Several things account for that amazing total: Plentiful sunshine, rich soil, ample labor and high-quality irrigation water. With mild winters, little danger of hard frost and more than days of sunshine a year, Yuma County enjoys the longest growing season in the country. And while the winter is notable for the emerald and ruby patchwork formed by vast vegetable fields, something is either growing or happening in Yuma county fields even during the hottest months of the year.
One reason is because those fields - sediments deposited by the Colorado River over millions of years - have some of the most fertile soil in the country.
But with less than three inches of rainfall annually, water was the missing component, though the mighty Colorado flowed nearby. The Bureau of Reclamation's first big water project in the West gave nature a hand, with construction of the first dam on the Colorado and completion of the Yuma Siphon - delivering water through a huge tunnel built under the riverbed - in , the same year Arizona became a state.
Now, of the , acres of land utilized for agriculture in Yuma County, per cent are irrigated with Colorado River water delivered by one of the county's seven irrigation districts. Every single field in the county is also laser-leveled and graded using GPS technology, making Yuma's irrigation network one of the most efficient in the world.
In fact, about 90 percent of all the leafy vegetables grown in the U. To put this in perspective, the Yuma area is home to nine salad plants that produce bagged lettuce and salad mixes. During peak production months, each of those plants processes more than two million pounds of lettuce per day. There are also 23 cooling plants in the Yuma area, where powerful refrigeration units bring truck-sized loads of vegetables from field to shipping temperatures in less than half an hour.
Crops harvested here in the morning can be in Phoenix by afternoon and on the East Coast in three to four days. Field note: Lettuce sold by the head is usually field-packed into cardboard boxes, that bound for salad plants are harvested into blue plastic bins. But lettuce is just part of the story: More than different crops are grown in the Yuma area, including many grown to seed here because of our perfect growing conditions.
For example, Yuma County ranks number one in Arizona for lemon, tangelo and tangerine production, and for watermelons and cantaloupes; a local cattle company usually has more than , head of beef cows on its lot.
Lettuce and leafy greens are in such high demand, they are produced year-round. Beginning in November, harvest of leafy greens will transition to the southern growing areas of California near Imperial County, south of Los Angeles and in Arizona, mostly near Yuma. Throughout the winter months, these southern growing areas of California and Arizona are the primary source of lettuce and leafy greens.
During these months, your lettuce could be from almost any growing region in California or Arizona. Both programs utilize state government auditors to verify their members are following a set of required food safety practices. Both also require lettuce companies to have a traceback system that allows them to track their products back to the field where it was grown. Over the past two years, romaine lettuce has been implicated in a few foodborne illness outbreaks.
The exact cause or source of these outbreaks still remains unknown. The seedbed used for lettuce should be tilled until it is soft and friable to a depth of 10 to 12 inches.
Depending on soil texture, this is accomplished by moderate to extensive disking. Once the soil is of a desirable consistency, rows are listed, beds are shaped, and lettuce is seeded two rows per bed. In Arizona, lettuce beds are typically 8 to 10 inches high on 40 to 42 inch centers and are almost always laid out in a north-south direction. This orientation minimizes temperature and light differences between the two rows on each bed.
A great number of varieties are successfully grown in the lower desert of Arizona. New varieties are released or removed from commercial production each year, making it impractical to list them in this publication. The variety a grower may use is based both upon physiological considerations and personal preferences for a particular variety.
All varieties are bred for early vigor, size, earliness and uniformity of maturity, shape, texture, pest resistance and so forth. For more specific information regarding varieties in your area, consult your local county Cooperative Extension Office or a seed dealer. For planting purposes, the lettuce season is typically divided into three categories, early late-August-September 30; known as the 'Empire' slot , mid October ; known as the 'Winterhaven' slot , and late October December 10; known as the 'Van Guard' slot.
The primary physiological considerations for selecting a variety are seed germination temperature, length of growing season, and day length sensitivity. The prevention of germination due to high temperatures is known as thermoinhibition. A thermoinhibited lettuce seed will germinate when returned to more suitable temperatures.
However, continued exposure to high temperatures may induce a secondary dormancy, called thermodormancy. A thermodormant seed will not germinate even when returned to non-inhibiting temperatures. Germination of lettuce may be promoted by light and inhibited by dark; this is known as photodormancy. Specifically, far-red light inhibits germination while red light abundant sunlight promotes germination.
