5 "Must Dos" for growing a quality greenhouse lettuce crop

Lettuce seems to be the crop of choice for growers looking to start producing controlled environment food crops. Many of the new controlled environment growers that have begun operating over the last five years have started with producing lettuce and leafy greens.

"For many people producing lettuce is one of the easiest crops when it comes to providing the right amount of water, light and cool temperatures," said Ramón Melón Martinez, an agronomic consultant and expert hydroponic leafy green grower. "This changes when trying to grow a consistent quality crop year round. There is the pressure of trying to grow without making any interruptions in the production system for multiple years and trying to increase the production level on a yearly basis.

"In most cases lettuce is a plant that is sold complete, so there is no room for mistakes that will affect the look of the product. And because of the way lettuce is marketed to retailers in year-round programs, growers don't have the option of explaining why plant leaves may look slightly burnt or why plants are wilting five days after harvesting. This requires a strong discipline in crop and scheduling management be implemented in the growing facility to maintain continuous production success. This discipline needs to be focused on following these five must dos."

1. Limit the hiccups

Lettuce plants are very resilient and are able to overcome specific production issues.

"The question is how many hiccups or issues are the plants able to tolerate and what are the most important hiccups that have to be overcome?" Martinez said. "It depends on the greenhouse production system. Lettuce cultivation in a greenhouse has the challenge of adapting the internal conditions to fluctuations in the weather and production system operation. 

"Any stress on the plants, be it from weather conditions or a clogged irrigation filter, if it is repeated over the production cycle, it can generate structural damage to the plant composition. This could lead to a shortage of calcium in the growing cells with different severities or collapse of the meristem preventing proper development of the plants."

Plant stress is a hiccup that could lead to a shortage of calcium in the growing cells with different severities or collapse of the meristem preventing proper development of the
plants. Photos courtesy of Ramón Melón Martinez

Martinez said hiccups can happen for numerous reasons and there are multiple ways of avoiding them, mostly by ensuring a good energy balance on the crop and stable fertigation and irrigation. It is quite likely that if production conditions are not stable, the end result is damaged plants.

2. Maintain the proper leaf area index

As the industry moves toward mobile production systems the goal is to have lettuce plants at the same physiological stage year round.

"The challenge for growers will be keeping all the plants moving at the same rate of growth," Martinez said. "Whatever production system is used it is critical to understand that the climate needs to be adjusted to the physiological state in which the plants are at a specific time.

"The easiest way to do this with lettuce cultivation systems is to focus on leaf area index (LAI) to adjust the growth of the plants. A high LAI promotes the growth rate of the plants if conditions ensure no disease pressure. For a high LAI the microclimate surrounding the plants is more stable which allows the stomata to remain open for a longer time as there are less humidity fluctuations."

However, there is a limit to LAI management.

A high leaf area index (left photo) can produce leaf stretching and lead to a higher disease pressure. A low LAI (right) produces compact plants that require longer development time.

"Plants competing for space will have to stretch to search for the light," Martinez said. "In that specific moment two things are happening: First, there is a loss of energy because instead of developing new leaves, the old leaves are elongating. Secondly, there is postharvest loss. Elongation is accomplished by intracellular water accumulation. The more water in the cells causes a shorter postharvest life.

"A high LAI can produce stretching and lead to a higher disease pressure. A low LAI produces compact plants that require longer development time. The perfect LAI means there is a good balance between plant growth and plant morphology."

Maintaining the right LAI requires monitoring the plants from the seedling stage. If seedlings are allowed to stretch, then the plants will always be stretched.

"In spring there can be a rapid acceleration of growth," Martinez said. "When lettuce leaves start touching this can have a physiological effect on the plants. The leaves start stretching for light. This stretching comes with a cost to the plants and it comes with water accumulation as well. Plants with leaves that are at the wrong angle for light interception have a longer growth cycle. These plants will also have a higher water content making them less resistant to pests and diseases."

3. Manage irrigation piping, water and the root zone

The root environment and water biology play a critical role in plant development.

