the environmental control system in your greenhouse will help you maximize production. In addition, maintaining a properly pruned plant will help support a healthy crop and ensure good yields.

Celina is an assistant professor in the Environmental Horticulture Department at the University of Florida, IFAS. cgomezv@ufl.edu