Editing Frost (section)

==Effect on plants==

===Damage===
[[File:Feuilles-avec-glace-leaves-with-ice-1.jpg|right|thumb|Frost on the grass of a public park in November]]
[[File:Geography of Ohio - DPLA - aaba7b3295ff6973b6fd1e23e33cde14 (page 31) (cropped).jpg|thumb|right|Map of average first killing frost in Ohio from "Geography of Ohio," 1923]]
Many plants can be damaged or killed by freezing temperatures or frost. This varies with the type of plant, the tissue exposed, and how low temperatures get; a "light frost" of {{convert|-2|to|0|C|F}} damages fewer types of plants than a "hard frost" below {{convert|-2|C|F}}.<ref name="BI_Frost">{{cite web |url=http://www.botanicalinterests.com/articles/view/26/Frost-Tolerance-of-Vegetables |title=Frost Tolerance of Vegetables |publisher=Botanical Interests |access-date=Nov 12, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131113010216/http://www.botanicalinterests.com/articles/view/26/Frost-Tolerance-of-Vegetables |archive-date=2013-11-13 }}</ref><ref>{{Cite journal |last1=Beerling |first1=D. J. |last2=Terry |first2=A. C. |last3=Mitchell |first3=P. L. |last4=Callaghan |first4=T. V. |last5=Gwynn-Jones |first5=D. |last6=Lee |first6=J. A. |date=April 2001 |title=Time to chill: effects of simulated global change on leaf ice nucleation temperatures of subarctic vegetation |url=http://doi.wiley.com/10.2307/2657062 |journal=American Journal of Botany |language=en |volume=88 |issue=4 |pages=628–633 |doi=10.2307/2657062|jstor=2657062 |pmid=11302848 }}</ref>

Plants likely to be damaged even by a light frost include vines—such as beans, grapes, squashes, melons—along with [[nightshade]]s such as tomatoes, eggplants, and peppers. Plants that may tolerate (or even benefit from) frosts include:<ref>{{cite web |url=http://www.grow-it-organically.com/fall-vegetables.html |title=Fall vegetables vs. Summer vegetables |url-status=live |archive-url=https://web.archive.org/web/20131113011159/http://www.grow-it-organically.com/fall-vegetables.html |archive-date=2013-11-13 }}</ref>
*root vegetables (e.g. beets, carrots, parsnips, onions)
*leafy greens (e.g. lettuces, spinach, chard, cucumber<ref name="JHR_2-2013">{{Citation|last=Klosinska|first=Urszula|date=27 February 2014|publisher=de Gruyter|title=Low temperature seed germination of cucumber: genetic basis of the tolerance trait|journal=Journal of Horticultural Research|volume=21|issue=2|pages=125–130|display-authors=etal|doi=10.2478/johr-2013-0031|doi-access=free}}</ref>)
*[[cruciferous vegetables]] (e.g. cabbages, cauliflower, bok choy, broccoli, Brussels sprouts, radishes, kale, collard, mustard, turnips, rutabagas)
Even those plants that tolerate frost may be damaged once temperatures drop even lower (below {{convert|-4|C|F|disp=or}}).<ref name="BI_Frost"/> Hardy perennials, such as ''[[Hosta]]'', become dormant after the first frosts and regrow when spring arrives. The entire visible plant may turn completely brown until the spring warmth, or may drop all of its leaves and flowers, leaving the stem and stalk only. Evergreen plants, such as pine trees, withstand frost although all or most growth stops. [[Frost crack]] is a bark defect caused by a combination of low temperatures and heat from the winter sun.

Vegetation is not necessarily damaged when leaf temperatures drop below the freezing point of their cell contents. In the absence of a site [[nucleation|nucleating]] the formation of ice crystals, the leaves remain in a [[supercooled]] liquid state, safely reaching temperatures of {{convert|-4|to|-12|C|F}}. However, once frost forms, the leaf [[cell (biology)|cells]] may be damaged by sharp ice crystals. [[Hardening (botany)|Hardening]] is the process by which a plant becomes tolerant to low temperatures. See also [[Cryobiology]].

Certain [[bacterium|bacteria]], notably ''[[Pseudomonas syringae]]'', are particularly effective at triggering frost formation, raising the nucleation temperature to about {{convert|-2|C|F}}.<ref>{{cite journal |vauthors=Maki LR, Galyan EL, Chang-Chien MM, Caldwell DR |title=Ice Nucleation Induced by Pseudomonas syringae |journal=Applied Microbiology |volume=28 |issue=3 |year=1974 |pages=456–459 |pmid=4371331 |pmc=186742 |doi=10.1128/aem.28.3.456-459.1974}}</ref> Bacteria lacking ice nucleation-active proteins ([[ice-minus bacteria]]) result in greatly reduced frost damage.<ref>{{cite journal
|last= Lindow |first= Stephen E. |author-link= Steven E. Lindow |author2=Deane C. Arny |author3=Christen D. Upper |title= Bacterial Ice Nucleation: A Factor in Frost Injury to Plants
|journal= Plant Physiology
|volume= 70
|issue= 4
|pages= 1084–1089
|date= October 1982
|pmid= 16662618
|doi= 10.1104/pp.70.4.1084
|pmc= 1065830}}</ref>

