Pollination and pollinator varieties in olive
98% of olive flowers drop. Whether the remaining 1-2% holds depends on having a pollinator in the grove. Gemlik is only partially self-fertile: in a single-variety grove harvest falls to a third.
An olive tree in an on-year opens thousands of flowers. 98% of them drop. Only 1-2% set fruit.
That ratio is normal — it is how the olive works. But how much of that 1-2% holds depends on something most growers are unaware of: is there a pollinator variety in the grove?
How the olive is pollinated
The olive is wind-pollinated (anemophilous). It needs no bee or other insect; the wind carries the pollen.
The problem is not in the pollen travelling but in the travelled pollen being of use.
A common condition in olives is self-incompatibility. Although the sexual organs and cells of a flower develop perfectly well, fertilisation fails when the flower is pollinated by pollen from the same variety.
This is entirely genetic, controlled by incompatibility genes (S genes).
The mechanism: in self-pollination the pollen tube usually fails to reach the ovule, unable to grow down the style far enough for fertilisation. In cross-pollination the tube grows faster and reaches the ovary.
The result: yield falls in a grove planted with a single variety.
The self-fertility index
How much fruit a variety can set on its own is measured as a ratio:
t; R = fruit set from self-pollination ÷ fruit set from open pollination
And that ratio gives three classes:
| R value | Class | What it means in practice |
|---|---|---|
| 0.00 - 0.15 | Self-infertile | a pollinator is obligatory |
| 0.15 - 0.30 | Partially self-fertile | yield drops sharply without a pollinator |
| 0.30 - 1.00 | Self-fertile | can bear alone, though cross-pollination still helps |
That last note matters: research reports that cross-pollination benefits even self-compatible varieties. A "self-fertile" label does not mean "no pollinator needed".
Five varieties, measured results
Five varieties were tested in a grove planted in 1990 at the Koruklu Talat Demirören Research Station
of the GAP Agricultural Research Institute.
The method is direct: while the flowers were still at the balloon stage they were isolated with
paper bags (self-pollination), and other shoots on the same tree were left open (open pollination).
Fruit was counted four times — 20 days after full bloom, in midsummer and at harvest.
The resulting classification:
| Variety | R index | Class | When planting |
|---|---|---|---|
| Delice | 1.13 | self-fertile | sufficient on its own |
| Nizip yağlık | 0.34 | self-fertile | a pollinator still helps |
| Domat | 0.29 | partial | pollinator needed |
| Gemlik | 0.29 | partial | pollinator needed |
| Yuvarlak halhalı | 0.14 | self-infertile | pollinator obligatory |
For the count-by-count detail and regional assessment of the trial, see south-eastern Anatolia's nameless olives.
The real message: an early count misleads
The most instructive part of the trial is its timing. Early fruit set under self-pollination is sometimes
even higher than under open pollination. But the olive later drops what it cannot carry.
Which is why walking the grove in May and calling it a good year is premature. The difference emerges at
harvest, and in self-infertile varieties it can reach sevenfold.
The count-by-count figures and the full comparison table:
South-eastern Anatolia's nameless olives
The Gemlik question
The finding with the most practical consequence concerns Gemlik.
Gemlik is partially self-fertile (R = 0.29), and at harvest its yield from its own pollen is about one third of what it achieves with foreign pollen.
Why that matters has to do with what certified-sapling support produced: recent years saw intensive planting of Gemlik in Turkey and a rapid expansion of table-olive plantations. We touched on that trend in the article on table olive quality.
If those groves were established as single-variety Gemlik, then even with healthy trees, correct husbandry and a suitable climate, yield will remain structurally low. And that is not a deficiency fertiliser or irrigation can fix — it is a genetic incompatibility.
The remedy is simple and cheap: mix a suitable pollinator variety into the grove.
How to choose a pollinator
A pollinator variety should be chosen against four criteria:
1. Economic productivity — the pollinator tree should bear a crop too, not just occupy space
2. Pollen quality — it must produce abundant, viable pollen
3. Compatibility with the variety to be pollinated — they must be genetically compatible
4. Matching flowering time — they must bloom in the same period; if one opens earlier or later, the pollen is wasted
The fourth is the most often overlooked. A good pollinator can be entirely useless simply because it flowers at the wrong time.
Turkey's unmeasured varieties
The study notes one further gap:
t; Research to date on the reproductive biology of olive varieties in Turkey has covered, broadly, 14 varieties.
Given that Turkey has 100 registered olive varieties, the self-fertility status of most of them is unknown.
And in the study's own words: uncertainty remains about the appropriate pollinators for the varieties in already-established groves.
This is another face of the same gap seen in south-eastern Anatolia's unmeasured genotypes: we have the varieties, but not the measurements.
Summary for anyone planting a grove
- 98% of olive flowers drop anyway; protecting the 1-2% that hold is what counts
- Planting a single-variety grove is risky — especially when that variety's self-fertility is unknown
- For self-infertile or partially fertile varieties a pollinator is obligatory
- Even self-fertile varieties benefit from cross-pollination
- Do not neglect matching flowering time when choosing a pollinator
- Known results: Delice and Nizip yağlık self-fertile · Domat and Gemlik partial · Yuvarlak halhalı self-infertile
Related: olive varieties · Gemlik · Domat · alternance
Sources
- Korkmaz, Ş. & Ak, B.E. (2018) — Determination of self-fertility status of some olive cultivars grown in the GAP Region, Harran Journal of Agricultural and Food Science 22(4): 471-477 (trial conducted 2015)
- Moutier, N. (2002) — the self-fertility index and its classification
- Martin (1990) — flower drop rates in olive
- Cuevas, J. & Polito, V. (1997) — pollen tube growth
- Lavee, S. (1998) — criteria for selecting pollinator varieties
- Sibbett & Osgood (1994) — benefit of cross-pollination in self-compatible varieties
- Eti, S. (2009) — sexual incompatibility and S genes
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