Feel­ing the frost

The Orchardist - - Contents - By Erica Faber

Re­cent frost events have af­fected some grow­ers with vary­ing de­grees of dam­age to fruit, leaves, branches and flower buds. So, what we can do to min­imise frost dam­age and be bet­ter pre­pared.

FROST DAM­AGE

The amount of frost dam­age ex­pe­ri­enced de­pends on the cul­ti­var or root­stock’s sen­si­tiv­ity to freez­ing and the length of time the tem­per­a­ture is be­low the “crit­i­cal dam­age” tem­per­a­ture.

Frost dam­age re­sults not from the cold tem­per­a­ture but mainly from ice for­ma­tion in­side plant tis­sue.

Av­o­cado leaves ap­pear wilted dur­ing pe­ri­ods of low tem­per­a­ture.This is the tree’s nat­u­ral pro­tec­tive re­sponse to freez­ing tem­per­a­tures and does not mean the leaves have been frozen. Leaves will be firm and brit­tle and of­ten curled when frozen be­com­ing flac­cid af­ter thaw­ing. If the frost dam­age is not too se­vere, they grad­u­ally re­gain tur­gor and re­cover, leav­ing dark “water stain” marks on the leaves.

A se­vere frost will af­fect fruit, flow­ers, twigs, branches and leaves. Leaves will col­lapse and dry out and may re­main on the tree for sev­eral weeks. If, how­ever the leaves are rapidly shed, this is a good sign and means that the twigs and branches have not been se­verely dam­aged!

Symp­toms of frost dam­age can take up to a week or longer to show the full ex­tent of the dam­age. Plant tis­sue that freezes gen­er­ally ap­pear dark green and water soaked at first, later be­com­ing brown and necrotic.

FROST PREVEN­TION AND MINIMISATION

SITE SE­LEC­TION

First and fore­most is site se­lec­tion. Cold air is heav­ier than warm air and will ‘flow’ down slopes and set­tle in the low­est ar­eas, pro­gres­sively fil­ter­ing back up the slopes as more cold air ac­cu­mu­lates in th­ese low-ly­ing ar­eas. Avoid plant­ing in th­ese ar­eas if pos­si­ble.

ROW ORI­EN­TA­TION

When plan­ning your or­chard, run your rows up and down the slope if pos­si­ble, to al­low for max­i­mum air drainage and don’t plant in the low­est ar­eas. If the gra­di­ent is too steep aim for some down­ward slope to the row. Avoid rows run­ning di­rectly across a slope which trap cold air in pock­ets. Care­fully plan row de­sign in or­chards with gen­tly un­du­lat­ing slopes, as they can lead to lo­calised pock­ets of cold air.

SHEL­TER­BELTS / WINDBREAKS

Any­thing that re­stricts the flow of cold air e.g. a dense wind­break, will re­sult in a build-up of cold air within the or­chard caus­ing a lo­calised frost event. En­sure shel­ter­belts are skirted to at least knee height to al­low drainage of the cold air out of the or­chards.

WEED/GRASS MAN­AGE­MENT

When long grass or weeds are present in an or­chard, sun­light is re­flected from the sur­face. Less heat en­ergy is ab­sorbed by the soil and re­leased at night, mak­ing it more prone to frost dam­age. Mow­ing or spray­ing the tree rows with an ap­pro­pri­ate her­bi­cide are meth­ods of con­trol.

SOIL MOIS­TURE LEV­ELS

Ther­mal con­duc­tiv­ity and heat con­tent of soils are af­fected greatly by the soil water con­tent. On a daily ba­sis heat is trans­ferred into and out of ap­prox­i­mately the top 30cm of the soil. When the soil is wet, heat trans­fer and stor­age in the up­per soil layer is im­proved, so more heat is stored dur­ing day­light for re­lease dur­ing the night.

