Lettuce Handbook

Greenhouse Hardware

Of fundamental importance to hydroponic lettuce production are the physical components of both the germination area and the pond area.  It is necessary to have not only an idea of the physical components associated with each area, but also a good understanding of their purposes.

Germination Area
The first 11 days of lettuce production takes place in the germination area.  Seedlings develop best under constant lighting conditions with specific, closely controlled temperature, relative humidity, carbon dioxide, and irrigation.  These conditions can best be met in a controlled area with the following equipment:

• Ebb and Flood Benches

• Solution Tank and Plumbing

• Lighting

• Aspirated Box

• Sensors

Ebb and Flood Benches

This is a photo of an empty Ebb and Flood bench while the bench is flooding for sub-irrigation.

To uniformly supply the germinating seedlings with water and nutrients, Ebb and Flood benches (approximately 2.5 by 1.3 cm) are periodically (2 to 4 times per day for approximately 15 minutes) flooded.  These benches were specifically designed to supply water and nutrients through sub-irrigation.  Through a pump and piping, the fertilizer solution is pumped into the Ebb and Flow bench.  The solution is then automatically drained after a given time period.

Humidity covers are used to provide a high humidity environment around the germinating seeds.

Solution Tank and Plumbing

A fiberglass tank (A) holds the nutrient solution used for sub-irrigating the seedlings.  Approximately 250 L of nutrient solution is sufficient to prime the system, fill the bench, and provide nutrient solution for the first 11 days of growth for approximately 2000 seedlings.  A small (Teel Model 1P808A 1/50 h.p.) pump (B) is used to pump the solution to the bench.  The piping (C) should be flexible to adjust to individual germination area needs.  A throttling or gate valve (D) is included to control the flow of the nutrient solution to the Ebb and Flow bench.  The bottom of the sub-irrigation bench (E) is visible in the photo above.

Lighting

Type
Cool white fluorescent (CWF) lamps (A) are recommended, but High Pressure Sodium (HPS) can also be used.  Heat generated by the lamps must be dissipated from the germination area in order to maintain the temperature set points.  Use of incandescent lamps (B) is discouraged because the red light emitted from these lamps causes the seedlings to 'stretch'.  Fluorescent lamps are rich in blue light, which cause short and squat seedlings.

Configuration
The lamps should be configured for a uniform distribution of light over the entire growing area.  Light intensity is maintained at 50 micromoles/m^-2s^-1 of PAR (Photosynthetically Active Radiation) during the first 24 hours the seeds are kept in the germination area.  For the remaining 10 days, the light intensity is maintained at 250 micromoles/m^-2s^-1.  The photoperiod (or day length) is 24 hours.  Shorter photoperiods are acceptable if the light intensity is increased to provide the same total daily accumulated light (~22 mol/m^-2d^-1).  Light output of CWF lamps decays over time.  Thus, it is important to measure the light output of the lamps regularly.  If the light intensity drops below an acceptable level (e.g. 200 micromoles/m^-2s^-1), new lamps should be installed.  A quantum sensor can be used to measure the amount of PAR.

Every three months 25% of the lamps should be replaced.  The lamp life is about 7500 hours, and by 5000 hours about 50% of the light output is lost.  If the lamps were replaced all at one time, there would be an enormous increase in total light output between installation and replacement.  Replacing only a portion of the total lamps means some lamps are working at their maximum light output and some are declining in efficiency, and the fluctuation in total light output is minimized.  This exchange system will make for more uniform lighting, which is critical for uniform production.

Aspirated Box

This is an example of an aspirated box which houses and protects the computer sensors from light or localized temperature fluxes.  The position of the box should be close to the plant canopy to measure the environmental parameters at the plant level.  This may not be possible in all germination areas.  The box is equipped with a small fan (A) which draws air past the sensors (B).  Sensors are located upstream from the fan.

Sensors
See "Sensors" under Computer Technology for full details.

Pond Area
The concepts involved in the pond area are the following:

• Pond Size
• Pond Solution
• Construction
• Pond Design
• Lighting
• Turbulator Fan
• Aspirated Box

Pond Size
For example, for the production of 1000 heads per day a 660 m² growing area is required. The lettuce plants are grown in the pond area for 21 days. This includes one spacing of the plants at Day 21, from 97 plants m^-2 to 38 plants m^-2.

Pond Solution
Equal portions of Stock Solutions A and B are added to RO water to achieve an EC of 1200 microS cm^-1.

Construction

The pond may be sunken in the greenhouse floor, with the pond surface just above the floor or a containerized pond with concrete walls can be constructed. The pond floor can be layered with sand to cushion any sharp edges from puncturing the polyethylene lining. A heavy plastic (for example, 0.5 mm poly) liner is then installed as the major barrier for leak protection. Proper precautions should be taken to avoid leaks.

Design
The pond area is designed to allow for one plant spacing on Day 21.  To facilitate the spacing process, multiple ponds run in parallel.  The plants are grown in one of the ponds between days 11 and 21.  After respacing (from 97 plants m^-2 to 38 plants m^-2) the plants are moved to one of the remaining ponds where they will be grown for two weeks (day 21 through day 35).

Lighting
Uniform light distribution is required in the Pond Growing Area.  A supplemental light intensity within the range of 100-200 micromoles/m^-2s^-1 (for a total of 17 moles/m^-2d^-1 of both natural and supplemental lighting) at the plant level is recommended.  High pressure sodium (HPS)lamps are used to supply light.  These lamps are relatively efficient, have a long life, and slowly decay in output over time.  Independent lighting consultants have specialized software to determine proper number and placement of lamps needed for a specific and uniform light intensity.  It is critical to have the correct lighting system installation.

Because the CEA lettuce program is production-intensive, lighting and electrical power usage is high.  Local utility companies should have information on special rates and rebate programs for new industries and Controlled Environment Agriculture facilities.

Turbulator Fan

An overhead fan is used to blow air vertically down onto the lettuce plants.  The airflow increases plant transpiration.  This increase in transpiration increases the transport of nutrients, especially calcium, from the roots to the young, fast-growing lettuce leaves.  The greater rate of nutrient transport provides sufficient amounts of calcium to the leaves and, therefore, prevents tipburn.  Without this airflow, lettuce must be grown under reduced light levels, which slows the rate of growth.

Aspirated Box

The aspirated box located in the pond area has the same function as the aspirated box in the germination area.