An indoor garden or greenhouse provides many opportunities for growth enhancement and increased yields as well as a pleasant environment to enjoy, however, a growing area is often annoyingly limited and space optimization then becomes of increasing importance. Given that a well-controlled growing environment comes at a cost with regards to energy inputs, equipment, and upkeep, maximizing the use of that limited space to give a high level of output is the objective of many growers. With the use of artificial lighting technology, particularly highly efficient and low-heat-output LEDs, space optimization in growing areas has become considerably easier. In greenhouses where only natural light is available there are also many system-design options to maximize productivity without sacrificing produce quality.
A single-plane hydroponic system is one that is commonly seen in many commercial greenhouses, particularly those growing taller plants such as tomatoes, capsicum, and cucumbers. These systems allow maximum interception of light from above by each plant and generally space is optimized as the plant canopy develops upwards for several feet. Smaller-framed plants such as lettuce, herbs, and strawberries, however, don’t necessarily optimize growing space when grown on a single vertical plane, particularly where there are light levels sufficiently high to support multi-level cropping.
Moveable Bench/Channel Systems
One design for single-plane systems, used in commercial greenhouses — largely for lettuce and other leafy greens — is the ‘moveable bench’ or ‘moveable channel’ system. This involves large benches of nutrient film technique (NFT) channels that can slide along rails on which they are supported. The main advantage of this is it reduces the need for numerous access pathways within the crop as occurs with fixed-channel systems. The NFT channels, that start off planted with small seedlings, are initially placed directly side by side and as the plants develop and require a larger spacing, the channels are gradually slid further and further apart to give each plant more room to develop. Mature crops are then harvested from one end of the system, while newly planted channels are introduced at the opposite end. Plant spacing increases as each channel moves along the production system. With NFT systems, the nutrient-delivery emitters are designed to be flexible to accommodate the change in gully spacing with plant growth. Moveable channel systems allow more plants to be produced per unit area than fixed bench systems. Such simple systems can be adapted to smaller growing areas and indoor gardens, where the moveable channels are adjusted by hand as plants increase in size, while still maintaining a single horizontal plane which maximizes overhead light interception.
The most commonly used vertical hydroponic systems are those that either incorporate a tower, column, or stacked design where plants are positioned around the sides often from floor to just under the light source, to staggered systems that allow for a more uniform pattern of light interception. Where the light source is only coming from directly overhead, planted tower or columns often run into problems with insufficient light in the lower levels of the system while plants at the top may end up too close to light source.
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With indoor vertical systems, and in some greenhouses, the uneven light interception can be assisted with the use of supplemental side lighting, however, this also takes up additional space. Vertical systems, however, can be successfully used under high natural light conditions provided the design takes light penetration, the effect of shading of upper plants on lower plants and other factors such as air flow into account. While vertical systems do significantly increase the plant density per unit area, if light is insufficient on the lower levels of these systems, produce quality can be reduced and overall yields will be lower than expected. Some crops, like strawberries, may seem well suited to a vertical or tower system, however, if light levels are limiting on the lower levels of the system, berry quality in terms of brix and flavor will suffer.
Staggered or tiered systems, often housing NFT channels, are another way of making use of vertical space, and have fewer issues with light penetration and intensity on the lower levels. Tiered systems are commonly used for small plants like lettuce, salad greens, strawberries, and culinary herbs. Basic-tiered systems using only natural light or overhead lamps may have between two to four tiers depending on crop type and light intensity. NFT channels may be in tiers directly one above the other or staggered into an ‘A frame’ or similar arrangement to maximize light interception on each tier. Indoor gardens have the advantage that lighting can be flexible within the tiered system, so plants are not solely dependent on a single overhead light source in the growing area. These systems have long been used for raising hydroponic crops such as seedlings, vegetative propagation, microgreens, baby salad leaf, and other short and compact plants. Each shelf or tier has hydroponic production channels or shallow beds with an overhead light source attached to the underside of the next tier frame.
Light levels need to be carefully controlled and assessed for these types of systems to provide sufficient and even intensity over the entire cropping surface without the risk of burning sensitive leaves as they grow upwards towards the light source. Tiered and staggered systems also need consideration when it comes to plant access — growers need to be able to harvest, plant, maintain, and view all the plants on each tier which can become increasingly difficult if the tiers are shallow. Another important factor with tiered systems is air flow and many poorly designed systems can become overcrowded to the point where air circulation is inhibited. This causes a number of common issues with vertical systems such as stagnant air reducing growth rates due to a lack of carbon dioxide (CO2) replenishment at the leaf surface and increased humidity levels around the base of the plants creating a higher occurrence of disease. Air flow is also essential for uniform heating and cooling around the crop.
While NFT systems with lightweight channels are a commonly utilized method of space optimization in vertical and staggered systems, other solution culture methods such as DFT (deep-flow technique), aeroponics, ebb and flow, and substrate systems are also used. Vertical stack, column, or tower systems are often substrate based and may incorporate drip irrigation or similar nutrient delivery methods. These may be either recirculating or closed systems, collecting the nutrient from the base of the tower system for reuse or open systems, where the nutrient waste is drained away after passing through the column. Coconut fiber, perlite, and stonewool are commonly used substrates in vertical systems, allowing good levels of moisture retention between irrigations as well as high levels of root zone oxygenation. Tall vertical systems can be prone to oversaturation of the growing substrate and root zone on the lower levels of the tower or column, and additional drainage and careful selection of a freer-draining substrate are important system design considerations.
Plants for Space Optimization Systems
Small-framed plants such as herbs, lettuce, salad greens, microgreens, and strawberries are the most commonly grown crops in vertical or other space optimization system designs, and these allow higher crop densities to be maintained. However, there are other crop options such as the use of dwarf or bush varieties of hot and sweet capsicum, cucumber, and tomato, particularly in moveable bench and staggered systems. Genetically compact growth habits or mini varieties bred for container production can be high yielding and a good option for a space optimization system, provided sufficient light intensity is provided to support high-quality fruit production. Plants may also be pruned and trained to fit the system and prevent excess upward growth, this is particularly relevant with cut and regrow salad and herb crops. Where a number of different lettuce types are being grown in tiered systems, it is advisable to have the colored types (red and brown varieties) on the upper levels as higher light is required for good coloration, and the green types on the lower levels.
Smaller indoor gardens can take advantage of space optimization designs just as easily as larger commercial greenhouses, however, some modification may be required depending on the space available. Growing channels can be wall mounted where that is an option, provided an air gap is provided between the wall surface and the system to allow air to flow all around the plant. These types of systems are particularly attractive and productive with crops that can be trained to trail downwards such as rosemary and thyme or ‘tumbler’ tomatoes and many flowering annuals.
Space optimization in both small indoor gardens and larger greenhouse environments services the vital purpose of maximizing yields and productivity from a limited area. However, it is not as simple as cramming in as many plants as possible within a small space as light then becomes the limiting factor for productivity and restricted air flow can lead to high humidity and disease outbreaks. Careful consideration of the height, design, light penetration, and intensity and air flow must be given to ensure a space optimization system is going to give maximum benefits.
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