A newsletter reader recently asked why an older ornamental tree which had produced for years might suddenly stop for 5 years.  Olive trees can produce olives for hundreds of years. Olives are naturally alternate bearing with small crops every other year and for no apparent reason they can go for 2 - 3 years without fruit.

Fruit Develops on the Second Year Growth

Fruit development starts the previous summer on that year's new growth. If you prune every year then there will never be a  second year's growth for fruiting.

Winter Temperatures

Olive trees require a minimum winter temperature to bloom in the spring.  Global warming may some day hit the olive industry as temperatures definitely affect crop set and size. A local winter temperature  rise due to development around the tree such as buildings and asphalt which act as a wintertime heat sink, can inhibit fruiting.

In one experiment at UC Davis, olive trees were kept in containers in a greenhouse over the winter but a branch from one tree was allowed exposure to cold through a hole in the wall. The next year none of the trees bloomed or fruited except the single branch exposed to winter temperatures.

Low spring temperatures during bloom may slow the growth of pollen toward the ovary. As the unfertilized ovary is viable for only a short time, this can result in no fertilization.  High temperatures may also slow pollen growth, especially in Manzanillo.

Severe heat in late April this year in Central California during olive bloom and crop set may be responsible for this year's small olive crop.   The California Olive Committee (COC) estimates the 2004 olive crop to be 75,000 tons,  roughly 18,000 tons of Sevillano and 57,000 tons of Manzanillo, which is 31% below the 2003 crop.

Pollination

Many olive varieties cannot self pollinate. Olive pollen is primarily carried by wind and can go long distances.  Bees may play a minor role in pollination.  If a solitary tree in an urban area stops producing it might be because someone removed the nearby tree that was pollinating this tree. 

Mechanical Disruption of Blooming

If the tree is blooming but no olives develop, this could be because of a mechanical disruption to fruit set.  Wind or rain can knock off the blooms just at the critical time of the year.  If you don't want olives on your tree, spraying with a high power water jet to knock off the blooms will usually work.

Abscission Agents

Even if the flowers are pollinated, fruit may fail to form if sprayed with an abscission agent. These are sprays that keep fruit from setting so that olives do not develop.  Farmers who sell to the canning industry apply such sprays so that fewer but bigger (and more profitable) olives will grow.  Olives can be a messy fruit, leaving blackish stains on parking lots and carpets. Ornamental trees in parking lots or apartment complexes are sometimes sprayed so that no olives will develop.

Abscission agents are referred to as ethylene-releasing compounds (ERCs).  Examples are ethephon and etacelasil which sell under the trade names Alsol and CGA-15281 and naphthaleneacetic acid (NAA) marketed as Liqui-Stik Concentrate (EPA reg #34704–382) by Platte Chemical Company. See Spray Thinning Guidelines

Other

Excessive watering and fertilizing will result in mostly vegetative growth and fewer fruit.

Pests, weed competition, and lack of nitrogen can also reduce fruiting.