The upcoming flowering stage of avocado production is arguably the most important phase that results in fruit set.
By now, the fat, round flower buds that were sitting tightly in the leaf axil have extended and developed into panicles full of closed flowers. This is called the cauliflower stage. In some orchards, there may even be the odd open flower.
A mature avocado tree can produce in excess of 1 million flowers with fruit sets 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 1) functioning in the female phase with only the female parts being receptive. The flower then closes after 2-6 hours and reopens in the afternoon of day 2 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 1 in the female phase, closes and reopens from the morning to afternoon of day 2 in the male phase. This synchronises with the A-cultivar flowers opening in the female phase and pollen transfer from the B cultivar to the A cultivar can then take place.
This synchronisation however seldom occurs perfectly and is mostly only synchronised at warmer temperatures of minimums of 15°C and maximums of 25°C.
Different B-flowering cultivars also flower at different stages of the season so need to be compatible with the flowering time of Hass if used as a polliniser eg. Fuerte flowers earlier in the season and would therefore not be a successful polliniser choice for Hass.
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%) tested in a trial, was 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 2. In climates with high humidity and cooler temperatures, the stigma is still receptive and viable on day 2 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 8-12 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.
Once pollen is transferred many conditions have to 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 that 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.
Fertilisation and fruitset are affected by a number of factors including temperature, humidity, nutrient status, abnormalities in the embryo, and water stress.
STEPS TO ENCOURAGE OPTIMAL FLOWERING, POLLINATION AND FRUIT SET
With so many influencing variables 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:
- NUTRIENTS – This is a period of very high nutrient demand and withdrawal due to the existing crop as well as the development of flowers, nectar production etc. Nutrient uptake through the roots is still low due to the cool temperatures affecting the rate of transpiration as well as root activity. Support the tree with foliar applications 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. The optimal timing for these flower sprays, particularly boron is when 50% of the trees are at 50% cauliflower stage, 25% at earlier flower development and 25% approaching full bloom. Consider supporting your trees during a heavy flower with foliar products high in amino acids and enzymes.
- BEEHIVES – Improve pollination by ensuring you have adequate beehives present from 10% flowering. The bees take time to settle in and resume full activity and strength. Place hives in small groups spread throughout the orchard, in sheltered, sunny areas. Pruning improves bee activity as more open canopies and orchards result in warmer orchards and easier flight paths throughout the tree and orchard. Ensure there is water available. 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 honey bees to treated blossoms and encourages foraging behaviour. The bees stay focused on working the blossoms harder. This increase in foraging intensity improves crop pollination.
- IRRIGATION – 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.
- 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.
- HARVEST PLANNING – 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. This can be done by doing a select pick/harvest of your best quality and biggest fruit at the beginning of the harvest season and leaving the balance of the crop for later in the harvest season. This also enables the smaller fruit to size up better. It is especially advisable on trees that are stressed i.e. trees that have been frosted and are carrying frost damaged fruit or trees with poor tree health. Fruit and flowers on a tree are a significant energy and nutrient drain on the tree and a stressed tree will decline further if its crop load is not well managed.