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Growing towers are vertical tubes through which nutrient-rich water is diffused from the top, usually through a drip emitter, thereby creating ‘rain’ inside the tower as it drips over the plant roots that are suspended in the air. The towers, or columns, may either be hollow or filled with a substrate that provides support for the roots and aids in water dispersal. In its simplest form, a growing tower may be a section of PVC pipe with holes cut into the sides. In their comparative study of lettuce grown in a hydroponic tower system and a conventional horizontal NFT system, Touliatos et al. 2016 found that the tower system produced 13.8 times more crop than the horizontal system, calculated as a ratio of yield to occupied floor area. However, the mean fresh weight of the lettuces grown in the horizontal system was significantly higher than that of the lettuces grown in the vertical system. While crop productivity was uniform in the horizontal system, shoot fresh weight decreased from the top to the base of the tower, most likely as a result of gradients in nutrient availability and light intensity. Similar light gradients have been reported in other greenhouse trials using hydroponic tower systems (Liu et al. 2004; Ramírez‐Gómez et al. 2012). Strawberries grown in vertical PVC towers filled with perlite at a plant density of 32 plants/m2 produced a marketable yield of 11.8 kg/m2; however, yield per plant was reduced by 40 g with every 30-cm decrease down the height of the tower, as a result of suboptimal light conditions in the lower sections of the tower (Durner 1999). The diameter of the towers will also have an effect on plant growth. Water content values in tall and narrow towers will be lower than in shorter and wide towers having an equal volume of growing medium per unit length, and the roots of the plants will be subjected to larger daily temperature variations which may affect nutrient uptake and disturb the carbohydrate metabolism in the root, resulting in inhibited growth (Heller et al. 2015).
The Tower Farms aeroponic system (Figure 1) is modular: a three metre high tower could grow 52 leafy greens, herbs, or fruiting crops, or 208 microgreens. Each food-grade PVC tower is equipped with a 50 W small pump and a timer which turns the pump on for 3 minutes and off for 12 minutes on a continuous cycle. Although technically each tower has a footprint of less than 1 m², 2 m² per tower would include enough space for the towers, the dosing station, the aisle spacing, and the propagation bench area. In Europe the Tower Farms system is distributed by Ibiza Farm.
Figure 1: The Tower Farm system https://ibiza.farm/
In their survey of commercial aquaponics producers, Love et al. (2015) noted that almost one third of these used growing towers. However, comparative data on yields from aquaponic tower systems and conventional horizontal aquaponic systems are lacking. ZipGrow is a vertical hydroponic technology designed for high-density vertical crop production by Bright Agrotech, which operates a 400-tower vertical aquaponic system in Laramie, Wyoming (Figure 2). Their spacing density is one tower per every 0.7 m2. The crop is planted in a channel which runs the length of one side of each rigid UV-resistant PVC square tube. The plants grow in the company’s own patented growing medium called Matrix Media, which is made from recycled water bottles and a silicone oxide binder. The growing medium, which is irrigated from the top using drippers, provides many benefits to the aquaponic system. Firstly, it has an extremely high biological surface area of about 82-88 m2/m3, which allows the system to have very high nitrification rates and fosters healthy plant growth. Secondly, it has a void ratio of 91% due to its fibrous nature. This high porosity creates a highly aerobic environment for the plant roots and oxygen enrichment of the nutrient water trickling through the tower, and also allows for high percolation rates. Additionally, because of the aerobic environment, solids can collect and decompose on the media without creating an anaerobic microenvironment (Michael 2016). In Europe the ZipGrow system is distributed by Refarmers. A standard 5 ft (152 cm) tower provides mechanical and biological filtration for 0.7 to 1.1 kg of mature fish. A stocking density of between 12 kg and 15 kg per m3 is recommended.
As noted above, most tower systems experience a lot of light loss. This is especially true of 4-sided systems, which experience almost 90% light loss from the top front of the tower mass to the bottom rear of the tower mass, even when spaced generously. When ZipGrow towers are massed and managed properly, however, light loss is very low, even at 0.5-0.8 square metres per tower. There are three configurations that a grower can use, depending on their facility and crop types: massed configuration, line configuration, and facing aisles. Growers can also conserve light by using conveyor cropping (Figure 3).
