AKA Sex in the Orchard, Sibling Rivalry, The Wrath of Mother Nature, and Alien Invasions

Our subtitle may sound like the newest novel from Tom Clancy or Stephen King, and if you were an olive, you might just feel that way. A while back, a subscriber asked why an older ornamental tree that had produced olives for years might suddenly stop producing for five years. The fact is, we still do not understand all of the physiological and genetic science of our beloved olive, but we can share some background to help you understand what happens in the olive orchard at this exciting time of year, and speculate on the fate of the aforementioned tree.

Our wily but beloved olive trees can produce olives for hundreds of years. Yet olive trees typically alternate between bearing small crops and normal crops every other year. For no apparent reason, they can also go for 2 - 3 years without bearing fruit.

While there is still more to learn about the olive tree’s reproductive process, our answer begins with understanding that this year’s fruit development began last summer. This fact also helps us understand why care must be taken when we prune our orchards.

FLOWERING

Olive trees produce fruiting shoots called inflorescences, which originate at the axil of a leaf. Each inflorescence will typically contain 10 to 30 flowers depending on the cultivar. The number of flowers that then mature into olives is dependent on a number of factors.

Stress Factors

Various stress factors can inhibit flower (as well as fruit set and fruit )development:

• Nitrogen - not enough or too much

• Pruning – too much or too little

• Water – when scarce, the tree favors leaves over fruit

• Pest infestations

Temperature

The temperature should be not too hot, not too cold, and not too constant.

• Olive trees require winter cold to achieve normal blooming and fruiting

• The optimal degree of cold winters is dependent upon the origin of the olive cultivar (e.g. Eastern Mediterranean vs. Southern Europe)

• Ideal flowering in California occurs when temperatures fluctuate daily between 35° –40° F minimum and 60° to 65° F maximum

• Strong winds, hot temperatures, and freezing temperatures can negatively impact fruit set

• Particularly cold springs will negatively impact harvest by delaying blooming and increasing flower abnormalities

• Localized winter temperature rises due to development around the tree such as buildings and asphalt can act as a wintertime heat sink, thereby inhibiting fruiting.

BLOOMING

Before flowers bloom, they go through a differentiation period. No, this is not some modern politically correct exercise, but it does have to do with the sexuality of the blooms. This generally takes place March through May and results in one of two results (although there is some new research indicating that there may be a third possible result).

• Perfect Flowers: Flowers which have both stamen (the male part) and pistil (the female part)

• Staminate Flowers: Flowers which have only stamens and lack a pistil (which apparently has been aborted)

Why are there staminate flowers and not just perfect (hermaphroditic) flowers? There are a number of theories and reasons, but here are two main points: 1) It takes a great deal of energy to develop the pistil (or ovule - think of this like the human ovum). The tree has to moderate the number of pistils it is able to develop with the nutrients available to it; 2) In order to pollinate the pistil, the tree is dependent on a high volume of stamens – stamens don’t have the same nutrient requirements as the pistils, so the tree exercises her “nutrient economizing” by terminating pistils (ovules) as needed.

Orchard stress, in particular water stress, can negatively impact this portion of development. In competitions between flowers and leaves, the leaves win, and the first floral part to fail is the pistil. A moist soil profile during this period can help prevent this kind of stress.

Pistil abortion occurs naturally, and is not perfectly understood. As alluded to above, it may be a result of a lack of nutrition in the tree. Pistil formation must compete with vegetative growth for nutrients at a time when both are working their hardest. In addition, following a big harvest year, the tree’s nutritional resources have been depleted. Yet, if only 1% - 2% of the flowers are able to set fruit, the grower will still enjoy a satisfactory crop. If a tree is blooming, but no olives develop, this could be because of a mechanical disruption to fruit set. Wind, rain, and/or hail can knock off the blooms just at this critical time of the year.