"Modern hydroponic production systems are based on continuous operation that tends to overlook the constant release of carbon to the environment," Martinez said. "Plants release about 20 percent of fixed carbon in root exudates that sustain microbial populations which colonize on the roots. The microbiota around the roots provide the plants with increased nutrient solubility, fixed nitrogen, competitive suppression of pathogens and plant growth promoting molecules.

"If this hydroponic microbial community, which is not as abundant as soil microbiota, is not balanced and controlled it can lead to a proliferation of pathogens or have a secondary effect on nutrient uptake that can make it harder for plant development. This is why it is essential to monitor, control disinfection and bacterial addition and provide water filtration to maintain continuous operation."

Martinez said there has been a big push from the industry to disinfect production systems in order to control pathogens.

The microbiota around the roots provide plants with increased nutrient solubility, fixed nitrogen, competitive suppression of pathogens and plant growth promoting molecules.

"Many of the mobile production systems are one irrigation system with some growers having two," he said. "It is complicated to clean these systems without stopping production for two to three weeks. This may be easier to do during the summer when more crops are coming from the field.

"In North America right now there is not a high differentiation among consumers for hydroponic greenhouse lettuce vs. field-grown lettuce. But food safety issues are becoming more important, which means having a continuous supply of lettuce year round is becoming more critical. As a result, growers need to pay close attention to what the plants are doing and what is happening with the water and the piping becomes more critical."

One of the issues than can arise in irrigation piping is the formation of biofilm.

"Biofilm is an anaerobic type of bacteria that grows in different layers in the piping," Martinez said. "Biofilm normally requires calcium and magnesium deposits on the inside of the pipe. These deposits become an attachment point for biofilm bacteria to start to develop. The biofilm can start to effect the root growth and the plants start to grow more slowly.

"If growers have problems with root diseases the plants are still salable. That's not the case if there are diseases on the leaves. Plants with bad roots may take longer to finish, but they can still be sold. If there are diseases on the leaves, chances are those plants are not going to be salable. If the plants have a healthy root system, then most disease issues can be overcome. I'm always more careful with the plant roots and observing them more often. If the plants have poor roots, then there are going to be problems."

4. Start with a production budget

Martinez said there is nothing worse than starting a greenhouse lettuce operation without having secured the supplies necessary to produce the crop and knowing how that crop is going to be marketed once it's harvested.

"When a new operation starts it is hard to know all the numbers, but starting with an approximation causes the grower to focus much closer on crop management and inspection," he said. "Production budgeting needs to focus on the crop time required to reach harvest weight. This information needs to be constantly recorded to make budget changes. Once a one-year crop cycle is finished it can be used as template for the coming years updating possible improvements or using data analysis to review variations in the production.

"The budget process is constantly being refined. Growers need to continuously review their budgets every six to 12 months so that they make adjustments. These reviews should be driving growers to continuous improvement."

Production budgeting needs to focus on the crop time required to reach harvest weight. Once a one-year crop cycle is finished it can be used as a template for the coming years. This allows for updating possible improvements or using data analysis to review variations in the production.

Martinez said as important as it is to forecast production, it is equally important to be able to forecast sales. 

"Growers need to be able to forecast when crops should be started and when they will finish," he said. "They have to know the crop cycle so that if there is more finished product coming they're ready to start marketing that product and plan when crops will be ready to market."

5. Aim for high germination rates

With the amount of lettuce seed that is being used in greenhouse production systems it is really important to look at germination rates. A one percent loss in germination can be very costly.

"Commonly overlooked, an even germination rate is key to a stable operation," Martinez said. "Understanding that different types of lettuce may have different times for seed emergence is critical to optimize production. Two days at a constant 66.2şF (19şC), a saturated substrate and greater than 95 percent relative humidity should ensure a good germination rate for most lettuce varieties. Lower germination rates have a major impact on transplanting.

"It takes time to figure out the problems associated with poor germination. Having poor germination rates is going to affect yields and will impact the morale of the workers sowing the seed and transplanting the seedlings. There can be multiple reasons for germination problems, including the seed, the substrate and the irrigation. In some cases, it takes trying to germinate four to five crops to determine what is causing germination problems."

Source and All Photo Courtesy of Urban Ag News

Source: Urban Ag News