The leaf-lettuce varieties are often photodormant. Both thermodormancy and photodormancy may be alleviated by priming. Priming, also know as osmopriming or osmoconditioning, is a controlled hydration process in which the amount of water seeds imbibe is controlled by an osmoticum, typically polyethylene glycol. During priming the seed imbibes enough water to start many of the physiological processes associated with germination, but not enough to cause the radicle to protrude, or the seed to germinate.
In addition to alleviating thermo- and photo-dormancy, priming shortens the germination time and synchronizes germination, thus leading to more uniform stands.
Although only certain varieties are primed, virtually all lettuce seed is pelletized Fig. During this process an inert material, usually diatomaceous earth, is coated around individual lettuce seeds. This allows singularization of the seed during planting.
Although necessary, both the priming and pelleting process lead to shortened shelf life, compared to a seed that has not been similarly treated. The cause of this is not currently known, but new technologies are being developed which minimize these negative effects. Figure 2.
Pelletized right and raw lettuce seed left. Seed deterioration is a naturally occurring phenomenon, but steps may be taken to minimize this process. Seed should be kept at cool temperatures and not exposed to high relative humidity. The amount of time seed is allowed to remain in the back of pickups, on loading docks, or on a dry seed bed, should be minimized, as it will contribute to accelerated seed deterioration resulting in lowered vigor, and eventually low overall germination.
In Arizona, most lettuce is seeded using a StanHay type planter Fig. These are precision planters that utilize rubber belts with small holes spaced to deliver a single pelletized seed. However, some growers prefer to plant to stand to reduce thinning costs. Pelletized seed is planted at the rate of 6 to 8 pounds per acre which should result in about 6 to 12 seeds per foot of row.
This will result in a plant population of 20, to 26, plants per acre after thinning. The seeding rate is largely a function of the planting date, method of irrigation and variety to be planted. The crop should be thinned, after the first two true leaves have developed, to approximately 10 to 14 inches between plants depending upon the head size of the variety planted.
Planting date is primarily a function of the variety and desired harvest date. Planting begins as early as late August and continues into the middle of December.
Plantings prior to October 15 are considered to be fall lettuce, and later plantings are considered spring lettuce. Harvesting takes place primarily from December through March. Optimum germination and growing temperatures vary depending upon the variety planted. Figure 3. Lettuce planter. Lettuce passes through six distinct development stages: seed, cotyledon, seedling, rosette, cupping and heading periods.
The seed stage occurs from pre-planting to emergence. Once exposed to water and appropriate temperature, the seed will begin germination, usually requiring as little as 12 hours for fall planted varieties or up to 7 days for winter planted varieties.
Once the plant sheds the seed coat and emerges from the soil, it enters the cotyledon stage. The cotyledon stage last until the plant is able to better establish its roots.
Once the root has grown a couple of inches, the seedling begins to grow upwards and the first true leaf emerges. From emergence from the soil to the first true leaf usually requires approximately 7 days for fall planted lettuce and 20 days for winter planted lettuce.
The seedling stage occurs from the first true leaf until the plant develops a distinct circular cluster of leaves known as a rosette. The rosette stage for fall planted lettuce will generally last 25 days, but may last as long as 50 days for winter planted lettuce. Cupping begins when the tips of the inner leaves begin to curl inwards on the edges. Cupping signifies that the beginning of head formation is near, and will usually last about 7 days for fall planted lettuce and 14 days for winter planted varieties.
Heading begins once the cupped leaves begin to overlap and cover the growing point of the plant. Head formation will continue until the crop is ready for harvest, which generally last about 30 days for fall planted varieties and about 45 days for winter planted varieties. Fall planted lettuce may require as little as 65 days from the beginning of germination to harvest, while winter planted lettuce will require as long as days.
Lettuce produced on low desert soils requires from to pounds of nitrogen N to the acre for optimal yields. The actual rate will vary depending upon residual soil N, soil texture, irrigation and rainfall. Split applications of N are usually more efficient than a single preplant application because N in the soil is subject to leaching, denitrification gaseous loss to the atmosphere , and other mechanisms of loss during the growing season. Generally, 50 pounds N per acre is considered adequate preplant N.
Stand loss and stunting may result from excessive amounts of ammonium-N especially on light or sandy soils. Subsequent applications of N can be applied by sidedress or water-run and usually start after thinning and cultivation. Typically following thinning, lettuce will be cultivated and sidedressed with N. The amount and frequency of N application can be adjusted based upon a pre-sidedress soil nitrate-N test or a plant midrib tissue test.
