By Erica Faber – Technical Manager 

Understanding avocado flowering can be confusing and can have a significant impact on your orchard’s production. For our newer growers, as well as our seasoned growers, I thought I would use this opportunity to revisit and feature a topic that is arguably one of the most important stages of growing avocados –
flowering, pollination, fruit set and ultimately orchard productivity.

Pruning, moisture management and nutrition are also important at this critical time. It is also the time of year we are harvesting and planting out new orchards so I have included links to plenty to read up on below to support your actions.

A mature avocado tree can produce more than one million flowers with fruit set ranging from 0.3-0.5%. The avocado flower is very unique in that each flower has a distinct male and female phase which is nature’s way of encouraging cross pollination between different cultivars and ensuring species diversity. This however does not preclude self and close pollination within the same cultivar as is demonstrated in orchards of only one cultivar or individual trees in home gardens.

Briefly explained, avocado cultivars can be divided into either A or B flowering cultivars.

For A-flowering cultivars e.g. Hass, the flower will open for the first time (day one) functioning in the female phase with only the female parts being receptive. The flower then closes after two to six hours and reopens in the afternoon of day two in the male phase, with the male flower parts now being functional and shedding pollen.

The flower remains open and finally closes for the last time the next morning. If pollination and fertilisation was successful, the flower will remain on the tree and begin to form a fruit. If fertilisation was not successful, ethylene will be produced at the base of the ovule and the flower will abscise (fall off).

For B-flowering cultivars e.g. Zutano, the flower opens on the afternoon of day one in the female phase, closes and reopens from the morning to afternoon of day two in the male phase, synchronising with the opening of the A-cultivar in its female phase. Pollen transfer from the B cultivar to the A cultivar can then take place.

Flower in female phase with stigma exposed and stamens lying flat against tepals.

Flower in male phase with stamens upright and anthers releasing pollen.

This synchronisation however seldom occurs perfectly and is mostly only synchronised at warmer temperatures of minimums of 15oC and maximums
of 25oC.

The cooler New Zealand climate and variable spring weather result In delayed and irregular opening in the female phase with a greater overlap occurring between the female and male phases.

The pollen transfer within the same tree or neighbouring trees of the same cultivar during this overlap period result in close pollination. Isozyme analysis of fruit has supported this and shown that most of the fruit (up to 85%) is fertilised with pollen from the same varieties’ own pollen i.e. Hass pollen for Hass fruit.

In higher temperatures with lower humidity, the stigma viability is short lived and it will appear brown or shrivelled on day two but in climates with high humidity and/or cooler temperatures, the stigma is still receptive and viable on day two when the flower is in the functioning male phase making self-pollination possible.

Regardless of the flowering behaviour or rather misbehaviour, without pollen transfer, pollination and fertilisation cannot take place.

Pollen is transferred by a variety of insects, by wind and even by gravity.

The honeybee however takes the lead role and is the principal pollinator. To ensure the best possible pollination and fruit set we need to ensure that there are enough bees within the orchard. With heavy flowering and cooler climates, more hives per hectare are required and eight to twelve hives in these instances are advised.

Bumblebees come in at a good second place as an effective pollinator for avocados and are less averse to cold, windy, and wet weather conditions. A 1:4 ratio of bumble to honeybee hives is recommended for optimum pollination.

The story does not end there. Once pollen is transferred, many conditions must be met before fruit set can occur.

To ensure optimum fertilisation, research has shown that the more pollen that is transferred to the stigma the better the chance of fertilisation.

At least 20 golden pollen grains appear to be the magic number as some pollen doesn’t germinate, some pollen tubes don’t have the “vigour” to make it all the way to the ovule and some pollen tubes grow too slowly and the ovule loses its viability.

Once the pollen grain adheres to the sticky stigma, it starts to germinate, producing a pollen tube which grows down through the style, enters the ovary, finally reaching the ovule. It can take anything from a few hours to 24 hours from germination to fertilisation. Fertilisation then initiates the development of the ovary into a mature avocado fruit and the ovule into the seed.

There are also various other factors that affect fertilisation. The cooler the temperature, the slower the pollen tube growth rate. Temperature and humidity also affect the viability and receptiveness of the stigma to receive pollen. The higher the temperature and lower the humidity, the quicker the stigma will shrivel. If the stigma shrivels before the pollen tube has finished its growth to the ovules, fertilisation will not occur.

The starch content of the floral tissue and nutrient status also has an influence on the capacity of the flower to develop successfully into a fruit and has an influence on early fruitlet abscission.

