How to Manage Pests

UC Pest Management Guidelines

Lettuce

Irrigation Of Head And Romaine Lettuce

(Reviewed 4/17, updated 4/17)

In this Guideline:

Head lettuce requires sufficient water to replenish moisture lost through evapotranspiration (ET). Irrigation also serves to cool the crop during hot weather through evaporation and to leach accumulated salts from the root zone. Most of the water used for lettuce production is during the last 30 days of the crop, when evapotranspiration rates are highest. The amount of water needed to grow lettuce depends mainly on:

• the weather during the crop cycle,
• salinity of the soil and irrigation water,
• physical properties that affect the water-holding capacity of the soil, and
• irrigation practices.

Soil Texture
Loam-to-clay loam textured soils are best suited for lettuce production. Sandy textured soils require more frequent irrigations or more water applied than loam soils to maintain similar production levels. Clay textured soils with good drainage are also used for lettuce production. These soils are often ripped or deep chiseled to help improve drainage.

Water Salinity
Water salinity levels above 1 deciSiemens per meter (dS/m) have been shown to reduce lettuce growth and yield, and may lead to a build up of salt near the soil surface. Soil salinity levels greater than 2.1 dS/m in a soil paste extract can reduce lettuce yield. Extra water is often applied to leach out salts that build up in the soil. A leaching fraction of 30% (30% more than amount applied to meet the crop ET demand) will assure that the soil salinity level approximates the salinity of the irrigation water. Lettuce is most sensitive to salt during germination and stand establishment. In the southern deserts, water is applied with overhead sprinklers to leach salts from the soil surface before and during germination.

Irrigation Methods
Irrigation methods for lettuce include furrow, sprinklers, and drip.

Furrow. With furrow irrigation, it is important not to over saturate the beds—excess moisture favors the development of bottom rot. Gated pipe is also used to deliver water, especially near harvest. Gated pipe allows uniform application of water down furrows and maintains dry access into the head of the field for field equipment. Surge valves can be used with gated pipe to improve uniformity and reduce run-off in fields with furrows longer than 600 feet.

Salinity can be more challenging to manage with furrow than with sprinkler or drip because salts tend to accumulate on the bed tops. In situations where salt accumulation is a potential problem, apply extra water by sprinklers during pre-irrigation and germination to leach salts from the beds before switching to furrow irrigation. Under furrow irrigation, salts will tend to accumulate in the center of the bed. Alternate furrow irrigation can often move the salt front towards the shoulders of the bed, beyond the plant row.

Sprinklers. Lettuce can be irrigated to maturity with solid-set, hand-move, linear-move, or permanently buried sprinkler systems. Solid set sprinklers are often set up in a field on a 30 ft x 30 ft to 30 ft x 40 ft grid. The uniformity of sprinklers decreases greatly in wind speeds above 5 mph. Shifting the location of the lateral lines between irrigations with hand-move sprinklers and between field operations with solid-set sprinklers will help even the distribution of moisture in the field, preventing growth patterns. Self-propelled linear-move sprinklers reduce the labor costs associated with handling sprinklers. Because these sprinklers are moving, water must be applied at higher rates than would be applied with solid set and hand move sprinklers, which can increase run-off.

Drip. Surface-drip permits growers to water frequently during the phase of rapid vegetative growth. One drip line is installed between 2 plant rows on 40-inch (1 m) wide beds, or 3 drip lines are installed between 6 or 5 plant rows on 80-inch (2 m) wide beds. The drip lines are typically retrieved before harvesting, and wound on reels for reuse on subsequent crops. Tape ends can be joined using specialized splicing equipment. Most growers can use drip tape with a wall thickness greater than10 mil for 8 to 12 crops before it needs to be replaced.

Drip can potentially distribute water more uniformly than furrow and sprinklers, and has helped growers attain uniform growth in fields with variable soil textures by maintaining similar soil moisture levels in all areas of the field. Drip can be managed to maintain higher soil moisture levels than can be attained with sprinklers and furrow by applying less amounts water more frequently.

The use of drip irrigation reduces potential run-off that is typically associated with furrow and solid set sprinklers. Drip can be managed to minimize leaching of nitrate-nitrogen by fertigating weekly with low rates of fertilizer and applying less water more frequently than can be achieved with sprinkler and furrow systems. High bicarbonate, manganese, or iron levels in ground water in some areas of the central coast can plug drip emitters. Periodic injections of acid are used to remove bicarbonate and iron precipitates.

IRRIGATION SCHEDULING

Schedule irrigations by monitoring soil moisture tension and estimating water use of the crop using ET data. The amount of water to apply should be equal to the cumulative ET since the last irrigation event and an additional amount to account for the distribution uniformity of the irrigation system and other factors such as for scheduling errors or for leaching of salts.

