Editing Lettuce (section)

== Cultivation ==
[[File:Park Lane Nurseries - geograph.org.uk - 583201.jpg|thumb|right|A lettuce farm in the United Kingdom]]
A [[Hardiness (plants)|hardy]] annual, some varieties of lettuce can be [[Overwintering|overwintered]] even in relatively cold climates under a layer of straw, and older, [[heirloom variety|heirloom varieties]] are often grown in [[cold frame]]s.<ref name="Weaver173">Weaver, pp. 172–173.</ref> Lettuces meant for the cutting of individual leaves are generally planted straight into the garden in thick rows. Heading varieties of lettuces are commonly started in flats, then [[transplanting|transplant]]ed to individual spots, usually {{convert|20|to|36|cm|abbr=on}} apart, in the garden after developing several leaves. Lettuce spaced farther apart receives more sunlight, which improves color and nutrient quantities in the leaves. Pale to white lettuce, such as the centers in some iceberg lettuce, contain few nutrients.<ref name="Weaver175" />

[[File:Planche Lactuca sativa.jpg|thumb|left|A transplanted bed of lettuce in a [[polytunnel]]]]
Lettuce grows best in full sun in loose, [[nitrogen]]-rich soils with a [[soil pH|pH]] of between 6.0 and 6.8. Heat generally prompts lettuce to [[bolt (horticulture)|bolt]], with most varieties growing poorly above {{convert|24|C}}; cool temperatures prompt better performance, with {{convert|16|to|18|C}} being preferred and as low as {{convert|7|C}} being tolerated.<ref name="Bradley129">Bradley, p. 129.</ref> Plants in hot areas that are provided partial shade during the hottest part of the day will bolt more slowly. Temperatures above {{convert|27|C}} will generally result in poor or non-existent germination of lettuce seeds.<ref name="Bradley129" /> After harvest, lettuce lasts the longest when kept at {{convert|0|C}} and 96 percent humidity. The high water content of lettuce (94.9 percent) creates problems when attempting to preserve the plant&nbsp;– it cannot be successfully frozen, canned or dried and must be eaten fresh.<ref name="UI" /> In spite of its high water content, traditionally grown lettuce has a low [[water footprint]], with {{convert|237|liters}} of water required for each kilogram of lettuce produced.<ref>{{cite book |last=Holden |first=Joseph |date=2013 |title=Water Resources: An Integrated Approach |url=https://www.routledge.com/Water-Resources-An-Integrated-Approach-1st-Edition/Holden/p/book/9780203489413 |publisher=Routledge |page=335 |isbn=9780203489413 |access-date=12 November 2019 |archive-date=5 June 2020 |archive-url=https://web.archive.org/web/20200605143046/https://www.routledge.com/Water-Resources-An-Integrated-Approach-1st-Edition/Holden/p/book/9780203489413 |url-status=live }}</ref> Hydroponic growing methods can reduce this water consumption by nearly two orders of magnitude.

Lettuce varieties will [[Hybrid (biology)|cross]] with each other, making spacing of {{convert|1.5|to|6|m|in|abbr=on|-1}} between varieties necessary to prevent contamination when saving seeds. Lettuce will also cross with ''[[Lactuca serriola]]'' (wild lettuce), with the resulting seeds often producing a plant with tough, bitter leaves. [[Celtuce]], a lettuce variety grown primarily in Asia for its stems, crosses easily with lettuces grown for their leaves.<ref name="Weaver175">Weaver, pp. 175–176.</ref> This propensity for crossing, however, has led to breeding programs using closely related species in ''Lactuca'', such as ''L. serriola'', ''[[Lactuca saligna|L. saligna]]'', and ''[[Lactuca virosa|L. virosa]]'', to broaden the available [[gene pool]]. Starting in the 1990s, such programs began to include more distantly related species such as ''[[Lactuca tatarica|L. tatarica]]''.<ref>{{cite journal | title = Phylogenetic relationships among ''Lactuca'' (Asteraceae) species and related genera based on ITS-1 DNA sequences | author1 = Koopman, Wim J.M. | author2 = Guetta, Eli | author3 = van de Wiel, Clemens C.M. | author4 = Vosman, Ben | author5 = van den Berg, Ronald G | journal = American Journal of Botany | year = 1998 | volume = 85 | pages = 1517–1530 | doi = 10.2307/2446479 | jstor = 2446479 | issue = 11 | pmid = 21680311 | doi-access = free }}</ref>
[[File:Lettuce mix.jpg|thumb|A selection of lettuce cultivars]][[File:Yangzhou - supermarket - woju - P1070028.JPG|thumb|The lettuce variety [[celtuce]] is grown for its stem, used in [[Chinese cooking]].]][[File:Buttercrunch butterhead lettuce.jpg|thumb|A butterhead lettuce]]
[[File:Salata Prajzerica (Croatia).jpg|thumb|''Prajzerica'', a lettuce variety from [[Croatia]]]]