Many flowers are also not capable of developing into mature fruit due to abnormalities in the embryo or embryo abortion. Water stress, nutrient imbalances, carbohydrate reserves and levels and ratios of different hormones also influence fruit set and abscission.

It is no wonder that the flower to fruit ratio is so low in avocados, but there are some steps we can follow to
ensure optimal flowering, pollination, fertilisation and fruit set.

What you can do to improve fruit set:
Application of nutrients
This is a period of very high nutrient demand and withdrawal due to the existing crop as well as development of flowers, nectar production etc. You can optimise fertilisation and fruit set by applying foliar / flower sprays of specific nutrients. For example, boron is especially important for pollen germination, pollen tube growth, fertilisation and fruit set. Zinc is important during reproduction for protein synthesis. If you don’t apply regular foliar sprays on your orchard, then ensure you time at least one Boron, Zinc and seaweed foliar spray when 50% of the trees are at 50% cauliflower stage, 25% at earlier flower development and 25% approaching full bloom. Flowers are in fact more efficient at absorbing foliar sprays than leaves. Consider also applying foliar sprays to support any heavy/over flowering or stressed trees with products high in amino acids and enzymes.

Read your trees!
By now, the fat round flower buds that were sitting tightly in the leaf axis have extended and developed into panicles full of flowers ranging from closed, open or “spent”. In some orchards, depending on your region or microclimate, flowering may be more advanced and you will want to regularly assess your flowers from the beginning of September to gauge when to get your bees in. Do not go off historical dates as each season will differ.

Find a warm sunny spot in your orchard and examine the flowers on the Northern quadrant. Closed flowers does not mean that flowering hasn’t commenced. Examine the shape of the bud closely. Unopened buds appear round and have a greenish yellow tinge. Flowers that are in their first or second day of opening are more oval and slightly more yellow and spent flowers become elongated and are elliptical in shape and more of a mustard yellow. These spent flowers are no longer viable and will in time fall off or if pollinated, pin head fruit will start to appear as the outer petals fall off.

Spent flowers (flowers in centre of image).

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Beehives in by 5-10% flowering
Improve pollination by ensuring you have adequate beehives present from at least 5-10% flowering. The bees take time to settle in and resume full activity
and strength.

Growers are often conservative on the number of hives per hectare. Depending on your planting density and flowering intensity, aim for eight to twelve hives per hectare.

Place hives in small groups spread throughout the orchard, in sheltered, sunny areas. The only way for fruit to occur is to move pollen onto flowers and lots of it so to encourage your bees to stay busy pollinating your flowers, place a source of water close to the hives so that the bees do not have to travel far in search of water. Place a branch or something that floats in the water trough so that the bees can safely drink and not drown.

Pruning improves bee activity as more open canopies and orchards result in warmer orchards and easier flight paths throughout the tree and orchard.

Contact your apiarist if any spraying needs to be done and use bee friendly products or spray at night.

If, over pollination the weather conditions are not favourable for bee activity, consider applying a pheromone spray like Bee-Scent”.

Bee-Scent™ is a pheromone-based liquid formulation that attracts honeybees to treated blossoms and encourages foraging behavior. The bees stay focused on working the blossoms harder. This increase inforaging intensity improves crop pollination.

With heavy flowering and cooler climates, more hives per hectare are required and eight to twelve hives in these instances is advised.

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Monitor and manage moisture levels
Any water stress over fruit set results in higher fruitlet drop and consequently lower production. Although our rainfall is high, we unfortunately often have dry periods over this critical time.

Should you be fortunate enough to have irrigation, now is the time that this investment will pay off. Use moisture meters to measure soil moisture levels to ensure that you are applying the required amount of water.

The correct placement of irrigation meters is important. Position your meters on the northern quadrant halfway between the trunk and dripline. Use a tree that is representative of the irrigation block.

When irrigating, regularly check for blocked or missing nozzles, leaks in the lines or poor water pressure. Remember to do seasonal maintenance and check that the irrigation system is working properly before you need to start up irrigation for the season again.

Regular pruning
A regular pruning programme will ensure adequate light into the canopy which in turn results in flowers and fruit throughout the canopy and a more even crop load on the entire tree and not just predominantly the top or the periphery. It also results in better bud quality as by pruning you essentially are also “reducing” the crop load by removing limbs that would flower and fruit. A form of flower pruning before the tree expends the energy producing the actual flowers.

