May 2018
Agdex 251/15-1
Development of Greenhouse
Lettuce Growing Techniques
in Alberta
L ettuce (Lactuca sativa) has excellent potential as
a year-round greenhouse crop. Lettuce, a good
source of Vitamin A, E and folacin, is considered a
healthy food choice.
Background
Lettuce is one of the most commonly grown
greenhouse vegetables in Alberta, after cucumbers,
tomatoes and peppers, and the primary product of
greenhouse lettuce is the head or leaf.
According to Statistics Canada, the Canadian
greenhouse production of lettuce in 2014 was
19.9 hectares. Alberta consumed 21,357 tonnes of Figure 1. Greenhouse lettuce growing systems
lettuce in 2011, about 20 to 30 per cent of Canadian
production, yet only 5 per cent of that total was in styrofoam boards that float in a pool of nutrient
produced in Alberta. solution.
Alberta greenhouse producers could increase lettuce For greenhouse producers, re-circulating aeroponic
production and replace imports. Profitable greenhouse and hydroponic systems allow for a very effective use
production depends on growers using production of growing space and nutrients, and these systems are
practices that maximize productivity and quality while well suited to the greenhouse production of high value
minimizing costs. crops such as lettuce.
Lettuce growing systems What is aeroponic?
Around the world, different styles of growing systems Lettuce can be grown aeroponically in A-frames with
are used for greenhouse lettuce (Figure 1). a nutrient solution sprayed on the bare roots (Figure
In Alberta, some large commercial greenhouses use a 2). Aeroponics is a system where the plant roots are
raft system for producing lettuce. The lettuce is grown suspended in the air and saturated with a fine mist
of nutrient solution at designated time intervals. Using aeroponics, the plants are totally suspended in
This technology is designed to minimize water air and can access up to 100 per cent of the available
consumption. oxygen in the air (Figure 3).
Figure 2. Roots suspended in air in an aeroponic system Figure 3. Plant roots are open to the air
With aeroponics, growers can produce lettuce year Some key elements to note:
round, with the potential for more plants per area as • growing method promotes root metabolism and
well as faster growth and bigger lettuce heads while plant growth
using less water and nutrient supply. • plants grow longer and faster: accelerates biomass
growth and reduces rooting time
The development of high performance, automated • use of fine droplets, 20 to 100 microns, produces
aeroponic systems for lettuce production signifies an excessive root hairs
economic advantage for Alberta-based greenhouse • large droplets, over 100 microns, leads to less
production for several reasons: oxygen being taken up by the plants
• improved water use efficiency (low requirement of • misting time: 1 to 2 minutes at different interval
water to operate against conventional hydroponic times up to 3 minutes
methods) • root formation, number of roots, root length all
• more growing cycles per year to reduce costs increase as dissolved oxygen increases
associated with greenhouse production (for • root zone temperature: cool nutrient solution
example, labour) holds more dissolved oxygen (DO) for root uptake
• improved integration of automation, re-circulation – nutrient solution carries dissolved oxygen
and space utilization in plant culture greater than 7 ppm (parts per million), but nutrient
solution temperature must be below 20 °C
These strengths may provide a cost-effective • lack of oxygen (O2) can cause root cell death and
sustainable production system for greenhouse lettuce. increase the risk of root disease such as pythium
Potential of aeroponics technique Root oxygenation and role of oxygen
Aeroponics is a research tool for nutrient uptake, Good aeration of the re-circulating nutrient solution is
monitoring of plant health and optimization of essential to provide oxygen to plant roots and reduce
crops grown in a closed environment. An aeroponic both the incidence and severity of root rot diseases.
computerized control system will monitor key When oxygen levels are low in the root zone, the roots
elements within the crop management targets during do not take up the nutrients required for growth. The
the growing season: more oxygen present (greater than 7 ppm), the better
the nutrient uptake and the better the root system.
• watering spray intervals (time between spray
applications) However, root zone temperature is very important.
• spray duration (length of time for each spray If the root zone temperature is high, then the oxygen
application) level will go down. The movement caused by the flow
• control strategies (nutrient injection and feed
recipes)
2
of water in the aeroponic system is usually enough to
keep the oxygen level high in the solution.
