Budburst – an exciting time of year
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.
PASSIVE FROST PROTECTION
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.
MANAGING FROST LOSS
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.
MONITOR NUTRIENT LEVELS
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.