Production performance is also higher on renewed wood.

Early season harvest
In heavy flowering on years when the tree is also carrying a crop, it is perhaps advisable to reduce the crop load on the tree. Fruit and flowers on a tree are a massive energy and nutrient drain on the tree and a stressed tree will decline further if its crop load is not well managed.

Encourage spring flush
Monitor your flowering to ensure a good proportion of both determinate and indeterminate flowering. Without adequate spring flush, not only will you leave fruit exposed to sunburn and less resources for optimal fruit sizing and general tree performance, but you will also compromise next year’s crop as there will not be enough flowering wood to ensure consistent yields. In this case, consider revising your nutrient programme to push.

Flowering on well-managed regrowth.

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The pruning window is closing so ensure you have booked your contractors to get your pruning done. Pruning ensures better production and more consistent cropping. Marking the limbs before the contractor arrives will save time and money and the job can get done quickly. Alternatively ensure your contractor has a good understanding of what pruning method you are following.

Remember that pruning is not just about getting light into the canopy but also ensuring good regrowth that will optimise future production.

Ensure pruning cuts are made at an angle so that rainwater runs off and also treat pruning cuts to avoid infection and secondary rots.

The pruned limb as well as any exposed limbs will need to be treated against sunburn. Sunburn on exposed branches and new pruning cuts as well as secondary rots and cankers can cause irreparable damage affecting tree health, performance and yield. Apply a 1:1 ratio of water and acrylic white paint (a cheap contractor grade will suffice) if painted on or dilute it down further if spraying it. Be careful of overspray and ensure that no paint is sprayed onto any fruit. Alternatively Surround can be used and sprayed onto any exposed limbs/branches. If using Surround, overspray onto fruit will not be a problem. Although the easier option, Surround however will not have the persistence of the paint/water option.

Remember to manage the pruning regrowth by selecting and tipping the shoots you want and thinning out the rest. This will not only improve and optimize the bearing structure and capacity of the tree but also ensure quicker return to production as well as improved flower bud quality.

The pruning window is closing so ensure you have booked your contractors to get your pruning done.

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Nutrition management is important going into fruit set as any deficiencies or imbalances will compromise the percentage of fruit you set. If you have managed winter nutrition well, you will have managed to maintain or elevate nutrient levels especially important for fertilisation and fruit set.

With increased soil moisture levels and temperatures, roots should be actively flushing and nutrient uptake increasing. You should have already applied your first solid fertiliser applications of the season, starting with lighter applications at the beginning of the season and building up the applications as the season and demands increase. Remember also that balanced application is critical. Avoid high nitrogen levels that result in excessive fruitlet drop, high pest pressure, reduced calcium uptake and an increase in postharvest diseases and shorter shelf life. Foliar applications over flowering as mentioned above is especially beneficial.

Winter, flowering progression and intensity as well as holding crop, can all have a detrimental impact on tree health. If you have noticed
tree decline, then assess your tree and rate it according to the Ciba Geigy chart found in this article – https://justavocados.co.nz/phytophthora-root-rot-control/.

This will enable you to understand what remedial action to take to gain back tree health and performance and improve production.

Click on the link below to refer to our Remedial programme for avocados with poor health for managing any stressed or poor health trees: https://www.facebook.com/groups/justavocadosgrowers/permalink/1384501125080255.

Spring is also the time where new plantings, replants or interplants commence. Soil sampling, land prep, corrections, pegging out and irrigation should have been completed in anticipation of planting.

On receipt of your trees, check to see that you are happy with the quality and that your trees are uniform and comply with the Industry High Health Scheme preplant checklist. Refer to our guidelines for detailed information on land preparation as well as preplanting and planting advice: https://www.facebook.com/groups/justavocadosgrowers/permalink/1385204891676545.

Harvesting is well under way with blanket clearances been issued in most of the regions. To ensure you maintain your best possible packout, please refer to our harvesting guidelines: https://justavocados.co.nz/harvesting -packout-and-fruit-deformities/, and our recent decision tree and video for harvesting around wet conditions: https://justavocados .co.nz/harvesting-around-rain/.

Although spring is a busy time on the orchard, take time to marvel at how wonderful nature is. Converting the energy from the sun and using all your orchard inputs and hard work, a tiny pinhead starts to develop progressing to the fruit in your hands as you harvest – what an awesome industry we are in!

For any orchard support or contracting work during this time, please contact our Grower Services Team or Avoworks www.avoworks.co.nz.