Seeds keep best when stored in cool conditions, and, unless stored [[cryogenics|cryogenically]], remain viable the longest when stored at {{convert|-20|C}}; they are relatively short lived in storage.<ref name="Kew" /> At room temperature, lettuce seeds remain viable for only a few months. However, when newly harvested lettuce seed is stored cryogenically, this life increases to a [[half-life]] of 500 years for vaporized nitrogen and 3,400 years for liquid nitrogen; this advantage is lost if seeds are not frozen promptly after harvesting.<ref>Davey, et al., p. 241.</ref>

=== Cultivars (varieties) ===
{{Anchor|Cultivars}}
[[File:Lactuca sativa 'Forellenschluss', Longwood Gardens 2023 01.jpg|thumb|Romaine lettuce 'Forellenschluss' from [[Austria]]]]
There are several types and [[cultivar]]s of lettuce. Categorization may sometimes refer to "leaf" versus "head", but there are seven main cultivar groups of lettuce, each including many varieties:
# Leaf&mdash;Also known as looseleaf, cutting or bunching lettuce,<ref name="WSU" /> this type has loosely bunched leaves and is the most widely planted. It is used mainly for salads.<ref name="UI">{{cite web|url=http://urbanext.illinois.edu/veggies/lettuce.cfm|title=Lettuce|publisher=University of Illinois Extension|access-date=25 March 2012|archive-url=https://web.archive.org/web/20120315133237/http://urbanext.illinois.edu/veggies/lettuce.cfm|archive-date=15 March 2012|url-status=dead}}</ref>
#* [[Red leaf lettuce]]&mdash;A group of lettuce types with red leaves.
# [[Romaine lettuce|Romaine]]/Cos&mdash;Used mainly for salads and sandwiches, this type forms long, upright heads.<ref name="UI" /> This is the most often used lettuce in [[Caesar salad]]s.<ref name="CDC">{{cite web|url=http://www.fruitsandveggiesmatter.gov/month/lettuce.html|title=Vegetable of the Month: Lettuce|publisher=Centers for Disease Control and Prevention|access-date=26 March 2012|url-status=dead|archive-url=https://web.archive.org/web/20120322151027/http://www.fruitsandveggiesmatter.gov/month/lettuce.html|archive-date=22 March 2012|df=dmy-all}}</ref>
#* Little Gem&mdash;a dwarf, compact romaine lettuce, popular in the UK.<ref>{{cite web |title=Lactuca sativa 'Little Gem' {{!}} lettuce (Cos) 'Little Gem' Annual Biennial/RHS Gardening |url=https://www.rhs.org.uk/plants/112416/i-lactuca-sativa-i-little-gem/details |website=www.rhs.org.uk |access-date=2022-11-14 |language=en-gb}}</ref>
# Iceberg/Crisphead&mdash;The most popular type in the United States. Iceberg lettuce is very heat-sensitive and was originally developed in 1894 for growth in the northern United States by [[Burpee Seeds and Plants]].  It gets its name from the way it was transported in crushed ice, where the heads of lettuce looked like [[iceberg]]s.<ref>{{cite news |title=Iceberg lettuce |date=July 3, 2007 |author=Renna |publisher=[[Tribune Publishing]] |work=[[Chicago Tribune]] |url=https://www.chicagotribune.com/news/ct-xpm-2007-07-04-0706290893-story.html |access-date=June 22, 2011}}</ref>  Today, it ships well, but is low in flavor and nutritional content, being composed of even more water than other lettuce types.<ref name="UI" />
# Butterhead&mdash;Also known as Boston or Bibb lettuce,<ref name="WSU" /> and traditionally in the [[UK]] as "round lettuce",<ref>{{cite web|title=Lettuce|url=https://realfood.tesco.com/glossary/lettuce.html|website=Tesco Real Food|publisher=[[Tesco]]|access-date=16 January 2017|archive-url=https://web.archive.org/web/20170118100805/https://realfood.tesco.com/glossary/lettuce.html|archive-date=18 January 2017|url-status=dead}}</ref> this type is a head lettuce with a loose arrangement of leaves, known for its sweet flavor and tender texture.<ref name="UI" />
# Summercrisp&mdash;Also called Batavian or French crisp, this lettuce is midway between the crisphead and leaf types. These lettuces tend to be larger, bolt-resistant and well-flavored.<ref name="WSU">{{cite web|url=http://vegetables.wsu.edu/winterlettuce.html|title=Winter Lettuce|publisher=Washington State University|access-date=26 March 2012|author=Miles, Carol|archive-url=https://web.archive.org/web/20120411080540/http://vegetables.wsu.edu/WinterLettuce.html|archive-date=11 April 2012|url-status=dead}}</ref>
# [[Celtuce]]/Stem&mdash;This type is grown for its seedstalk, rather than its leaves, and is used in Asian cooking, primarily Chinese, as well as stewed and creamed dishes.<ref name="UI" />
# Oilseed&mdash;This type is grown for its seeds, which are pressed to extract an oil mainly used for cooking. It has few leaves, bolts quickly and produces seeds around 50 percent larger than other types of lettuce.<ref>Katz and Weaver, p. 375.</ref>

