If you found our write-up on growing greenhouse tomatoes to be of interest, you might also like our article about growing greenhouse lettuce:
Hydroponic Lettuce: A New Way To Grow An Old Crop
Even though it’s the dead of winter, and frosty cold outside, within our greenhouse the temperature is in the seventies, well within the comfort zone of both man and plant.
And from one end of the greenhouse to the other, row after row of lush, emerald-green butterhead lettuce plants are soaking up the winter sunshine.
But the lettuce plants are not growing in the ground; instead, the plants are growing in long, plastic tubes mounted about waist high.
This is a hydroponic greenhouse, in which soil is neither required nor desired.
There isn’t even any soil in sight, for the floor of the greenhouse is completely covered with a ground cloth, which is white so that it will reflect sunlight back up into the plants.
How it Works
The hydroponic system we use to grow lettuce is really very simple, both in concept and in execution. We utilize a hydroponic method known as the nutrient film technique (NFT).
In this system, the lettuce plants are grown in rectangular, flat-bottomed tubes. Holes are spaced in the top of the tube at eight-inch intervals, with one lettuce plant growing in each hole.
Nutrient rich water is continuously pumped from a holding tank into one end of each tube. The nutrient solution flows from the input end of each tube to the drainage end, bathing the roots of the lettuce plants with a constant supply of both water and nutrients.
The quantity of solution entering each tube is restricted so that only a very shallow layer, or film of solution is flowing along the bottom of the tube.
As it drains from the tube, the nutrient solution is carried by a gutter drain back into the holding tank, where it is re-circulated by the pump.
In the wintertime, the nutrient solution is heated so that the root zones of the plants remain at an optimum temperature.
We built the framework that supports the tubes from galvanized pipe purchased from a local fencing supply company.
The framework must be assembled with precision to ensure that the tubes are supported with the proper gradient. The nutrient solution flowing through the tube must not flow too fast or too slow, and must flow at a steady rate throughout the length of the tube without any ponding.
All air drawn into the greenhouse for ventilation is filtered through a fine screen, which helps to exclude insects.
Any insect pests that do manage to become a problem are controlled with beneficial insects. For most bad bug species, there is a corresponding good bug species whose favorite snack just happens to be the bad bug you are trying to eradicate.
The beneficial insects are available through a number of commercial sources, and can be a very effective tool for controlling insect pests. We never spray so much as a drop of pesticide on our lettuce.
The NFT system, in conjunction with the shelter provided by the greenhouse works quite well for growing lettuce.
The enclosed structure of the greenhouse permits a certain degree of control over environmental variables such as temperature and humidity, which in turn helps to maintain a steady rate of growth. We’re able to harvest lettuce from October until it becomes too hot in early summer.
Each time a crop is harvested, another crop is started to take its place, so that at any given time there are multiple crops growing in various stages of maturity.
The Growing Process
Each lettuce plant begins life in a 1-inch foam Oasis cube. The Oasis cubes, which come in sheets of 162 cubes, are grown on a propagation table, separate from the main NFT system.
A timer controlled flood-and-drain system keeps the cubes moistened, and heat mats under the cubes maintain the ideal temperature for germination. The seeds usually germinate in two days, and are allowed to grow on the propagation table for no more than two weeks.
The sheet of Oasis cubes is then broken apart into the individual cubes, and each cube with its seedling is placed in a tube in the NFT system.
After spending one week in the nursery tubes, the plants are transferred into tubes with the holes spaced eight inches apart, called finishing tubes, where they remain until they are harvested.
When the plants are transferred from the nursery tubes into the finishing tubes, the finishing tubes are initially crowded together, with each tube touching its neighboring tube.
Once the plants have grown enough that they are beginning to touch plants in neighboring tubes, the tubes are moved apart to a spacing of six-inch centers, and then to a final spacing of eight-inch centers.
Spacing the tubes progressively farther apart as the plants grow helps to attain maximum utilization of the greenhouse interior space, growing far more plants than would be possible using only the final eight-inch spacing.
When the lettuce plant is harvested, it is removed from the tube, and the long roots are trimmed, but not completely removed.
The plant is then placed in a plastic clamshell or a plastic bag, which acts as a ‘mini-greenhouse,’ providing each plant with its own moist microenvironment.
We call our lettuce “living lettuce” because the roots, which are packaged with the plant, are able to continue supplying the plant with moisture, allowing the leaves to remain fresh, crisp and flavorful for an amazing length of time.
Our lettuce is as fresh as you can get without growing it yourself. But it’s also important to note that the chance of the lettuce being contaminated with something such as E-Coli (as has occurred in some organically grown crops) is extremely remote.
Compared to field grown crops, hydroponic greenhouse lettuce is grown in a relatively pristine environment. Pathogens that may exist in the soil are of no concern since the plants are never in contact with soil.
No System is Perfect
There are, of course, disadvantages that come along with the many advantages of our method of growing lettuce. It’s certainly not a set-and-forget system.
We must continuously be on the alert for problems with the re-circulating system.
Pumps can fail and feed tubes can clog, and if the system fails for a prolonged period on a hot day, it’s possible to lose every plant in the greenhouse.
It’s a labor-intensive system of growing, but also a very productive system, which uses only a tiny fraction of the land, water and other resources that a field-grown crop would require for the same quantity of production.
And it produces some wonderfully crisp, tasty and nutritious lettuce, which our customers sure do seem to appreciate!