Avocados in cool climates
Avocados, while essentially of semi-tropical origin, are adaptable to a range of climatic conditions. The variety Hass has demonstrated this capacity by being successfully grown from subtropical conditions in North Queensland to cool Mediterranean conditions in the South-West of Western Australia.
Growing avocados commercially outside of their optimum growing conditions, such as under cool Mediterranean conditions, presents a range of issues for commercial growers. Some include the temperature impact on flowering, pollination, fruit set and carbohydrate partitioning. Under cooler conditions, the avocado will also progressively tend towards more biennial bearing with corresponding fluctuating levels of spring leaf drop, root flushes and shoot flushes.
The overall yields of Hass avocados in the South-West of Western Australia are not as consistently high as growers would like. In 2006/07, the overall WA state avocado yield was just over 10 tonnes per hectare (Cutting 2007). However, growers consistently comment about achieving much higher yields for at least part of their orchards. The challenge is to identify the causes of low yields and to develop strategies to improve yields on a consistent yearly basis.
There is a range of theories as to why growers are struggling to consistently produce high yields from Hass avocados in cool Mediterranean conditions:
- poor pollination
- insufficient flowers
- insufficient carbohydrate production
- shoot growth competition with flowers.
Poor pollination and fruit set
Availability of viable pollen
Poor pollination can be the result of several issues, including lack of pollen, pollen transfer or pollen viability. Weather conditions during flowering have a significant effect.
Flowering in avocados is well documented, and the nature of the flowering pattern of type A and B flowering varieties generally well understood. Under ideal flowering temperature conditions of 25/20°C maximum/minimum, (Sedgley and Annells 1981) the A/B flowering patterns are quite typical which result in excellent cross-over of functionally male and female flowers between the A and B types.
Under these ideal conditions, the variety Hass (type A) can also have a short cross-over of functionally male and female flowers during the middle of the day within a single stand of Hass plantings (Figure 1). In this case, the functionally female stage remains open for a short period during the early stage of the functionally male stage, allowing for close-pollination (pollen from flowers of the same or nearby trees of the same variety) and satisfactory fruit set. As a result, many orchards have been planted as a monoculture of Hass without inter-planting a complementary type B variety.
Under cooler conditions, the flowering story becomes more complicated. Extensive investigations into the effect of temperature on flowering of avocados have been carried out (Ish-Am and Eisikowitch 1991, Sedgley and Annells 1981, Sedgley and Grant 1933, Sedgley and Alexander 1983). Cold temperatures alter the flowering cycle by delaying the normal opening and closing routine of the avocado flower, extending the overall period of flowering, delaying the release of pollen, slowing pollen tube growth and reducing the number of flowers open on a given day.
The impact of this for a single variety Hass orchard is that the expected period with both functionally male and female flowers open may be severely reduced, or eliminated altogether, thus reducing the possible period of close-pollination and reducing fruit set.
Investigations by Sedgley and Annells (1981) on Hass were under controlled and sustained temperature regimes. Under natural conditions, there are daily fluctuations in night and day temperatures during flowering resulting in regular changes in the floral sequence. As a result, there will be periods when pollination is much more likely than others.
The temperature effects and low average yields of Hass have led growers to reconsider the use of a suitable cross-polliniser variety to ensure sufficient pollen is available for pollination. However, the temperature story does not stop with Hass. Research by Sedgley and Grant (1983) into other varieties, both type A and B, has shown similar effects to different temperature regimes, though with some differences between varieties.
The delaying effect to type B varieties is so pronounced at low temperatures that the functionally female stage was often not recorded. This has a dual effect:
- If there are few functionally female flowers then the cropping potential of the type B varieties will be severely affected.
- Delaying of flower opening has been recorded to result in the peak pollen release period occurring during the night (Sedgley and Annells 1981).
Therefore, as the temperature regime is affecting the flowering cycle of flowers of both type A and B varieties, then the choice of a suitable cross-polliniser is not a simple case of choosing a known complementary flowering type. You will need to investigate the effect of the local temperature conditions on both varieties to see if they are indeed complementary. A single year of observations in 2009 in the South-West of Western Australia demonstrated similar results to those reported in the literature — see ‘Cross-pollinisers for Hass avocado’ for more details.
