High-wire vs. Modified-Umbrella Training Systems for Hydroponic Cucumbers

Farmers across Pennsylvania have adopted high-wire trellising systems for their hydroponic cucumbers. This trellising system is like the drop-and-lean system commonly used for hydroponic tomatoes. Plants are supported by a string and trained to a single stem by removing all lateral branches. When plants reach the overhead support wire, the string is lengthened and moved a few feet further along the wire. This creates more area on the string to support additional plant growth. This process is repeated as the plant again reaches the overhead wire.

Before high-wire trellising, farmers commonly used umbrella or modified-umbrella trellising systems. In the modified-umbrella system, plants are supported by a string and trained to a single main stem with lateral branches pruned after one or two leaves. Once the main stem reaches the overhead support wire, an ‘umbrella’ is formed by allowing the main stem and a lateral branch to drape downward.

The high-wire trellising system was developed in the Netherlands in the 1990s in a high-profile greenhouse with a top wire set about 11.5 feet above the crop. We wondered if there was an advantage to high-wire trellising in the low-profile greenhouses we see across Pennsylvania. We conducted an experiment to compare yields between cucumber plants grown using the modified-umbrella and high-wire hydroponic cucumber training systems. We thought using the high-wire system would result in yields equal to or lower than the modified-umbrella system and a more consistent weekly harvest.

 We grew ‘Socrates,’ a Beit Alpha cucumber, in greenhouses at Pennsylvania State University’s University Park campus in the spring, summer, and fall of 2019 (three repetitions) using either a modified-umbrella or high-wire training system with a top-wire height of 7 feet. We used a plant population of about 4 stems/11 ft². With our assistance, students in our “Hydroponics and Aquaponics” course recorded the number and weight of cucumbers harvested weekly. All data were pooled over the three seasons and analyzed using SAS’s GLIMMIX procedure. When probability values were less than or equal to 0.05, means were separated using the slice option to perform Tukey's multiple comparison test.

First, let’s look at the number of fruit we harvested. The interaction between the training system and the season was not significant for the number of marketable cucumbers. As shown in the table below, we harvested the most cucumbers from plants grown with the modified-umbrella versus the high-wire system and in the spring and summer versus fall experiments.

^*Values within a column followed by different letters within a trellising system or season are statistically significant at P≥0.05. Please read this article for more information on interpreting tables like this: Science, Tomatoes, and How to Read a Table.

Next, let’s look at the weight of the marketable fruit we harvested. The interaction between the training system and season was significant for the weight of marketable cucumbers. The table below shows that about twice as many cucumbers were harvested using the modified-umbrella compared to the high-wire trellising system. However, in the fall experiment, yields were not significantly different between training systems.

* Values with a column followed by different letters are statistically significant at P≥0.05.

Lastly, let’s look at the consistency of harvest. As shown in the figure below, the number of cucumbers harvested weekly was more consistent (flatter lines) when using the high-wire compared to the modified-umbrella system (arched lines).

The modified-umbrella trellising system resulted in more marketable cucumbers, roughly double the amount, than the high-wire system. Plants grown with the modified-umbrella system had the potential to produce more fruit because branches were pruned after producing two leaves, whereas branches were completely removed using the high-wire system.  Also, once the main stem reached the top wire in the modified-umbrella trellising system, two branches were trained to become each side of the ‘umbrella.’ This again created the potential for more fruit, as cucumbers could be produced on these branches.

Leaf area index (LAI) measures the amount of foliage per unit ground area. LAI is an indicator of plant photosynthesis, growth, and other factors. For cucumbers growing vertically in a greenhouse, the optimal LAI is between 3 and 3.5 for light penetration, photosynthesis, and air circulation. This equals about 13 to 16 leaves per plant (Xiaolei and Zhifeng 2004). We did not measure LAI. However, we observed fewer leaves with the high-wire system than with the modified-umbrella system because the top wire height and row length limited vertical growth. Scientists (Ding et al. 2020) report that cucumber plants with 15 leaves are about 5 ft tall with a high-wire trellising system. These plants have an LAI of about 2.69 using a plant population of 2.8 stems/11 ft². In our experiment, the top wire was set at 7 ft, indicating that we likely had at least 15 leaves. However, we used a plant population of 4 stems/11 ft², suggesting our LAI was near optimal for the high-wire trellising treatment. With the modified-umbrella system, the ‘umbrella’ branches increased stem density to 12 stems/11 ft², very likely increasing the LAI to above optimum, resulting in cucumber fruit being aborted and reducing the number of potential fruit.

 Another factor suggesting LAI was above optimum when using the modified-umbrella trellising system is found in the weight of the cucumbers. Cucumbers grown in the summer and spring weighed less than twice as much as using the modified-umbrella compared to the high-wire system; however, the number of fruit was roughly twice as high. This indicates that the weight of individual cucumbers and the weight of all the fruit from a single plant were higher using high-wire than the modified-umbrella trellising system. Scientists have found that when sunlight increases by 8%, cucumber fruit weight increases by 3% (Marcelis 1998).

Using wider plant spacing with the modified-umbrella treatment may have resulted in more light penetration into the plant canopy and a more optimal LAI. While the number of potential fruit would be less, fruit abortion would also be less, and more cucumbers could reach harvest. Fewer plants would also reduce production costs.

Weekly yield peaked 5 or 7 weeks after harvest started and lasted between 3 and 4 weeks using the modified-umbrella system. Weekly yield was more uniform using the high-wire system. Having uniform harvests can help predict and match yields to market demand. In the USA, approximately 30 – 40% of the food in our food supply is wasted (United States Food and Drug Administration 2023). One cause of on-farm postharvest food loss is fluctuating yields leading to produce quantities above market demand (Porat et al. 2018). Uniform harvests can decrease postharvest losses. Succession planting on 3- or 4-week production cycles can even out the harvest peak and result in a more uniform yield using the modified-umbrella trellising system.

With low (about 7 ft) top wires in low-profile greenhouses, farmers can grow cucumbers with a modified-umbrella or high-wire trellising system. The modified-umbrella trellising system will result in higher yields at equal plant populations (below 4 plants/11 ft²). However, farmers may prefer consistent weekly harvests, as we saw when using the high-wire trellising system. Additionally, in Pennsylvania, many farmers use the high-wire trellising system with plant populations higher than 4 plants/11 ft². This may increase LAI to above-optimal levels, leading to lower individual and per plant fruit weight. However, overall yields per unit area may be higher. Scientists have reported (Hao et al. 2010) that higher production costs due to having more plants are a disadvantage of high-wire trellising systems compared to conventional umbrella systems. Using twin-head plants in a high-wire trellising system with European cucumber resulted in equal and consistent yield and lower production costs compared to single-stem plants in a high-wire trellising system (Hao et al. 2010). Beit Alpha hydroponic cucumber farmers may want to try this.