Or­chard Man­age­ment

Bud­burst – an ex­cit­ing time of year

The Orchardist - - Contents - By John Wil­ton

I al­ways think that Septem­ber is a won­der­ful time in the or­chard with sum­mer­fruit in flower and early flow­er­ing va­ri­eties well into fruit­set.

Pipfruit bud­break is un­der­way too, so it’s time to as­sess re­turn bloom and check flower strength.

Tim­ing of bud­break, flow­er­ing and its spread is de­ter­mined by cul­ti­var, local mi­cro­cli­mate and whether or not dor­mancy break­ers have been ap­plied.

In my opin­ion we have ad­e­quate win­ter chill­ing for most de­cid­u­ous fruits. Rate of bud­break here is largely de­ter­mined by spring tem­per­a­tures and how con­sis­tent the warm­ing is. Un­like con­ti­nen­tal cli­mates, spring weather here is rather vari­able. Of­ten, we get a few days of spring, or per­haps even a hint of sum­mer with tem­per­a­tures reach­ing the mid 20ºC only to be fol­lowed by a re­turn to win­ter with tem­per­a­tures halv­ing for a day or two. If it’s clear, this is when dam­ag­ing frosts oc­cur.

Re­cent years have been mild, with few frost events. This does not mean that dam­ag­ing frosts are no longer a big risk. His­tory tells us that most of the crops we have lost were due to late frost that hit af­ter mid-Oc­to­ber. Some have been as late as early Novem­ber.


In re­cent years there has been a lot of cap­i­tal spent on frost pro­tec­tion, so the in­dus­try is in a much bet­ter po­si­tion now to deal with frosts.

Frost fans are by far the most widely used frost-fight­ing method. For top per­for­mance they rely on a warm air inversion layer within reach of the fan. Most frosts here are ra­di­a­tion frosts which oc­cur on calm, clear nights, when dense cold air set­tles close to the ground and warmer air heated by

long wave ra­di­a­tion from heat stored in the ground rises to form the inversion layer. Th­ese frosts usu­ally fol­low a southerly front which has dumped some snow on the ranges and cooled am­bi­ent air tem­per­a­tures.

If the southerly is a strong one pulling very cold air up from higher lat­i­tudes near the South Pole, am­bi­ent air tem­per­a­tures can fall to be­low zero overnight to give us an ad­vec­tion frost. Th­ese frosts do not have warm air inversion lay­ers so are much more dif­fi­cult to deal with. For­tu­nately, their in­ci­dence is rel­a­tively low in mid­dle New Zealand, but more com­mon in the south. As there is gen­er­ally no inversion layer, or a very weak one, frost fans are much less ef­fec­tive than in a ra­di­a­tion frost. The ad­vec­tion frosts we have are usu­ally not that cold com­pared with those ex­pe­ri­enced in the North­ern Hemi­sphere Con­ti­nen­tal cli­mates such as the one they had in Europe in April 2017 which had tem­per­a­tures well be­low crit­i­cal dam­age tem­per­a­tures. Ours usu­ally hover around crit­i­cal min­i­mum in­jury tem­per­a­tures. Frost fans can be fairly ef­fec­tive in th­ese frosts, par­tic­u­larly if they are used in con­junc­tion with a heat source such as un­der tree sprin­klers, or some strate­gi­cally placed stack heaters.

Even in the ab­sence of a heat source they will give some pro­tec­tion to a much re­duced area. The rea­son for this is that if the or­chard is well shel­tered, with lit­tle air move­ment, flow­ers and fruitlets ra­di­ate heat and be­come colder than the am­bi­ent air, which cools the air in con­tact with the plant. The

“Or­chard site, shel­ter and or­chard floor man­age­ment all im­pact on frost risk.”

frost fan stirs up the air within the tree mov­ing the cold air on which has a warm­ing ef­fect.

In spring 2002 we had an ad­vec­tion frost in Hawke’s Bay from which I was able to gather fruit set data from frost fans in an or­chard where there was no other form of frost pro­tec­tion. There were two ma­chines. The wind­ward ma­chine ad­e­quately pro­tected 1.39ha, and par­tially pro­tected a fur­ther 3ha. The lee­ward ma­chine, feed­ing off the wind­ward one ad­e­quately pro­tected 4.25ha, and par­tially pro­tected a fur­ther 3ha. Both ma­chines to­gether ad­e­quately pro­tected around 5.64ha. Had there been a good inversion layer or heat sup­plied its prob­a­ble that the two ma­chines to­gether would have ad­e­quately pro­tected twice that area. Fur­ther de­tails were pub­lished in the April 2003 Or­chardist.