The four main types in the Western world have been looseleaf, romaine, crisphead, and butterhead, with the others being intermediary or more exotic. The butterhead and crisphead types are sometimes known together as "cabbage" lettuce, because their heads are shorter, flatter, and more cabbage-like than romaine lettuces.<ref>{{Cite book|last=Rana|first=M. K.|url=https://books.google.com/books?id=QDgPEAAAQBAJ&q=The+butterhead+and+crisphead+types+are+sometimes+known+together+as+%22cabbage%22+lettuce,+because+their+heads+are+shorter,+flatter,+and+more+cabbage-like+than+romaine+lettuces&pg=PA173|title=Vegetable Crop Science|date=2017-10-02|publisher=CRC Press|isbn=978-1-351-65279-7|pages=173|language=en|access-date=28 September 2021|archive-date=28 September 2021|archive-url=https://web.archive.org/web/20210928054450/https://books.google.com/books?id=QDgPEAAAQBAJ&newbks=0&printsec=frontcover&pg=PA173&dq=The+butterhead+and+crisphead+types+are+sometimes+known+together+as+%22cabbage%22+lettuce%2C+because+their+heads+are+shorter%2C+flatter%2C+and+more+cabbage-like+than+romaine+lettuces&hl=en|url-status=live}}</ref>

=== Cultivation problems ===
[[File:Starr 081031-0356 Lactuca sativa.jpg|thumb|upright|A lettuce surrounded by weeds, which have crowded it to the point of [[bolting (horticulture)|bolting]]]]
{{see also|List of lettuce diseases}}