If your soils are dry or not at field ca­pac­ity and you are aware of an up­com­ing frost event, wet the soils one to three days prior to this. Even wa­ter­ing the evening be­fore a frost in­creases the heat given off by soil dur­ing the night. It is un­nec­es­sary to wet the soil deeply as most of the daily heat-trans­fer and stor­age oc­curs in the top 30 cm.

PLANT NU­TRI­TION & TREE HEALTH

Trees with poor tree health or nu­tri­ent de­fi­cien­cies are more sus­cep­ti­ble to frost dam­age. The re­la­tion­ship between spe­cific nu­tri­ent ap­pli­ca­tions and in­creased re­sis­tance to frost dam­age how­ever is ob­scure, and there are many con­tra­dic­tions. In gen­eral, though, nitro­gen fer­til­iza­tion be­fore a frost en­cour­ages growth and in­creases sus­cep­ti­bil­ity to frost dam­age.

To en­hance hard­en­ing of plants, avoid ap­pli­ca­tions of nitro­gen fer­til­izer in early au­tumn. Potas­sium has a favourable ef­fect on water reg­u­la­tion and pho­to­syn­the­sis in plants which could be a ben­e­fit un­der frost con­di­tions. How­ever, re­searchers are di­vided about the ben­e­fits of potas­sium for frost pro­tec­tion.

COP­PER AP­PLI­CA­TION

Con­trol­ling Ice Nu­cle­at­ing Bac­te­ria (INB) with cop­per-based prod­ucts is be­lieved to have some af­fect in pro­tect­ing plants when tem­per­a­tures drop and frost oc­curs. In the case of frost, th­ese bac­te­ria will freeze first on the sur­face of the plant. The plant tis­sue it­self does not ac­tu­ally freeze un­til the tem­per­a­ture reaches around -2°C to -4°C. If tem­per­a­tures drop fur­ther, dam­age will oc­cur ir­re­spec­tive of any ap­pli­ca­tion of cop­per sprays. It is also im­por­tant to note that ad­e­quate time must be given between ap­pli­ca­tion and a frost event (about 10 days), so that the ice-nu­cle­at­ing bac­te­ria can be de­stroyed. Re­search on this topic how­ever is not con­clu­sive.

AP­PLI­CA­TION OF HOR­TI­CUL­TURAL MIN­ERAL OILS

The ap­pli­ca­tion of pes­ti­cide oils to cit­rus is known to in­crease frost dam­age and ap­pli­ca­tion of oils should rather be avoided shortly be­fore the frost sea­son.

CHEM­I­CAL FROST PROTECTANTS

Chem­i­cal sprays such as ad­ju­vants like Va­por­gard or frost protectants such as Ther­mo­max claim to im­prove frost har­di­ness of plants. Sci­en­tific lit­er­a­ture re­veals few ex­am­ples of ef­fec­tive spray on frost pro­tec­tion. How­ever, there have been sev­eral re­ports of field ap­plied sprays which have re­sulted in de­pressed freez­ing point for ex­cised leaf, flower and shoot tis­sue mea­sured un­der lab­o­ra­tory con­di­tions. Gen­er­ally, th­ese re­sults have failed to translate into con­sis­tent, com­mer­cially vi­able forms of frost pro­tec­tion. How­ever, anec­do­tal re­ports of suc­cess­ful field treat­ments con­tinue to cir­cu­late so weigh up the cost of the sprays ver­sus the eco­nomic loss due to a frost event and it may be worth­while try­ing. Tim­ing though is key.

IN­SU­LA­TION

Ap­ply frost cov­ers to young trees if they are planted in a frost prone area. Side and top cover is re­quired for pro­tec­tion against frost. Stem pro­tec­tors or trunk guards can also help pre­vent to­tal tree death by pro­tect­ing the main stem.

