Lettuce Handbook

Lettuce Production

This handbook is directed toward a daily production of 5 ounce (150 grams) heads of leaf lettuce.  The production of the lettuce crop is separated into two growing areas.  Seeds are started in a germination area where they germinate and grow for 11 days.  On Day 11, the plants are transported to the greenhouse and transplanted into the pond area where they are grown until harvest on Day 35.

Germination Area Stage
The Germination Area stage is scheduled for Production Days 0-11.

Day 0 - Sowing
Production begins with the making of the germination media.  The media fills 12 plug trays of 128 plugs each.  The media can be moistened with RO water so as to increase the mass of the media a little.  One lettuce seed is placed into each plug.  This can be done with an automated seeding machine.  The trays are then misted with RO water to soak the media thoroughly which settles the seeds into the media.  The 12 trays are placed in the Ebb and Flood bench within the germination area.  The trays are sub-irrigated with RO water for 1/4 hour every 12 hours.  For the initial 24 hours, lighting is maintained at 50 micromoles/m^-2s^-1 with a photoperiod (day length) of 24 hours to ensure good germination.  The temperature is set for 20C (68F).  The seed trays are covered with plastic humidity covers to ensure a high relative humidity which prevents desiccation.

Day 1 - Environmental Adjustment

A fertilizer solution is added to the sub-irrigation water 24 hours after sowing.  The EC of the water is maintained at 1200 microS cm^-1.  The pH of the solution is adjusted to 5.8 with possible addition of a base, potassium hydroxide (KOH).

The temperature is raised to 25C and the lights increased to 250 micromol m^-2s^-1.  These environmental factors are maintained for the remainder of the crops' time in the germination area.  Sub-irrigation continues for 1/4 hour every 12 hours until Day 6.  The photoperiod remains at 24 hours.

Day 2 - Decreasing Humidity

The humidity covers in place on Days 0 and 1 are removed on Day 2.  At this time, the seed has germinated and the radicle has started to penetrate into the soil, as can be seen in the above photo.  High humidity levels during the first two days of germination are to ensure the seed does not desiccate.  Low lights levels during the first 24 hours work in conjunction with the high humidity to prevent excessive seed drying.

See relative humidity under Computer Technology for more information.

Day 3 - Removing Double Seedlings

Any double seedlings should be removed from the plugs on Days 3 or 4 to ensure a uniform crop. Also, germination percentage can be determined to monitor seed quality and proper growing conditions at this stage. It is critical to have consistent environmental conditions and consistent plant growth during this stage.

Day 4

Day 5 - Seedling Selection

Seedlings are selected on Day 6, depending on the size and expansion of their first true leaf (~ 1 cm diameter). Seedlings which have unacceptable growth are discarded. Expect to discard 20-30% of the seedlings. Any seedlings with good growth at this time will be acceptable at the time of transplanting. Seedling selection is a vital process for the uniformity of the crop.

Day 6 - Increasing Watering Frequency

The lettuce seedlings have grown to such a size that they now require watering more frequently. The sub-irrigation system is scheduled for flooding four times per day, or every six hours, for 1/4 hr (15 min).

The following is a series of photos showing the growth of an individual lettuce seedling over a 5 day period.

Day 7

Day 8

Day 9

Day 10

Day 11

At this time, the leaves are beginning to overlap.  The roots of the seedlings have grown through the bottom of the plug tray.  When transporting the plugs to the pond area, avoid damaging these exposed roots.

Transplanting
On Day 11, the seedlings are transported to the greenhouse and transplanted into the pond.  Prior to transplanting, the seedlings are thoroughly sub-irrigated.  Transplanting can be scheduled to follow normal sub-irrigation periods in order to prevent desiccation during transfer.

The seedling plugs float in the pond in Styrofoam floaters.  Each plug is inserted into a pre-cut, square, centered 1.9 cm² hole in each 10 by 10 cm Styrofoam floater.  The floaters with seedlings are then placed and positioned in the pond.

Continue on to Pond Area Stage - Lettuce Production Days 11-35.