Soil nutrient deficiencies can cause a variety of plant problems that range from malformed plants to a lack of head growth.<ref name="Bradley129" /> Many insects are attracted to lettuce, including [[cutworm]]s, which cut seedlings off at the soil line; [[wireworm]]s and [[nematode]]s, which cause yellow, stunted plants; [[tarnished plant bug]]s and [[aphid]]s, which cause yellow, distorted leaves; [[leafhopper]]s, which cause stunted growth and pale leaves; [[thrip]]s, which turn leaves gray-green or silver; [[leafminer]]s, which create tunnels within the leaves; [[flea beetle]]s, which cut small holes in leaves and [[caterpillar]]s, [[slug]]s and [[snail]]s, which cut large holes in leaves. For example, the larvae of the [[ghost moth]] is a common pest of lettuce plants.<ref name="Edwards64">{{Cite journal|last=Edwards|first=C. A.|date=May 1964|title=The bionomics of swift moths. I.—The ghost swift moth, Hepialus humuli (L.)|journal=Bulletin of Entomological Research|volume=55|issue=1|pages=147–160|doi=10.1017/S000748530004935X|issn=1475-2670}}</ref> Mammals, including [[rabbit]]s and [[groundhog]]s, also eat the plants.<ref name="Bradley132">Bradley, pp. 129–132.</ref> Lettuce contains several defensive compounds, including [[sesquiterpene lactone]]s, and other natural [[phenols|phenolics]] such as [[flavonol]] and [[Flavonols#Flavonols glycosides|glycosides]], which help to protect it against pests. Certain varieties contain more than others, and some selective breeding and genetic modification studies have focused on using this trait to identify and produce commercial varieties with increased pest resistance.<ref>{{cite web|url=http://www.reeis.usda.gov/web/crisprojectpages/198636.html|title=Developing Multi-Species Insect Resistance in Romaine Lettuce|publisher=United States Department of Agriculture|access-date=5 April 2012|archive-url=https://web.archive.org/web/20131019193634/http://www.reeis.usda.gov/web/crisprojectpages/198636.html|archive-date=19 October 2013|url-status=dead}}</ref>

Lettuce also suffers from several [[Plant virus|viral diseases]], including [[Lettuce big-vein disease|big vein]], which causes yellow, distorted leaves, and [[lettuce mosaic virus|mosaic virus]], which is spread by aphids and causes stunted plant growth and deformed leaves. [[Aster yellows]] are a disease-causing bacteria carried by leafhoppers, which causes deformed leaves. Fungal diseases include [[powdery mildew]] and [[downy mildew]], which cause leaves to mold and die and [[Fungicide use in the United States#Bottom rot|bottom rot]], [[Fungicide use in the United States#Lettuce drop|lettuce drop]] and [[gray mold]], which cause entire plants to rot and collapse.<ref name="Bradley132" /> Bacterial diseases include ''[[Botrytis cinerea]]'', for which [[UV-C]] treatments may be used: Vàsquez et al. 2017 find that [[phenylalanine ammonia-lyase]] activity, phenolic production, and ''B. cinerea'' [[plant disease resistance|resistance]] are increased by UV-C.<ref name="Urban-et-al-2018">{{cite journal | last1=Urban | first1=L. | last2=Chabane Sari | first2=D. | last3=Orsal | first3=B. | last4=Lopes | first4=M. | last5=Miranda | first5=R. | last6=Aarrouf | first6=J. | title=UV-C light and pulsed light as alternatives to chemical and biological elicitors for stimulating plant natural defenses against fungal diseases | journal=Scientia Horticulturae | publisher=[[Elsevier]] | volume=235 | year=2018 | issn=0304-4238 | doi=10.1016/j.scienta.2018.02.057 | pages=452–459 | bibcode=2018ScHor.235..452U | s2cid=90436989}}</ref> Crowding lettuce tends to attract pests and diseases.<ref name="Weaver175" /> Weeds can also be an issue, as cultivated lettuce is generally not competitive with them, especially when directly seeded into the ground. Transplanted lettuce (started in flats and later moved to growing beds) is generally more competitive initially, but can still be crowded later in the season, causing misshapen lettuce and lower yields. Weeds also act as homes for insects and disease and can make harvesting more difficult.<ref>{{cite web|url=http://www.ipm.ucdavis.edu/PMG/r441700111.html|title=Lettuce: Integrated Weed Management|work=UC Pest Management Guidelines|publisher=University of California|date=August 2007|access-date=3 April 2012|archive-url=https://web.archive.org/web/20120707190840/http://www.ipm.ucdavis.edu/PMG/r441700111.html|archive-date=7 July 2012|url-status=dead}}</ref> Herbicides are often used to control weeds in commercial production. However, this has led to the development of [[herbicide-resistant]] weeds in lettuce cultivation.<ref name="Davey222" />