Figure 3: Configurations and cropping regimes for ZipGrow towers https://info.brightagrotech.com/hubfs/blog-files/Infographics/ZipGrow\_Tower\_Spacing\_Guide\_- Bright_Agrotech.pdf
A 1.5 metre ZipGrow tower can grow 8-10 lettuce sized plants or 5-8 basil-sized plants, depending on the variety. Mass configurations of towers hanging in rows on a rack are usually the best option for commercial producers looking for high yields. When the towers are massed and managed properly, 0.7 m2 per tower is more than enough to get good crops with natural light. 50 cm of space between rows allows access the towers. The towers can also be mounted on walls (Figure 4).
Figure 4: Wall-mounted ZipGrow system https://commons.wikimedia.org/wiki/File:Urban_Vertical_Farm_With_Woman_%26_Child.jpg
The ZipGrow system was used in the GrowUp Box, a shipping container community aquaponic farm with a rooftop greenhouse in central London (Figure 5). The GrowUp Box has a footprint of only 14 square metres and can produce over 435 kg of salads and herbs and 150 kg of fish annually.
Figure 5: The GrowUp Box https://www.timeout.com/london/things-to-do/growup-box-tours
In the US, NaturePonics has developed BooGardens (Figure 6), a vertical system using bamboo grown in Indonesia and the Philippines which can be used for residential and commercial aquaponic, hydroponic or aeroponic applications. The bamboo harvested to make the towers regrows and can be reharvested three years later, making it the most sustainable growing tower system currently on the market.
Figure 6: BooGardens commercial aquaponics unit http://www.natureponics.net/boo-gardens/
A variation of growing towers is the stacked pot system, such as that produced by Verti-Gro for hydroponic cultivation. The five litre EPS pots, which provide insulation for improved root growth, can be stacked up to ten pots high, with each pot providing enough space for four plants. The pots are mounted on rotation plates on a PVC riser, which means that they can be turned easily for even reception of light (Figure 7). The system, which was patented in 1994, has been subjected to a number of scientific evaluations. Stacks of 6 pots were found to perform significantly better than stacks of 7 or 8 pots, both in terms of biomass, yield and fruit quality, because the composition of the nutrient solution changed as it passed through the column and negatively affected plant growth in the lower section (Al-Raisy et al. 2010). Light can also be an issue: the intensity of sunlight reaching the plant canopy at the bottom of a tower of seven pots was only 10% of that reaching the top, and the suboptimal light conditions in the middle and bottom sections adversely affected strawberry plant growth and fruit yield. Plants in these sections did not develop an optimal number of branch crowns, and subsequently produced less fruit compared with plants in the top section (Takeda 2000). Fruit quality is also influenced by the position of the plants on the tower, with those in the top tier having higher total soluble solids (TSS) and lower titratable acidity compared to those produced on lower tiers (Murthy et al. 2016). A comparative study of hydroponic strawberry production using stacks of four Verti-Gro pots and two types of horizontal system found that the lower light intensity at the base of the tower, and consequent lower photosynthetic rate, resulted in lower numbers of fruit, lower fruit weights, and fewer marketable fruits compared with the horizontal systems. Low light levels cause stamen sterility and poor pollen quality, and hence a reduction in fertilization rate, which can contribute to malformed fruit production (Karimi et al. 2013).
The benefits of being able to grow high densities of plants in growing towers need to be balanced with the amount of space that is required to provide an even spread of light as well as the row space required for management and maintenance. Row width must ensure that produce is not compromised by moving items such as trolleys and scissor lifts. The grow lights will impede people’s movements and thus they either need to be part of the growing structure, or retractable or movable so that workers can readily undertake tasks, or the planting structures will need to be movable and the lights remain static.
Figure 7: Verti-Gro system https://www.vertigro.com/Verti-Gro-4-Tower-System-Automatic-p/vgk-16agp.htm
Stacked pot systems are most suitable for growing large and heavy plants, such as fruiting crops. Grow with the Flow aquaponics farm in Denton, Nebraska, uses towers made from stacked pots to grow tomatoes and cucumbers, as well as herbs (Figure 8).
Figure 8: Growing towers in the Grow with the Flow aquaponic greenhouse https://commons.wikimedia.org/wiki/File:Vertical_Tower_Aquaponic_System.jpg
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