Aeroponics system design
An A-frame shaped aeroponic system of one movable
bench can be built with mounted, fixed spray nozzles
connected with 2 cm (3/4 in.) diameter PVC plastic pipe
(see Figure 2). In total, the growing area would be 9 m2,
with the supporting A-frame reaching 121 cm at the
peak.
In an aeroponic production system, the plants are
suspended in an enclosed environment by misting
the plant roots with an atomized, nutrient-rich water
Figure 4. Hydroponic system to grow plants
solution using the nozzles (each nozzle puts out 6.8
litres (1.8 gallons) of solution per hour) while the rest
of the plant (canopy) sits above the setup. The demand for hydroponically grown products is
increasing fast around the world (Figure 5).
A nutrient-injection program can consist of two
stock tanks plus one acid tank. A high pressure pump
delivers nutrients to the roots with a flow rate of
1,363 litres (360 gallons) of solution per hour. Fertilizer
and irrigation water are delivered through a fully
automated aeroponic nutrient-injection system under
the control of a computer system in two nutrient
regimes.
Advantages of an aeroponics system
• water use efficiency (80 to 90% less water use in
aeroponics versus hydroponics)
• re-use of nutrient solution
• space: plants stack vertically, roof-topping
greenhouses
• less weight Figure 5. Demand for hydroponically grown produce is
• increase water uptake increasing
• excellent aeration
Disadvantages of an aeroponics system Hydroponics system design
• clogged foggers, pump failure or breakdowns In hydroponic floating raft technology, one movable
(pipe) going unnoticed may damage plants in a bench can be built with a total of 9 m2 growing area.
short time The nutrient-rich solution can be delivered to the
• regular maintenance schedule, filter needs to be plants by filling the benches, draining them and
kept clean then pumping the solution back to the same bench
continuously.
Advantages of a hydroponic system
What is hydroponic? • can grow anywhere and produce crop year-round
Hydroponics is the method growing of plants using • planting out and automated harvesting is easier
water and the essential nutrients required without the than with aeroponic system
presence of soil (Figure 4).
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Disadvantages of a hydroponic system The feed target (ppm) of the nutrient solution should
• if a root disease appears, all plants in the system be maintained at the following levels:
will be affected, and plant death will occur rapidly • nitrate nitrogen 165 ppm
• uses more space and water compared with • phosphorus 50 ppm
aeroponic system • potassium 210 ppm
• calcium 180 to 200 ppm
• iron 3 to 5 ppm
• manganese 0.5 ppm
Growing conditions • zinc 0.1ppm
Temperature and humidity controls are also important • boron 0.5 ppm
for successful lettuce production. • sodium molybdate 0.05 ppm
Temperature: Temperature is an important factor at
all stages of lettuce growth. Lettuce is a cool weather
crop, so long days and hot summer temperatures can Cultivars
cause the crop to “bolt” or go to flower. Butterhead lettuce
1. Seed company - RIJK ZWAAN
During crop growth and production, cooling fans, • Alexandria:
ventilation and curtain shade cloths are used to ◦ has good size and performance
maintain a daytime temperature of 16 to 18 °C and ◦ popular cultivar in commercial greenhouse
a nighttime temperature ranging from 11 to 12 °C, lettuce in Alberta
depending on growth stage.