Soil Moisture Tension
High soil moisture tensions reduce the growth rate of lettuce. Use tensiometers or watermark electrical resistance blocks (Irrometer Co. Riverside CA) to measure soil moisture to assure that the crop is not stressed for moisture. Place these instruments in the plant row at depths of about 8 and 18 inches. Readings are in units of tension (kiloPascals [kPa} or centibars).Because soil properties vary spatially within a field, make moisture readings in three or more areas, representing the head, middle, and tail of the field.

Estimating Water Use
Water use by the crop (ETc) can be estimated using reference evapotranspiration data (ETo) adjusted with a crop coefficient (Kc) for lettuce, using the following equation: ETc ≡ Kc ´ ETo. The California Irrigation Management Information System (CIMIS) coordinated by the California Department of Water Resources provides daily estimates of reference evapotranspiration for most production regions of California (www.cimis.water.ca.gov). The CIMIS data can be entered into a spreadsheet worksheet also available from this site to determine an irrigation schedule.

Crop Coefficients for Irrigating the Different Growth Stages
During the first 2 to 3 weeks of the crop, canopy cover is less than 5% and most water loss is a result of evaporation from the soil surface during and after irrigating with overhead sprinklers. The crop coefficient on the day a field is irrigated is approximately 1.0 to 1.1, and about 0.6 to 0.4 the following day depending on the soil properties. The crop coefficient will usually decline to less than 0.2 by the second day after irrigating. In general, a crop coefficient between 0.3 and 0.5 can be used for overhead sprinklers during germination, where higher values would be typical for frequent irrigations.

After the crop is thinned the crop coefficient is closely related to the percentage of ground shaded by the canopy. At a maximum canopy cover of 80 to 90%, the crop coefficient is between 1.0 and 1.1. A detailed model of the crop coefficient and canopy growth for lettuce can be accessed from this website.

IRRIGATION AT THE DIFFERENT GROWTH STAGES

Preplant and Germination
Most fields are pre-irrigated with 2 to 4 inches (5-10 cm) of water, depending on initial conditions, to soften soil for listing and seedbed preparation. Both seeded and transplanted lettuce are sprinkle-irrigated frequently (every 2 to 3 days) until seedlings emerge or are established (usually 6 to 10 days in late spring and summer). Hand-move or solid-set sprinklers are used for stand establishment. Because lettuce is shallowly seeded (about 1/8 inch depth), frequent irrigations are required to prevent the seed from desiccating, and under hot conditions (soil temperatures greater than 80°F), to cool the soil surface through evaporation, thereby preventing heat-induced dormancy of non-primed seed. Thermodormancy can also be prevented by starting the first irrigation in the late afternoon so that the seed can imbibe water and germinate during the cooler hours of the night. Irrigating excessively during germination can cause unnecessary run-off, leading to losses of sediments and associated nutrients and pesticides in the tail water. On some soil types, the impact of sprinkler water droplets on the soil surface causes a crust to form that impedes the emergence of the seedlings,decreases infiltration, and increases run-off.

Drip use for germination. A subset of growers bury drip tape to a depth of 2 to 3 inches (2.5-7.6 cm) before planting, and use the tape for germinating the seed. An advantage of using the drip tape for germination is that the lettuce can be drip irrigated during the entire crop cycle, rather than the only the last 4 to 5 weeks before harvest, which is the case for drip installed on the surface of the bed due to the thinning and cultivation activities that would damage the tape. The drip tape is extracted from the soil after harvest, and then wound on to spools and reused for another crop. Germinating the crop successfully using drip tape requires that moisture move laterally and vertically from the tape to the seed line. Planting slightly deeper than normal and rolling the beds to improve contact between seed and soil aggregates can improve the ability of the seed to imbibe moisture.

Postemergence
After emergence the crop is irrigated less frequently than during germination. Because the crop has a shallow root system after emergence (less than 8 inches), soil moisture tensions should be maintained below 15 to 20 cbars (15 to 20 kPa) at depths between 8 and 12 inches. Some fields are furrow irrigated after emergence is complete. One strategy to minimize salt accumulation in the seed lines is to irrigate alternate furrows.

Post-thinning
After thinning, irrigation schedules must often be adjusted to accommodate field operations such as side-dressing fertilizer, weed cultivation, and pesticide sprays. In fields that will be irrigated with surface drip, tape is usually installed after the last cultivation of the bed top. Because water use of the crop is still low immediately following thinning, the interval between irrigations is often more than 7 days so that most field operations can be completed without conflicting with the irrigation schedule.

As the plants become larger and canopy cover increases, daily water use of the crop increases. Irrigations usually need to become more frequent to prevent the lettuce from becoming stressed for moisture. Most of the root system is concentrated between the surface and 1 foot depth. Roots between 12 and 18 inches become more active as the crop matures. Ideally, irrigations should be frequent enough to maintain less than a 20 to 30 cbar (20 to 30 kPa) soil moisture tension at depths between 8 and 12 inches. Soil moisture tension at depths between 18 and 20 inches should be less than 15 cbars (15 kPa).