Or­chard site, shel­ter and or­chard floor man­age­ment all im­pact on frost risk. Cold air drifts to and set­tles in low parts of the or­chard. Sites with good air drainage suf­fer less frost risk than flat sites where cold air ponds. River val­leys usu­ally have lower risk than land locked basins, sim­ply be­cause there is air move­ment along the val­ley. The river also acts as a heat source and will in­flu­ence frost lev­els along­side its banks.

Shel­ter will also in­flu­ence frost risk. Of­ten dur­ing frost events cold air flows off the land to­wards the sea, which on a frosty night is warmer than the land. Thick shel­ter­belts on the wind­ward side of a block can be de­signed to shed the cold air around the or­chard. Con­versely, a thick shel­ter belt on the down­wind side of an or­chard block can pond cold air be­hind it, lead­ing to heav­ier frosts than if the cold air was al­lowed to es­cape through or un­der the shel­ter belt.

Ground cover man­age­ment can make a huge dif­fer­ence to frost risk. The air above firm, damp soil will be con­sid­er­ably warmer in a ra­di­a­tion frost, than the air above pas­ture land, or a grassed or­chard. This is be­cause the soil is ex­posed to

di­rect sun dur­ing the day so can ab­sorb the suns heat and wa­ter has very good heat re­ten­tion prop­er­ties.

Dur­ing the frost sea­son the or­chard floor should be kept weed free and the grass sward mown down as short as prac­ti­cal.

An ex­cep­tion to this rule is where wa­ter sprin­kling is be­ing used for frost pro­tec­tion. In this sit­u­a­tion you need a large sur­face area on which to freeze wa­ter be­cause your heat source is from the la­tent heat of wa­ter freez­ing.

Un­less soil drainage is very good us­ing wa­ter for frost pro­tec­tion can do more harm than good through soil wa­ter log­ging. The other prob­lem with us­ing wa­ter is that sprin­kling for frost pro­tec­tion needs to com­mence while air tem­per­a­tures are above freez­ing point. At that time, it’s not cer­tain whether or not tem­per­a­tures will fall to dam­ag­ing lev­els dur­ing the frost. De­cid­u­ous tree fruit crops are rel­a­tively hardy com­pared to more frost ten­der crops such as grapes and ki­wifruit, so will usu­ally tol­er­ate tem­per­a­tures in the re­gion of -1.5 to -2.0°C be­fore sig­nif­i­cant dam­age oc­curs. Many frost events fail to reach th­ese tem­per­a­tures in which case frost in­jury may have been min­i­mal and much less dam­ag­ing than wa­ter log­ging the root sys­tem.


As al­ready men­tioned, pipfruit in par­tic­u­lar have rel­a­tively good frost tol­er­ance. This is mainly be­cause here in New Zealand blos­som pe­ri­ods are long, of­ten four to six weeks, so un­til well into petal fall there is a range of dif­fer­ent flower stages present so it has to be very se­vere frost for us to wipe out most of the crop. Frost up un­til late bloom stage will take out the more ad­vanced flow­ers and their set­ting fruit leav­ing later flow­ers still able to set fruit. Un­for­tu­nately, it’s the early flow­ers that have the most fruit po­ten­tial in re­gard to fruit size, yield and qual­ity. Th­ese are usu­ally the king fruit on spurs and ter­mi­nals. When th­ese are lost the tree com­pen­sates by set­ting fruit on the weaker later flower, usu­ally aux­il­iary buds of one-year wood. This late set fruit only has 70 to 80 per­cent of the siz­ing po­ten­tial of the crop taken out by the frost. This late set fruit usu­ally ripens around seven to 10 days be­hind strong spur and ter­mi­nal bud fruit. As frost in­jury is usu­ally not uni­form across a block the range of ma­tu­rity in the crop will widen lead­ing to mixed ma­tu­rity prob­lems at har­vest.

In ad­di­tion to crop loss, rus­set in­ci­dence in rus­set prone va­ri­eties such as Scired or Fuji is much higher on frosty sites.

You do not have to have ob­vi­ous crop loss for frost pro­tec­tion to pay. The re­turn is of­ten in grow­ing a bet­ter qual­ity crop.


Th­ese days we rely heav­ily on crop load for vigour con­trol.

In a mod­ern in­ten­sive plant­ing los­ing a crop from frost can be a dis­as­ter. For this rea­son, it’s nec­es­sary to have frost pro­tec­tion in place by the sec­ond or third year af­ter plant­ing. Crop loss at this crit­i­cal stage of the or­chards canopy de­vel­op­ment can re­sult in ex­ces­sive tree vigour, which ex­pe­ri­ence shows may take sev­eral more grow­ing sea­sons to re­cover from.