Lettuce grown in soils with a nitrate-N concentration of 20 ppm or greater in the top 12 inches will generally not respond to additional N fertilizer. Midrib samples are also useful during the second half of the growing season after the 8 to leaf stage.
Generally, lettuce is not considered deficient in N if midribs nitrate-N concentrations exceed ppm. A second series of cultivation and sidedressing is usually conducted approximately 14 days following the first such operation, usually near early head formation. Additional sidedress applications of N may be applied if adverse growing conditions are encountered. Recent research indicates that the use of controlled release fertilizers may represent a viable alternative to split application of conventional soluble N sources.
There is considerable variation in N release rates and costs of controlled release N fertilizers, and individual products should be closely evaluated before they are utilized.
Phosphorus P fertilizer should be broadcast or banded 2 to 3 inches below and beside the seed row immediately before planting. Studies have shown that P applied later in the season is less effective than that applied preplant. Rates of P applied can be adjusted using a preplant P soil test. Lettuce in Arizona will require from to lbs P2O5 per acre. Tissue tests are considered inconsequential because it is extremely difficult to correct P deficiencies mid-season.
Nevertheless, these tests are useful tools to diagnose P deficiencies. Generally, whole plants with leaf tissue P concentrations of less than 0.
Potassium fertilization is generally not needed for lettuce production in Arizona. Most soils used for lettuce production in Arizona have exchangeable K levels, a clay mineralogy mica , and K release rates that make a K response improbable. Additionally, most lettuce is irrigated with Colorado River water that contains approximately 5 ppm K or 15 lbs K per acre foot of water.
Although some growers commonly use micronutrient fertilizers, most recent research suggests that lettuce produced in Arizona does not show a positive yield and quality response. Therefore, the routine use of soil or foliar applied micronutrients cannot be economically justified. Micronutrient fertilization should be based on the actual diagnosis of a deficiency. Cooperative Extension bulletin number "Fertilizing Head Lettuce in Arizona" provides tables for recommended fertilizer rates based upon soil and midrib analysis and detailed sampling procedures.
Contact your local county Cooperative Extension office for a copy of this publication. Generally, 38 to 50 inches of water per acre are required to produce a desirable lettuce crop, but this varies dramatically with soil type, slope of field, temperatures and planting window.
Irrigation water is delivered via canals from the Colorado River, or from deep wells, in Yuma County. There are two distinct irrigation stages in the low desert of Arizona. The first irrigation stage involves seed germination and stand establishment. Under these conditions, impact sprinklers are essential to not only provide water, but also to reduce the temperature of the seed bed though evaporative cooling Fig.
Figure 4. Sprinklers are commonly used to germinate lettuce seed when temperatures are hot. When used, sprinkler pipe is placed in the field immediately after planting. The pipes are typically 3 inches in diameter and are spaced 40 ft apart. Each pipe contains 12 inch risers spaced 30 ft apart, mounted with a 4 inch long impact sprinkler head.
Water pressure is maintained at 60 to 80 psi, depending on the temperature and evaporation rate. Worn orifices or inadequate water pressure may result in excessively large water droplet sizes that may wash out seed planted in the immediate vicinity of the sprinkler head. The wedge drive reduces droplet size, increases evaporative cooling and speeds the sprinkler's rotation.
Under cooler conditions, evaporative cooling may not be an important consideration, but sprinklers are often used to deliver uniform moisture to maximize stand uniformity. Under these conditions, sprinklers are turned on as needed to maintain a moist seed bed.
After stand establishment, the field is allowed to dry and the sprinkler pipes removed. For effective germination, beds should be moistened thoroughly but not to the point that they become super saturated and anaerobic. The field is generally maintained in a flooded condition until the lettuce has uniformly emerged. The second irrigation stage involves furrow irrigation for crop maintenance following stand establishment.
Usually an irrigation is made to soften the soil prior to thinning and cultivation. Afterwards, adequate moisture should be maintained throughout plant development.
During the second irrigation stage, the furrows are typically filled within approximately 1 to 2 inches from the top of the beds and then the water is cut off. Care is taken to avoid saturating the middle of the bed to the point where the top of the bed becomes wet or "blackened.