2. Seed company - BAYER
Sudden changes in temperature can favour disease • Frank:
development by causing condensation on leaves or
increasing the incidence of tip burn. Lettuce is highly ◦ great shaped lettuce with a well filled head
sensitive to freezing, which damages the leaves. and a good balance of outer leaves
Humidity: Excessive humidity will reduce transpiration, 3. Seed company - ENZA ZADEN
resulting in decreased nutrient transport. A relative • Fairly:
humidity (RH) of 70 to 80 per cent is generally targeted ◦ has remarkable reliable performance under
during lettuce production. different conditions
◦ high tolerance to internal tip burn
Oak leaf lettuce
Nutrient solution management 1. Seed company - ENZA ZADEN
Nutrient solution management allows for accurate • Pagero:
control of the fertilizer feed solution, which has a ◦ has highly attractive, nice fresh green
significant effect in increasing productivity and lettuce colour
crop quality. ◦ big-sized cultivar has very regular heads
Two factors have a significant effect on the growth of Romaine lettuce
greenhouse lettuce: concentration of nutrient salts 1. Seed company - BAYER
and pH, so these elements need to be tested and • Intred:
monitored regularly. In the winter, lettuce should ◦ unique internal red colour
receive a concentration of nutrient salts EC (Electrical ◦ very nice heart shape, medium size, good
Conductivity) from 1.9 to 2.2 mS/cm and in the shelf life
summer, 1.6 to 1.8 mS/cm. A pH of 5.8 for the nutrient
solution is optimum. 2. Seed company - BAYER
• Thumper:
Fertilizer and acid are added to the feed tank to
maintain pH and the appropriate nutrient levels for ◦ compact little lettuce with a good uniform
each cultivar and stage of crop development. head shape
◦ very stable
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Physiological disorders newly developing cells, so tip burn symptoms begin to
appear. These symptoms will appear even when plenty
of water-soluble calcium is in the nutrient solution.
Tip burn
Tip burn happens in both areoponic and hydroponic Methods to prevent this physiological disorder:
systems and with cultivars under supplemental winter
• Calcium levels in the nutrient solution must be
lighting (Figure 6).
high enough for sufficient calcium uptake by roots,
and transpiration rates must be high enough to
enable sufficient translocation of calcium ions to
the growing tips.
• Increasing ventilation and air circulation with fans
will increase transpiration and help keep moisture
from accumulating around the leaf surfaces.
• Use low temperature and low light with the
formation of the lettuce head.
• Reducing the application of nitrogen, harvesting
the lettuce slightly before maturity and
maintaining proper humidity will help prevent tip
burn.
Figure 6. Tip burn on greenhouse lettuce plant
Cost effectiveness and
productivity
Tip burn of young, inner lettuce leaves is caused The initial cost of building an aeroponics system
by a calcium (Ca) deficiency and is characterized by is higher than the initial cost required to build a
browning of the edges and tips of the leaves. Calcium hydroponics system.
is an immobile element and must be taken up in the
transpiration stream. Aeroponics: butterhead lettuce produces 360 heads/
cycle at 12 cycles per year, which will produce 4,320
Environmental conditions that reduce the transpiration heads per year.
rate – such as sudden temperature changes, too high
relative humidity (RH) too low light or low temperature Hydroponics: butterhead lettuce in the same
– can result in tip burn. conditions will provide 1,980 heads per year.
Under high RH, the plants are unable to evaporate There are significant differences between the
enough water to enable the transport of minerals automated aeroponic and hydroponic systems
(like Ca) to growing plant cells. In this situation, (Figure 7).
calcium uptake becomes insufficient to reach the
Aeroponic System Hydroponic System
Nutrient Solution Nutrients in mist of water droplets Nutrients in H2O solution
Water Use Efficiency Low re-circulating mist of droplets High re-circulating flow of water
Yield 360 lettuce heads in 9 square metres 180 lettuce heads in 9 square metres
Risk Factor High Medium
Figure 7. Differences between aeroponic and hydroponic systems
5
Depending on the cultivar, time of the year and
supplemental winter lighting, the yield/productivity of
the two systems will be quite different (Figure 8).
Yield (Number of Heads Harvested per Cycle per 9 m²)
400
360
350
Number of Lettice Heads
300
250
200 180
150
100
50
0
Aeroponic System Hydroponic System
Figure 8. Lettuce yield/productivity differs between areoponic
and hydroponic systems
Conclusion
Alberta’s greenhouse industry is positioned for growth
through access to new technology. An increasing
number of leafy crops like lettuce are being grown
hydroponically in more and more places around the
world. Aeroponics offers another way to increase
greenhouse lettuce production in Alberta.
Prepared by
Nabeel Mohammed
Alberta Agriculture and Forestry
Greenhouse Section
Crop Diversification Centre South
Brooks, Alberta
More information
Alberta Ag-Info Centre
Call toll free 310-FARM (3276)
Website: agriculture.alberta.ca
The development of this factsheet was supported
in part by Growing Forward 2, a federal-provincial-
territorial initiative.