Frost in­jury, like hail, al­ways looks much worse im­me­di­ately af­ter the event than a week or two later. Within a mat­ter of hours af­ter the event it’s pos­si­ble to see in­jury. Frosts around and im­me­di­ately af­ter bloom usu­ally kill the ovule which turn brown within a cou­ple of hours. Flow­ers in­jured in this way do not set. In young fruitlets the de­vel­op­ing seed are also killed by frost and th­ese fruitlets drop. Un­less the blos­som pe­riod is very com­pacted there will usu­ally be enough un­dam­aged flower which will set to com­pen­sate for that lost in the frost. Late frost oc­cur­ring when fruit­let size is 10 to 15mm or larger tend not to kill the seed be­cause it is too well pro­tected by the grow­ing fruit tis­sue around it. This fruit usu­ally does not drop, but will show its in­jury as dis­tor­tion, rus­set e.g. frost rings around the ca­lyx end of the fruit, or crack­ing which is some­times in­ter­nal.

Oc­ca­sion­ally frost will af­fect chem­i­cal thin­ner be­hav­iour. NAA re­sponse can be in­creased as a re­sult of se­vere frost in­jur­ing fo­liage in the pre-bloom pe­riod. We had this oc­cur a few years back in one of our chem­i­cal thin­ning tri­als when the thin­ning re­sponse from blos­som treat­ments with NAA

“Gen­er­ally, it is a mis­take to aban­don chem­i­cal thin­ning fol­low­ing a frost.”

in them was more ag­gres­sive com­pared to other treat­ments in­clud­ing ATS and ethep­hon.

Ethep­hon blos­som thin­ners are also very sen­si­tive to frost if it oc­curs very close to their ap­pli­ca­tion. Th­ese days I would try to avoid blos­som thin­ner ap­pli­ca­tion near to frost events be­cause we have very good post bloom fruit­let thin­ners. Frost is li­able to dam­age tis­sue which may in­crease rus­set in­ci­dence when blos­som thin­ners are ap­plied as well.

If frost dam­age oc­curs in the bloom pe­riod wait and as­sess the in­jury be­fore con­tin­u­ing with the chem­i­cal thin­ning pro­gramme. Within a few days it should be pos­si­ble to see the level of loss so there is still plenty of time to ap­ply post blos­som thin­ners. Do not panic, time is on your side.

Gen­er­ally, it is a mis­take to aban­don chem­i­cal thin­ning fol­low­ing a frost. Post bloom thin­ners rely heav­ily on in­ter­fruit com­pe­ti­tion so if the frost has sig­nif­i­cantly re­duced fruit set their thin­ning ef­fect will be weak. Frost ef­fects in re­gard to re­duced fruit set will be quite vari­able with low frost prone ar­eas more badly hit than warmer ar­eas. Of­ten most of the crop loss will be in the lower tree, in which case the tree will com­pen­sate by set­ting even more fruit in the up­per tree. In the ab­sence of ef­fec­tive chem­i­cal thin­ning this will give you a huge hand thin­ning bill, not to men­tion the pos­si­bil­ity of the tree fall­ing into bi­en­nial bear­ing.


If there is sig­nif­i­cant crop loss, ex­cess vigour is likely to fol­low. Our ex­pe­ri­ence shows that once this oc­curs it will take more than one sea­son to bring tree vigour un­der con­trol. Fol­low­ing a se­vere frost vigour con­trol mea­sures need to be im­ple­mented to avoid ex­cess vigour.

If there is good ir­ri­ga­tion avail­able to man­age wa­ter stress and the soil de­void of stones, sin­gle side root prun­ing is prob­a­bly the best op­tion. This will put the brakes on run­away shoot

growth. It should be im­ple­mented as soon as its ob­vi­ous that the crop has been lost.

If the soil is un­suit­able for root prun­ing, trunk girdling is prob­a­bly the next best op­tion.

If the ex­cess vigour chal­lenge is not too great, a growth reg­u­la­tor pro­gramme may work and is well worth con­sid­er­a­tion. Re­galis ® is very ef­fec­tive for vigour con­trol if ap­plied with cor­rect tim­ing. Ethep­hon ap­plied dur­ing the main growth flush pe­riod also has mild to mod­er­ate vigour con­trol. Where vigour is high, nei­ther of th­ese growth reg­u­la­tors are ef­fec­tive enough, or need to be used at high rates which brings their own set of prob­lems. For ex­am­ple, with Re­galis ® there is good vigour con­trol dur­ing the three­spray pro­gramme pe­riod, af­ter which shoot growth will grow through the treatment lead­ing to a late growth flush. Just what ap­ple leaf curl­ing midge needs to cause prob­lems at har­vest.