SPRIN­KLERS

Over-plant or over-head sprin­klers pro­vide ex­cel­lent frost pro­tec­tion. The main dis­ad­van­tages with us­ing th­ese sprin­klers how­ever, are the high in­stal­la­tion cost and the large amounts of water needed. This will there­fore not be an op­tion where water avail­abil­ity is lim­ited. Ex­ces­sive use of

“Jack Frost is cer­tainly not a grower’s friend but some pos­i­tive frosty facts are that it can dis­rupt pest and dis­ease cy­cles, and im­prove soil struc­ture when mois­ture within the soil freezes, ex­pand­ing and split­ting open soil par­ti­cles.”

water for frost pro­tec­tion can also lead to soil wa­ter­log­ging and phy­toph­thora as well as nu­tri­ent leach­ing. The wet canopy can also ex­ac­er­bate fun­gal dis­eases on the fruit. Good man­age­ment and tim­ing is es­sen­tial.

For un­der-plant mi­cro sprin­klers, which ap­ply less water than con­ven­tional over­head sprin­klers, the goal is to keep only the ground un­der the plants near 0°C in or­der to con­cen­trate and en­hance ra­di­a­tion and heat trans­fer up­wards into the tree.

HEATERS

Heaters pro­vide pro­tec­tion by di­rect ra­di­a­tion to the plants and sur­round­ings and by caus­ing con­vec­tive mix­ing of air within the in­ver­sion layer. When heaters are op­er­ated, the heated air rises. As the heated air rises, it cools un­til it reaches the height where the am­bi­ent air has the same tem­per­a­ture. Then the air spreads out and, even­tu­ally, the air de­scends again. A cir­cu­la­tion pat­tern is cre­ated. How­ever, if the in­ver­sion is weak, the heated air cools, but it rises too high and a cir­cu­la­tion pat­tern is not pro­duced. As a re­sult, heaters may not pro­vide ad­e­quate pro­tec­tion when there is lit­tle or no in­ver­sion. Mak­ing fires too hot will also make heaters less ef­fec­tive be­cause the heated air rises above the in­ver­sion ceil­ing and the cir­cu­la­tion pat­tern is not cre­ated. Heaters can be as rudi­men­tary as metal drums filled with burn­ing wood or as so­phis­ti­cated as propane heaters.

HE­LI­COPTERS

Dur­ing cold and still con­di­tions, the down draught from a he­li­copter fly­ing slowly over the or­chards causes the slightly warmer air lay­ered above the in­ver­sion layer to cir­cu­late down­wards amongst the trees, re­duc­ing the like­li­hood of frost dam­age. A he­li­copter should pass over the en­tire crop ev­ery 30 min­utes dur­ing mild freezes and more of­ten dur­ing se­vere freezes. How­ever, if there is lit­tle or no in­ver­sion, he­li­copters are not very ef­fec­tive. Re­mem­ber to no­tify neighbouring or­chards or bet­ter yet, com­bine the flight to share the costs.

NEW IN­NO­VA­TIONS

For good old fash­ion kiwi in­ge­nu­ity re­fer to http://www. towand­blow.co.nz/ for a por­ta­ble diesel fu­elled wind ma­chine for frost pro­tec­tion!

FORE­CAST­ING AND MON­I­TOR­ING

Fore­cast­ing the min­i­mum ground tem­per­a­ture and how the tem­per­a­ture might change dur­ing the night is use­ful for frost pro­tec­tion be­cause it helps us as grow­ers de­cide if pro­tec­tion is needed, when to take ac­tion and what ac­tion to take. Fore­warned is fore­armed! Go to http://about. metser­vice.com/our-com­pany/learn­ing-cen­tre/ frost/ for more info on mon­i­tor­ing and fore­cast­ing. Jack Frost is cer­tainly not a grower’s friend but some pos­i­tive frosty facts are that it can dis­rupt pest and dis­ease cy­cles, and im­prove soil struc­ture when mois­ture within the soil freezes, ex­pand­ing and split­ting open soil par­ti­cles. So much for the pos­i­tives...

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