Preharvest
Water use of lettuce is usually highest during the 2-week period before harvest. As water uptake of the crop accelerates, irrigations may need to be longer or more frequent than during the previous few weeks. In drip fields, under-irrigating can restrict the wetted volume of soil where roots extract moisture and nutrients. When corky root disease (Rhizomonas suberifaciens) is present, more frequent irrigations are required to compensate for the plants' degraded root system.

Excess water and fertilizer sometimes causes heads of iceberg type lettuce to become large and puffy, reducing the crop value. Some growers hold water back for a short period 1 to 2 weeks before harvest to increase the density of the heads and/or to slow the growth of large heads, which may crack and sunburn. Water is sometimes cut back near harvest for some romaine varieties to minimize cupping of leaves in the head, which can lead to tip burn.

Although the roots are most concentrated in the upper foot of soil, roots in the 12 to 24 inch soil layer also extract a significant amount of moisture. Monitoring of soil moisture at 18- to 20-inch depths can assure that applied water is reaching the deep roots. Soil moisture tensions should return to low values (less than 15 to 20 cbars) at the 18 to 20 inch depths after an irrigation. Schedule irrigations when soil moisture tensions are greater than 25 to 35 cbars (25 to 35 kPa) at depths of 8 to 12 inches during this period.

The last irrigation is often timed to allow wet areas of the field to dry sufficiently so that heavy equipment can access the field at harvest.

IRRIGATION AND WATER USE IN DIFFERENT GROWING REGIONS

Central Coast
Although lettuce acreage under drip has increased substantially on the Central Coast during the past 10 years, sprinklers remain the dominant irrigation method; furrow systems are used in some areas where the soil texture and slope are conducive to uniform irrigations.

Water use varies on the Central Coast with proximity to the coast and with the date of the planting. Early plantings (February and March) tend to require less water than summer plantings (June and July), depending on the winter rainfall pattern and the days between planting and harvest. Crops grown near the coast where temperatures are moderate and fog is typical may require 30% less water than crops planted inland. The amount of water typically used for lettuce on the Central Coast in summer is 1.5 to 2 acre-feet per acre for a sprinkler-irrigated lettuce crop, 2 to 2.5 acre-feet per acre for a furrow irrigated crop, and about 1 to 1.5 acre-feet per acre for a drip-irrigated crop.

Southern Desert
In the desert region, most growers use sprinklers for the first 5 to 7 days or until seedlings emerge and then switch to furrow irrigation for the remainder of the season. Furrow remains the main method of irrigating lettuce after emergence, although some growers are using drip.

Lettuce crops are planted from mid-September to mid-December. Although daily water use is higher for late summer plantings than late fall plantings, the late summer plantings are harvested 30 days sooner than the late plantings. The amount of water typically required to grow lettuce using furrow irrigation in the southern deserts is around 3 to 3.5 acre-feet of water per acre. Irrigation water from the Colorado River can have moderate levels of salts; therefore, growers in this region are accustomed to pre-irrigating fields with sufficient amounts of water to leach salts from the surface layer of soil.

Southern San Joaquin Valley
In the Central Valley, irrigation is transitioning from furrow to drip, which is becoming a very common method of irrigating lettuce from thinning to harvest. Subsurface drip is used in this region in other annual crops rotated with lettuce.

Lettuce is planted in mid-summer for late fall harvest and in late fall for early spring harvest; hence, water use can vary substantially between different seasonal plantings. The amount of water per acre used to grow lettuce in the San Joaquin Valley is around 1 to 1.5 feet for a mid-summer planting irrigated with sprinklers followed with surface drip after thinning and about 20% less water for a late fall planting that is harvested in spring (March to April).

IRRIGATION AND WATER USE OF HEAD, ROMAINE, AND LEAF LETTUCE

All lettuce types are typically germinated with sprinklers and subsequently irrigated with drip, sprinkler, or furrow. Also, all lettuce types require frequent irrigations to minimize moisture stress. As mentioned earlier, however, growers will hold back water for a short period on iceberg lettuce near harvest to improve the quality of the heads. (Iceberg and romaine tend to have deeper root systems than leaf lettuce varieties.)

The primary difference in water requirements among head, romaine, and leaf types of lettuce is mainly a result of the differences in days from planting to maturity. For example, iceberg lettuce usually matures in 63 to 68 days, romaine in 60 to 65 days, red and green leaf lettuce in 50 to 55 days, and butter leaf lettuce in 55 to 60 days during summer.

Further Reading
Scheduling Irrigations: When and How Much Water to Apply, UC ANR Publication No. 3396. Gallardo, M., R. L Snyder, K. Schulbach, and L. E. Jackson. Crop growth and water use model for lettuce. Journal of Irrigation and Drainage Engineering. Nov/Dec 1996. 254-359.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450

General Information

M. D. Cahn, UC Cooperative Extension, Monterey County

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