During harvest, an irrigation may be necessary between cuttings if multiple harvests are necessary or profitable. Typically following thinning, lettuce will be cultivated for weeds, worked with a rotary spiker to remove some weeds, breakup the soil crust, sidedressed with N, and then furrowed out or "bolused. The bolus is used to restore the shape of the beds and allow for more uniform irrigation. The bolus implement consists of two chisels situated about 3 inches of the center of each furrow to breakup the soil pan.
The chisels are followed by a wedge to move displaced soil back onto the sides of the furrows, then followed by a large steel wheel to pack and shape the furrow.
Approximately 14 days following the first cultivation, near early head formation, a second series of spiking, sidedressing and bolusing is often performed. Economic losses due to weeds can be a serious problem in the production of lettuce. With planting taking place from late August through December, summer annual, winter annual and perennial weeds may be a problem. Weeds decrease crop yield and quality through competition for water, nutrients and sunlight.
In addition, many weeds harbor destructive insect pests and serve as alternative hosts for other organisms which cause crop diseases. Weeds commonly encountered in lettuce in Yuma, AZ in the late summer plantings include summer annual grasses such as watergrass or junglerice Echinochloa crus-galli or E. And groundcherry Physalis wrightii In the transition period between hot and cooler temperatures, lambsquarters Chenopodium spp.
And cheeseweed Malva spp. By late-October when temperatures cool, winter annual weeds are prevalent. Winter annual grasses include canarygrass Phalaris spp. And volunteer small grains.
Cruciferous weeds such as London rocket Sisymbrium irio , shepherdspurse Capsella bursapastoris and black mustard Brassica nigra are commonly encountered. Lettuce-related weed species are also prevalent under cooler conditions, including prickly lettuce Lactuca serriola and sowthistle Sonchus oleraceous. Cultural and chemical technologies are utilized for weed control in lettuce. Cultural practices including mechanical cultivation and hand hoeing are utilized from the fallow period prior to seed bed formation until first layby.
Herbicides have been used for weed management in lettuce for more than 30 years. There are four distinct timings of application for herbicides in lettuce: fallow, preplant, preemergence and postemergence. Before seedbed preparation, Roundup Ultra glyphosate , applied at 0.
Fields are laser leveled and then flushed with water to germinate weed seeds. These broadspectrum non-selective herbicides are applied after sufficient weeds have emerged. After a field has been laser leveled and prior to listing the seedbeds, Balan benefin , at 1. Per acre, is commonly applied and disked into the top 4 to 6 inches of the soil. Balan controls some grasses and some small-seeded broadleaved weeds Table 1.
Balan moves very little in the soil profile or within the plant tissue. Table 1. Efficacy of preplant and preemergence herbicides against commonly occurring weeds in lettuce.
Kerb pronamide , at 1. Per acre, can be used preplant, preemergence or postemergence. However, most applications are applied banded over the top of the seedbed just before or immediately following planting.
These preplant and preemergence applications should immediately be followed by sprinkler irrigation of 1 to 2 inches. However, Kerb is sensitive to movement in the soil with water, and excessive sprinkler irrigation will leach the material out of the weed seed zone top 1 inch of soil and render the product ineffective. Thus, the effectiveness of Kerb is greatest if used on lettuce that is not sprinkle irrigated. For postemergence applications, apply before or after lettuce emerges, but prior to weed emergence.
Kerb is most effective against cool season annuals, and best fits lettuce planted after September Table 1. Prefar bensulide ; at 5. Prefar is most effective against summer annual weeds, and is most efficient in lettuce planted in August and September. Preplant applications should be mechanically incorporated 1 to 2 inches before planting. Preemergence applications should be immediately followed by irrigation. When incorporating using sprinkler irrigation, wet the soil to the depth of 2 to 4 inches.
Furrow irrigation should thoroughly wet, or "blackened," the top of the bed. Prefar moves very little in the soil and should stay within the weed seed zone. Prefar can also be applied through sprinkler irrigation. Maximum weed control using preplant or preemergence herbicide is best achieved if the herbicide can be concentrated within the weed seed germination zone top 1 to 2 inches of soil.
