Greenhouse Production Systems
Substrate Culture Systems
Lay-Flat Bag Container Culture
Bag culture is a production system where greenhouse plants (e.g., vegetables) are grown in a soilless mix contained in a polyethylene bag (See Figure 14.13). The bag can be sealed around the mix, or it can be an open bag. The closed bags are laid flat on the greenhouse floor with plants growing from planting holes cut along the upper surface of the bag after it is laid flat in place. These are called “lay-flat” bags. Lay-flat bags contain the media, usually peat-based mixes, perlite, coconut coir, or rockwool, in a totally closed bag. Growers can make their own bags, but most often these bags are purchased prepared. Bags are typically made of UV-resistant polyethylene, with a black interior, and generally last for two years. The exterior of the bag should be white in regions of high light intensity levels, to reflect radiation and inhibit heating the growing medium.
Advantages and Disadvantages
Substrate culture in lay-flat plastic bags have shown advantages over other substrate systems, due to its light weight, reduction of evaporation and less risk of disease contamination. It has reduced installation costs, since the bags do not need rigid support on level surfaces. Several different materials have been used, alone or in mixtures, as substrates on bag culture, specially rockwool, perlite, peat, pine bark, and coir. The major disadvantages of the lay-flat bag system are the need for a leachate collection system and the need to replace the media every one to three cropping seasons.
Rockwool Culture
Rockwool is a widely used substrate in lay-flat bag systems for growing vine crops, especially tomatoes, cucumbers, and peppers. Rockwool slabs are enclosed in a white polyethylene sheet cover. They are available in a number of sizes raning from 35.5 x 12 x 2 inches (90 x 30 x 5 cm) to 35.5 x 18 x 3 inches (90 x 45 x 7.5 cm) (length x width x thickness) (See Figure 14.14). The trend is to use the wide slabs and grow four to six plants in them.
Floor Design. Before placing the growing slabs out, the greenhouse floor area should be disinfected and the soil surface (floor) leveled. Soil or sand makes the best floor as it can be easily formed to obtain good drainage. A slight slope toward the center between the two slabs will provide adequate drainage away from the slabs.
Rockwool Slab Layout. Rockwool culture is similar to other lay-flat bag culture in greenhouse layout and operation. Slabs are laid in twin rows set 2 to 2.5 feet (60 to 75 cm) apart for cucumbers. With tomatoes and peppers the slabs may be closer, generally 16 to18 inches (40 to 45 cm), depending on the specific plant row spacing required.
Irrigation System. A drip irrigation system with a fertilizer injector provides nutrients to each plant individually. The most common way of applying nutrient to rockwool slabs or large cubes is with the use of dippers. A simple drip irrigation system should use a dripper with a capacity of 0.5 gal/h (2 L/h), with one dripper per plant.
Transplanting. Transplants are started in small rockwool cubes, then the cubes are placed in larger transplant blocks, which are approximately 3 inches square. The blocks with the transplant are then placed in the greenhouse on the slabs where the plant will eventually root into the slab media. The rockwool slabs must be soaked with nutrient solution of EC 2.5 to 3.0 mS/cm and pH 6.0 for 24 to 48 hours before transplanting. This will adjust the pH and uniformly moisten the slabs. Place the slabs in their final position, with three drip lines entering a small slit on top of each slab at equal spacing. Operate the nutrient system until the slabs swell as they fill with solution.
Media Reuse. Successful media reuse requires significant time investment and care to produce small financial savings. For many growers, the risk is not worth the savings. Some attempts to reuse rockwool slabs have not been successful. One potential problem has to do with irrigation management and Pythium root rot potential. If slabs are not well sterilized, Pythium root rot can be a problem in the second crop. Reused slabs also contain considerable organic matter (old roots).
Click on the following topics for more information on greenhouse production systems.