Transfer of pollen
Viable pollen being available for transfer to a receptive stigma is just the first step in achieving effective pollination. The pollen must actually be transferred to the stigma, while it is still receptive, by some means and in sufficient quantity to result in the desired level of fruit set.
The more common view is that pollen transfer requires intervention of a small pollen vector, or pollinator. Most commonly discussed is the honey bee (Ish Am and Eisikowitch 1993), but a range of other vectors has also been monitored (Ish-am et al. 1999). An alternate view has been discussed by Davenport (1989, 2003 and 2007) whereby pollen is transferred by wind, both in the form of close-pollination and cross-pollination (pollen from flowers of nearby trees of a different variety).
Davenport (2007) has questioned the belief that pollination can only occur during the first stage of flower opening — that is, during the functionally female stage. That pollination can also happen during the second opening period, or functionally male stage. This again raises the question about the need for cross-polliniser varieties. Equally, there has been research showing proximity effects on yield of Hass with various cross-polliniser varieties when planted within an orchard (Fetcher et al. 2001).
Another complication with pollen transfer is rain or moisture, often associated with cooler weather during the flowering period in the South-West of Western Australia (Figure 2).
If we are to promote the benefits of honey bees to be our main pollinator, then we also need to consider the impact of cold or wet weather on bee activity.
A cold and wet winter will reduce the number and health of native bee populations. Growers don’t have to rely on native populations and can introduce healthy hives into their orchards at flowering time. But even healthy hives will be less active in cold or wet conditions. Bees prefer day temperatures above17°C (Dixon et al. 2007). Wet conditions are also not favoured. It is also possible rain may wash pollen from flowers, thus reducing the amount of pollen available.
Bees are attracted to the best and most accessible source of pollen and nectar. Cold weather will reduce the number of flowers open on an avocado tree (Sedgley and Annells 1981) and can delay the release of pollen. Plus the closing nature of avocado flowers between the independent functional stages may make avocado flowers less attractive to bees (Figure 3). The independent functional stages also add complication as bees often set out to collect either pollen or nectar. To do this they may actively seek out either functionally male or female flowers (Ish Am and Eisikowitch 1993). This reduces the chance of depositing pollen onto the stigma of a functionally female flower.
Focus has been on the European honey bee as the main pollinator of avocados in the South-West of Western Australia, but a range of other insects visit flowers including hover flies (Figure 4), native bees and common flies. There is little information as to the benefit, if any, these visitors provide in pollen transfer.
Temperature and pollen growth
Temperature not only affects the actual flowering cycle, but also affects pollen germination, the speed of the pollen tube growth as well the embryo growth rate (Sedgley and Annells 1981). However, while a lower temperature regime of 17/12°C (maximum/minimum) did slow the speed of all this, it did not prevent fertilisation or ovule growth, which still occurred within 6–24 hours.
Zamet (1990) suggested that 10°C was a critical temperature for avocados in Israel. That is, a period of five to six days with a minimum above 10°C increased the chance of a fruitlet not abscising. He found that the lower the number of favourable fruit set temperature periods during the flowering period, the lower the average yield per hectare.
In New Zealand, they tested an effective pollination period of two consecutive days of maximum temperature above 17°C and a minimum temperature above 11°C, but only demonstrated a very weak correlation between this and fruit set (Dixon et al. 2007).
It appears logical to accept that the cool and wet conditions experienced during avocado flowering are having an impact on the production capacity of Hass avocados. However, further investigations are warranted to accurately quantify these effects to assist in providing the best options for improving pollination.
The use of cross-pollination to take advantage of potentially limited good pollination events is one method of improving pollination and fruit set. However, it is not just a simple case of inter-planting with a known type B variety. You must select a variety that will consistently flower at the same time and in a truly complementary manner.
There is a possibility that even with significant cross-over of male and female flowers, from the use of a polliniser, that the climatic conditions are still not favourable for effective pollination due to effect of the climate on pollen growth or bee activity. Also, are European honey bees the only significant pollinators of avocados in the South-West region of Western Australia? All these issues require further investigation.