Additionally, a smooth, clod-free surface will maximize weed control when using Kerb or Prefar. Combinations of Balan and Kerb control most weeds. Use Balan preplant incorporated followed by Kerb applied preemergence and incorporated by irrigation , or apply both herbicides preplant and mechanically incorporate. Weeds related to lettuce such as prickly lettuce and sowthistle are not controlled. Other difficult to control weeds that may occur in lettuce include horseweed Conyza canadensis , fleabane Conyza bonariensis , cudweed Gnaphalium palustre , groundsel Senecio spp.
And broadleaf perennials and nutsedges Cyperus spp. Balan, Kerb and Prefar can cause injury to lettuce, especially under environmental conditions where lettuce emergence is slowed. Also, some lettuce varieties are more prone to injury than others. Refer to the herbicide label for details.
Poast sethoxydim used at 0. Per acre with a crop oil concentrate added will selectively control most annual and perennial grasses found infesting lettuce. Timing is critical for optimal control; apply Poast when grasses are small. Do not apply to grasses under stress.
Thorough coverage is required. Do not tank mix with other pesticides as efficacy may be reduced. Do not cultivate within 5 days prior to application or within 7 days following application. See the label for specific directions. There are many insecticides registered for use in lettuce, more than can be adequately addressed in this document. Refer to specific insecticide labels for details. Field crickets Gryllus spp. Description and Life History: These insects are annual pests in early planted sprinkler-irrigated lettuce fields in the low desert.
When they occur, they can quickly destroy most of a field. Problems are usually in fields planted closely to cotton or Sudangrass in August and September. Moving out of cotton, Sudangrass and desert flora, large numbers of these pests will migrate to seedling lettuce if available. Most damage occurs at night. They hide during the day in soil cracks, ditches, weeds, and under irrigation pipes. They are similar in appearance to many ground beetles. Darkling beetles normally have the tips of their antennae slightly enlarged, while ground beetles antennae are not enlarged on the tips.
Ground beetles are predators feeding primarily on other insects. They have very short elytra covering their wings, but their abdomen is not covered. Rove beetles are often confused with winged ants or termites.
When disturbed they will elevate their abdomens similarly to a scorpion. Rove beetles are insect predators or scavengers feeding on debris in the field. Damage: Cricket and darkling beetles will destroy a crop by eating the newly emerged seedlings. Although ground beetles and rove beetles do not feed on the plants and are usually considered beneficial insects, they often damage fall vegetable crops by digging and rooting up the seed and small seedlings.
Management and Control: These insects are difficult to monitor. Early-planted lettuce in close proximity to cotton or Sudangrass should be considered high risk fields and should probably be treated as soon as the seed begins germinating. Apply baits around field edges to control migrating populations and apply insecticides through the sprinkler pipe during germination and when the plants emerge.
Scout the field by looking under the sprinkler pipe to determine if control was achieved or if re-application is necessary. Description and Life History: Saltmarsh caterpillars are not normally a pest of fall grown vegetables but will often migrate as larvae from neighboring cotton or alfalfa. Large populations can be extremely damaging to seedling lettuce.
The larvae are usually yellowish brown in color and covered with long, dark black and red hair. Many people refer to them as wooly bear caterpillars. Full-grown larvae may be 2 inches long. Adult moths have white to yellowish wings and are peppered with many black spots. Their wing span is approximately 2 inches Eggs are laid in clusters of 20 or more on the leaves.
Damage: Most damage occurs to early planted seedling lettuce. Large populations of larvae will move out of newly defoliated cotton and devour the young plants. After thinning, saltmarsh caterpillars are generally not a problem. However, they should be included in counts for Lepidopterous larvae. On older plants damage is distinctive.
They prefer to feed in groups and will completely skeletonize several adjacent plants. Management and Control: Scout adjacent cotton fields prior to lettuce emergence. It is best to control saltmarsh caterpillars before they enter the field. If possible treat the population in the cotton field during or just before defoliation.
Saltmarsh caterpillars are particularly sensitive to Bacillus thuringiensis B. Physical barriers are effective at preventing larvae from entering a field. Saltmarsh caterpillars do not like to cross fence type barriers of aluminum sheeting or irrigation pipe.
These devises can be used to herd populations into holes containing cups of oil. Ditches filled with water containing liquid detergent or oil are also effective. Carbaryl can be sprayed around cotton fields or along ditches to kill migrating populations.
Description and Life Cycle: In the Southwest, the sweetpotato whitefly is one of the principal pests of crops Fig. It was not considered an important pest until the early 's, when extremely large populations became common on cotton, melons and lettuce throughout the Southwest.
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