Olive

Identification and origin

Olea europaea belongs to the olive family, Oleaceae. The species, which includes several subspecies, grows wild around the Mediterranean, in the Macaronesian islands, across much of northern, eastern, and southern Africa, in the Arabian Peninsula, and east to Iran, Afghanistan, Pakistan, the western Himalaya, and south-central China. Cultivated since about 2500 BC, it has turned invasive in Australia, New Zealand, Hawaii, and southern California.

It is an evergreen tree or large shrub, slow-growing, with a rounded crown. The bark is smooth and gray on young trees, becoming gnarled with age. The opposite leaves are narrow, elliptic to lance-shaped, and leathery, up to 3 in long: gray-green above, silvery beneath thanks to tiny scale-like hairs. Leaves two or three years old often drop as spring growth begins: this is normal.

In summer, small, fragrant white flowers appear in groups of 10 to 40, in panicles about 2 in long, on the previous season’s wood. The fruit, the olive, is an oval drupe about 2 in long, green and turning purplish black when ripe, with a single pit. In the ground the tree slowly reaches 15–29.5 ft tall; in a container it stays much smaller thanks to pruning.

Among common cultivars, ‘Arbequina’, from Spain, is self-fertile; ‘Koroneiki’ and ‘Mission’ are also widely grown. ‘Little Ollie’ stays compact, ‘Majestic Beauty’ bears few fruits, and ‘Swan Hill’ and ‘Wilsoni’ produce neither flowers nor fruit.

Light

The olive is a full-sun tree: it needs at least six hours of direct sun a day. Choose the warmest, most sheltered spot you have, facing south or against a sunny wall. In a pot, it spends the warm season outdoors in the sun and comes under cover as frost approaches. Under glass, in a greenhouse or sunroom, give it full light.

Watering

Once established in the ground, the olive tolerates long dry spells, because its small leaves limit evaporation. In a pot, the water reserve is much smaller. During the growing season, keep the mix lightly moist, watering thoroughly once the surface has dried, then let the excess drain away. Raise the container on pot feet to keep the drainage hole clear.

Too much water is far more dangerous than too little: soggy soil encourages root rot, and excess moisture during bloom makes the flowers drop before fruit sets. In winter, in a cool spot, water only enough to keep the root ball from drying out completely. Go by the mix and the weight of the pot, not the calendar.

Potting mix

The olive grows in almost any well-drained soil, including chalky soils up to a pH of about 8.5; sandy soils suit it best, and spots where water stands are to be avoided. In a container, use a peat-free, loam-based mix lightened with coarse sand or fine gravel to make it gritty.

It needs few nutrients and fruits poorly in overly rich soil. In a pot, mix controlled-release fertilizer granules into the mix at planting, then give a balanced liquid fertilizer once a month during the growing season. Too much nitrogen favors shoots at the expense of fruit.

Temperature

During the growing season, the olive does best between 68–77 °F. It then needs a cool winter: about two months between 39–50 °F trigger flowering the following spring. ‘Arbequina’ and ‘Koroneiki’ manage with mild winters, but an olive kept warm all year rarely blooms.

Above 86 °F during bloom, pollen germinates poorly; above 95 °F flowers can be killed, and around 104 °F leaves and fruit drop.

In a pot the roots are exposed to cold: bring the tree into a bright, frost-free place for winter and do not move it back outside until all danger of frost has passed. In the ground, protect it if temperatures are forecast to drop below 19 °F; a heavy frost cloth gives about 36 °F of protection. Areas that drop below 12 °F are unsuitable. It is grown in the ground in USDA zones 8a to 10b, with some cultivars in zone 7.

Repotting

Plant or repot in spring. A young olive does well in a pot 12–14 in across, or larger if the roots need it; move it to a wider container once the root ball fills the pot. Choose a heavy, stable container with drainage holes, fill it with the gritty mix, and stake the tree until it is anchored.

Pruning

Prune in early spring to keep a good shape and remove dead, diseased, or damaged wood. To limit the size of a potted olive, tip-prune the main branches each year, cutting back to a good replacement shoot. It takes hard pruning and is also trained as bonsai.

The olive blooms on the previous season’s wood and never fruits twice in the same spot: no new growth, no olives. Opening up the canopy limits fungal disease, but drastic pruning sacrifices the next crop.

Propagation

Sow seed in spring at 55–59 °F, but seedlings do not come true to the variety. To keep a cultivar, take semi-ripe cuttings in summer: cut a current-season shoot that is firming up at the base, strip the lower leaves, and insert it in a light mix in a warm, bright spot. Varieties grown from cuttings or grafts bear at about four years.

For a harvest, plant at least two compatible cultivars close together: many olives are partly self-sterile.

Common problems

• No flowers or fruit: a winter too mild or spent in the warmth, too little sun, excess nitrogen, severe pruning, no compatible pollinizer, or intense heat during bloom.
• Uneven crops: the olive naturally alternates a heavy year with a light one.
• Flower drop: excess moisture or heavy rain during bloom.
• Yellowing, wilting: root rot in waterlogged soil, or verticillium wilt.
• Galls on the stems: olive knot, a bacterial disease.
• Black coating on leaves: sooty mold, which grows on honeydew left by scale insects.

The same symptom can have several causes: always check exposure, watering, drainage, and recent temperatures before acting.

Toxicity

No toxicity is reported for dogs, cats, or horses, and the olive does not appear among the toxic plants listed by veterinary references. Raw olives, however, are extremely bitter and inedible without preparation: green ones must soak in salted water for several days, and black ones need dry-curing in salt for several weeks. Finally, its pollen is a common allergen, hence the appeal of sterile cultivars such as ‘Swan Hill’.