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Vincent Ciolino
December 15, 2011 | Farming, Olive Oil | Vincent Ciolino

Pollination is a fragile process easily disrupted by rain

Many factors affect the quality and quantity of fruit production in a particular vintage. The first few stages of the growing season (pruning, bud break, and pollination) all affect the quantity of the crop while the later stages primarily affect quality. This year, for the first time in our ten-year history, the quantity of both our grape and olive crops was severely affected by weather during pollination. Spring rains reduced our grapes by 25% and our olives by 99%. Let's examine the pollination process of both olives and grapes to understand what went wrong.


Comparatively speaking, grapevine flowers are tiny, green, and unobtrusive—so they are easy to overlook. Grapevines have hermaphroditic or perfect flowers, meaning each flower has both male parts (stamens) and female parts (pistils and ovaries). Thus every flower has everything it needs to make fruit—it doesn't require bees or wind or any other grapevines to pollinate. During fertilization, the pollen from the stamen of a perfect flower typically falls a short distance into the sticky stigma on top of the pistil. The germinated pollen grains travel to the ovary, where the fruit grows. It sounds simple but the timing of pollen maturity, ovary viability, and other aspects of flower development is critical—which makes for a fairly fragile pollination process.

Grapevine pollination can be disrupted by extreme temperature, excessive wind, and rain—thus greatly reducing the crop quantity. For grapevines, the optimal pollination weather means mild, wind-free days with no rain. Even so, typically only 30% of grapevine flowers are fertilized, thus about one-third mature into grapes. Rain reduces fertility by causing the pollen to clump together, while wind can blow the pollen or even the entire flower off. Cold or hot weather leads to poor pollination because the fragile timing of the process is disrupted. The flowers that aren't fertilized end up falling off, which is what we call shatter.

On the other hand, olive trees have two types of flowers: perfect flowers (both male and female parts), and staminate flowers (male flowers that lack pistils)—although only the perfect flowers can become olives. The ratio of perfect to imperfect flowers varies by tree, by variety, and by year although it is generally 1:1. Technically, a staminate flower initially had a pistil but then its growth stopped. The tree decides to halt the pistil growth in some flowers to avoid producing excess fruit during unfavorable years with fewer resources. Yet the additional pollen from the staminate flowers increases the chances that the perfect flowers are pollinated. In fact, although a mature olive tree will have a profusion of small white flowers (up to 500,000), only 1% of them will ever become olives in an optimal year.

The process of pollinating olive trees is more complex than grapevines because most olive varieties are somewhat self-incompatible, meaning they will set a better crop by cross-pollinating with other olive varieties. In fact, two of Montemaggiore's three varietals (Leccino and Pendolino) are self-sterile and absolutely require a pollen source from another variety. Only the Frantoio flowers can be pollinated with Frantoio pollen (although their fertility increases with pollen from either a Leccino or Pendolino tree). This incompatibility makes olive pollination more error-prone because pollen must travel from tree to tree, typically by wind (bees are not particularly attracted to olive flowers). For olive trees, optimal pollination weather means mild temperatures with gentle winds and no rain. Rain definitely puts a damper on olive fertility (excuse the pun) because it washes away the pollen and shortens the pollen's viable life-span.

So what went wrong with pollination in 2011? It wasn't the temperature or the wind, but it was rain this year. In the case of grapes, a cool wet May led to about a 25% reduction in the crop. In the case of olives, the rain in June caused a 99% crop reduction. We do have some olives, although most of our 800 trees have none, a handful have about 5 olives, and one or two have about 20 olives—barely enough to cure for eating.

As Vincent always says in regards to his beloved Chicago Cubs, there's always next year...


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