Does rainfall during olive flowering affect pollination and fruit set?

Provided your olive trees have good nutrition there is general consensus amongst olive growers that to achieve good pollination and subsequent fruit set you need two key criteria: -

i)                 Good soil moisture (but definitely not saturated - olives hate wet feet),

ii)                Warm (but not hot), sunny conditions with light breezes to ensure good pollination of this (largely) wind pollinated crop. 

This season (2023) leading up to flowering and fruit set has been particularly challenging at Manna Hill Estate due to the very low rainfall for Winter and Spring.  This was forecast by the Bureau of Meteorology (BOM) due to the strong El Nino and Indian Dipole events and hence not unexpected.  So not unsurprising that by late Spring (November) in a number of places the soil was starting to crack due to lack of moisture.   Typically, we don’t see these sorts of conditions till the end of Summer (if at all).  Given this, it is probably reasonable to assume that we have failed to achieve key criteria number i): - good soil moisture at flowering!

Cracking soils in late Spring as a result of the very dry Winter / Spring in 2023

And then on the day the trees started flowering it also started to rain – not heavy – but extremely persistent and consistent.  At the time of writing, it has been raining / drizzling pretty much consistently for over a week (with a total of ~60 mm of rainfall).  As our olive trees only flower for about 20 days in total we are nearly half way through this period without experiencing a single day where it hasn’t drizzled / rained pretty much consistently.  Intuitively you would think that a wind pollinated flower would struggle under these conditions as the rain would wash the pollen from the air - rather than wafting about pollinating flowers – but what does the research say? 

Influence of rainfall on pollen concentration in the atmosphere

There’s an interesting research paper published in the New Zealand Journal of Crop and Horticultural Science on “Influence of temperature and rainfall on timing of olive (Olea europaea) flowering in southern Italy” (reference here: https://doi.org/10.1080/01140670809510221).  Unfortunately for us, it validates what you would intuitively think: -

i)                 “Pollen dispersion and subsequent flower pollination are compromised by up to 80% when rainfall exceeds 8 mm and lasts for several days during flowering”.  (We have experienced 60 mm of rainfall during a period of 8 days so far).  And if we weren’t really sure of what the implications of this might be the researchers go on to say: -  “If the conditions during the flowering phenophase are not optimal, the consequence will be a considerable decrease of potential production.”

Sodden trees during the critical pollination period……

The study also had some other interesting findings relevant to olive growers viz: -

i)                 Olive flowering generally starts when the mean temperature (calculated with running averages on a 10-day basis) reaches values close to 16°C.

ii)                The first and second order flowers as the most fertile and characterised by the highest fruit set capacity are the first to open in an inflorescence.  The implication for us is that with rainfall from the first day of flowering this year we have lost the most fertile and highest fruit set capacity of the grove.

iii)              The olive's metabolic range is rather narrow, with an optimal temperature interval of only 10°C (20°-30°C)

So there we have it – likely not a good fruit set this year.  Just one of the delights of farming with ever-increasing climate variability!