===Protection methods===
[[File:Winterschutz.jpg|thumb|right|[[Roses]] with protection against frost – [[Volksgarten, Vienna]]]]
[[File:CuritibaFrostGeada.jpg|thumb|right|[[Curitiba]] ([[Southern Brazil]]) is the coldest of [[Brazil]]'s [[state capital]]s; the [[greenhouse]] of the [[Botanical Garden of Curitiba]] protects sensitive plants.]]
Typical measures to prevent frost or reduce its severity include one or more of:
*Deploying powerful blowers to simulate wind, thereby preventing the formation of accumulations of cold air. There are variations on this theme. One variety is the wind machine, an engine-driven propeller mounted on a vertical pole that blows air almost horizontally. Wind machines were introduced as a method for frost protection in California during the 1920s, but they were not widely accepted until the 1940s and 1950s. Now, they are commonly used in many parts of the world.<ref>wind machine references: http://www.fao.org/3/y7223e/y7223e0d.htm ; https://extension.psu.edu/orchard-frost-protection-with-wind-machines ; http://www.omafra.gov.on.ca/english/engineer/facts/10-045.htm ;</ref> Another is the [[selective inverted sink]],<ref>[http://www.rolexawards.com/laureates/laureate-20-guarga.html Selective Inverted Sink] {{webarchive|url=https://web.archive.org/web/20060318212517/http://www.rolexawards.com/laureates/laureate-20-guarga.html |date=2006-03-18}} Rolex Awards site (won award in ''Technology and Innovation'' category) 1998.</ref> a device which prevents frost by drawing cold air from the ground and blowing it up through a chimney. It was originally developed to prevent frost damage to [[citrus]] fruits in [[Uruguay]]. In New Zealand, [[helicopter]]s are used in similar fashion, especially in the [[vineyard]] regions such as [[Marlborough Region|Marlborough]]. By dragging down warmer air from the [[Inversion (meteorology)|inversion layers]], and preventing the ponding of colder air on the ground, the low-flying helicopters prevent damage to the fruit buds. As the operations are conducted at night, and have in the past involved up to 130 aircraft per night in one region, safety rules are strict.<ref>''Helicopters Fight Frost'' – ''Vector'', [[Civil Aviation Authority of New Zealand]], September/ October 2008, Page 8-9</ref> Although not a dedicated method, [[wind turbine]]s have a similar (although smaller) effect of vertically mixing air layers of different temperature.<ref name="Nature-2010.12.23">[http://www.nature.com/nature/journal/v468/n7327/full/4681001a.html Turbines and turbulence] {{webarchive|url=https://web.archive.org/web/20111201212633/http://www.nature.com/nature/journal/v468/n7327/full/4681001a.html |date=2011-12-01}}, ''[[Nature (journal)]]'', 468, 1001, 23 December 2010, DOI:10.1038/4681001a, published online 22 December 2010.</ref><ref name="Roy+Traiteur">Somnath Baidya Roy and Justin J. Traiteur. [http://www.pnas.org/content/early/2010/10/12/1000493107.abstract Impacts of wind farms on surface air temperatures], Proceedings of the National Academy of Sciences, Vol. 107, No. 42, October 19, 2010, p. 17,899.</ref><ref name="ScienceDaily-2005">[https://www.sciencedaily.com/videos/2005/1012-wind_farms_impacting_weather.htm Wind farms impacting weather] {{webarchive|url=https://web.archive.org/web/20100906101845/https://www.sciencedaily.com/videos/2005/1012-wind_farms_impacting_weather.htm |date=2010-09-06}}, Science Daily.</ref>
*For high-value crops, farmers may [[tree wrap|wrap trees]] and use physical crop coverings.
*For high-value crops grown over small areas, heating to slow the drop in temperature may be practical.
*Production of smoke to reduce cooling by radiation, now thought to be of little benefit.<ref>{{Cite book |title=Frost protection: fundamentals, practice, and economics |first1=Richard L. |last1=Snyder |first2=J. Paulo |last2=de Melo-Abreu|url=https://www.fao.org/4/y7223e/y7223e0d.htm |chapter=Chapter 7 - Active protection methods |publisher=[[Food and Agriculture Organization of the United Nations]] |year=2005}}</ref>
*Spraying crops with a layer of water releases latent heat, preventing harmful freezing of the tissues of the plants that it coats.
Such measures need to be applied with discretion, because they may do more harm than good; for example, spraying crops with water can cause damage if the plants become overburdened with ice. An effective, '''low cost''' method for small crop farms and plant nurseries, exploits the [[enthalpy of fusion|latent heat of freezing]]. A pulsed irrigation timer<ref name=Nurseries/> delivers water through existing overhead sprinklers at a low volumes to combat frosts down to {{convert|-5|C|F}}.<ref name=Nurseries>{{cite web|title=A practical method of frost protection|url=http://plantsale.com.au/Articles/FrostProtection|access-date=31 October 2011|url-status=live|archive-url=https://web.archive.org/web/20120320075831/http://www.plantsale.com.au/Articles/FrostProtection/|archive-date=20 March 2012}}</ref><ref name=SmallCrops>{{cite web|last=Selders|first=Arthur W.|title=Frost protection with sprinkler irrigation|url=http://www.wvu.edu/~agexten/hortcult/fruits/om101.pdf|publisher=West Virginia university|access-date=31 October 2011|url-status=dead|archive-url=https://web.archive.org/web/20111114032130/http://www.wvu.edu/~agexten/hortcult/fruits/om101.pdf|archive-date=14 November 2011}}</ref> If the water freezes, it gives off its latent heat, preventing the temperature of the foliage from falling much below zero.<ref name=SmallCrops/>