In order to initiate flowers and fruit, olive trees need a two-month period of cold weather (with temperatures below 10°C (50°F). They also need a fluctuation between day and night time temperatures. Plants kept indoors are therefore unlikely to flower.
Very dry soil conditions can also inhibit flowering, even if the tree is able to tolerate such conditions. Watering during dry spells between February and May is therefore crucial for fruit production. If olives are produced, avoid excessively dry conditions during the summer as this may cause the fruit to shrivel.
Prolonged cold weather (below 7.5°C or 45°F) can also inhibit fruit production.
Growing more than one cultivar will increase cross-pollination and improve yield, although even a single tree should produce some fruit, olives being self-fertile. Flowers are mainly wind-pollinated, and shaking the branches during flowering to release pollen helps to improve fruiting. Increased humidity at flowering time also helps fruit to set.
Fruit is produced at the tips of the previous year’s growth, so excessive pruning will also prevent fruiting. Thinning of the crop is recommended, reducing the fruit numbers to three or four per 30cm (1ft) of branch within three weeks of flowering, in order to ensure that the crop will ripen and not drop prematurely.
Ripe black olives can be picked and eaten raw, but taste quite different from commercially sold olives, which are usually picked unripe and green, or black and ripe, but are then cured to produce an edible product.