Ad­e­quate ni­tro­gen lev­els are nec­es­sary for good fruit set. Th­ese days, in or­der to max­imise fruit colour, we tend to run min­i­mum ni­tro­gen fer­tiliser pro­grammes. The idea is to try and run ni­tro­gen lev­els down to­wards har­vest, of­ten into the de­fi­ciency range where spring tis­sue lev­els will be too low for sat­is­fac­tory fruit set.

To avoid this prob­lem, ni­tro­gen lev­els have to be lifted back into the nor­mal range by the crit­i­cal fruit set stage. This is why we rec­om­mend sig­nif­i­cant amounts of ni­tro­gen, ei­ther as fo­liar or soil ap­plied in the post-har­vest pe­riod. The ini­tial spring growth flush and fruit set ni­tro­gen re­quire­ments come from the mo­bil­i­sa­tion of stored re­serves in the tree, not spring soil-ap­plied ni­tro­gen fer­tiliser.

A few years ago, many growers be­lieved that a urea spray im­me­di­ately be­fore a fore­cast frost event would min­imise frost in­jury fruit set losses. It’s doubt­ful if such a spray would do much to re­duce frost in­jury even though there were re­ports of bet­ter fruit set fol­low­ing the spray. My own opin­ion is that th­ese spring urea ap­pli­ca­tions were re­duc­ing ni­tro­gen de­fi­ciency and lift­ing tis­sue lev­els above the lev­els crit­i­cal for fruit set.

Th­ese days we are grow­ing trees in­ten­sively on dwarf­ing root­stocks with lim­ited root sys­tems, as well as push­ing yields up. Root sys­tems may be chal­lenged to ob­tain ad­e­quate nu­tri­ents to sus­tain th­ese heavy

crops. In north­ern Europe where th­ese in­ten­sive plant­ing sys­tems have been around for quite a while now a lot of the em­pha­sis is given to pre-bloom, and bloom nu­tri­ent sprays, par­tic­u­larly ni­tro­gen.

As in­sur­ance against weak fruit set due to low ni­tro­gen lev­els ap­pli­ca­tion of sev­eral low bi­urette urea sprays com­menc­ing around open clus­ter when there is suf­fi­cient leaf area for up­take, is worth con­sid­er­ing. Do not ap­ply urea sprays too close to ATS thin­ning sprays.

High fruit phos­pho­rus lev­els are associated with im­proved fruit colour. There is ex­per­i­men­tal ev­i­dence from Canada to show that sol­u­ble forms of phos­pho­rus fer­ti­gated over the four­week pe­riod fol­low­ing bloom lifts ap­ple phos­pho­rus con­cen­tra­tions and yields in young or­chards.


Early sea­son fo­liar anal­y­sis taken about three to four weeks af­ter bloom can in­di­cate nu­tri­ent sta­tus and are use­ful when used on a com­par­a­tive ba­sis to di­ag­nose prob­lems such as poor fruit set or pale fo­liage.

The ad­van­tage of early sea­son leaf test­ing is that where low lev­els of a par­tic­u­lar nu­tri­ent are found it can be quickly rec­ti­fied early in the fruit growth pe­riod.

As a rule, low ni­tro­gen lev­els are the nu­tri­ent most likely to be associated with fruit set and re­ten­tion prob­lems.

Fig 2. Early bloom in Scifresh. Note dif­fer­ent stages of bloom present. Late flower will com­pen­sate for early flower lost in frost, but this late flower fruit is usu­ally smaller.

Fig 3. The ad­vec­tion frost in Europe last year took the crop out in some or­chards.

Fig 4. This Royal Gala type near the green va­ri­ety in Fig 3 fared bet­ter in the frost by set­ting fruit on late flower in the up­per tree. Note the thin crop in the lower tree.

Fig 5. Heavy frost in the pre-bloom pe­riod can in­jure spur leaves. Th­ese dam­aged leaves can also in­crease NAA up­take caus­ing over thin­ning.Fig 6. Frost can cause crack­ing and split­ting in some va­ri­eties. Scifresh, Brae­burn and pears typ­i­cally show this symp­tom.

From top:Fig 8. Nec­tarine fruit dis­tor­tion and pit­ting at har­vest. Prob­a­bly caused by late spring frost.Fig 9. In­ter­nal crack­ing and brown­ing in nec­tarines prob­a­bly caused by late spring frost in­jury.

Fig 10. Fuji spring leaf level around three per­cent ni­tro­gen – good fruit set.

Fig 11. Pale leaves here in­di­cate low ni­tro­gen. Fruit­set very poor.

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