Grow Great Olives – My Smart Garden

For more tips to Grow Great Olives check out the workshop recording here.

Olive trees (Olea europa) have been grown in Australian for hundreds of years, but the backyard growing blitz really became popular with the wave of immigration from Italy and Greece back in the 1950s. This article looks at how olive trees grow, why they need pruning and how to do it, common pests and diseases of olive trees in Victoria, and how to know the best time to pick your olives.

Olive tree anatomy

Like all trees, olives have a root system, a trunk, and the “crown” or “canopy” of the tree which is composed of the branches, fruit producing twigs and leaves.

The root system of the Olea europa is quite shallow by comparison to most trees, they produce a root ball rather than driving a tap root deep into the soil. This makes the trees relatively easy to move, even when they are well established, but also means that they require more frequent irrigation than other trees that might be in your back garden.

The trunk of the tree can grow straight or twisted, depending on factors like wind exposure or ground movement during the growing years. The base of the trunk is often wider than the main trunk section because olive trees grow suckers from the base, which need to be pruned off regularly to make harvesting the olives easier. Olive trees have what’s called a “basitonic growth habit”, which means that they want to be a bush more than a tree! This is one reason why they sucker from the base, but also, when pruned, the trees will shoot new growth from the end of the branch closest to the trunk, rather than from the bud end of the branch like apples and other fruit trees.

Olive trees have two types of branches; structural and productive. The structural branches are those used to establish the shape of the tree, and usually fork from the trunk into a vase shape so that people can access the centre of the tree for harvesting or management purposes. The productive branches are those that carry the twigs where the flowers and fruit grow, or support the new growth that will become productive in the future.

Unlike deciduous trees, olives don’t lose all their leaves at once, but they will drop a proportion of the canopy each season. This is usually preceded by the leaves turning yellow over Autumn and Winter months.

The flowers of olive trees are produced in the Spring. They are wind-pollinated, and will produce a better crop if you have two or more trees of different varieties close by, but the pollen can travel considerable distances. Olive trees are also self fertile, but will produce more vestigial fruit (tiny round olives with no pits) if they self pollinate.

Why prune olive trees?

Pruning is undertaken to maintain a balance between the size of the root system and the size of the canopy. The roots can go a long way sideways, but they don’t go very deep, whereas the canopy can be both tall and wide. By reducing the canopy, the root system is better able to feed the tree effectively, but also to stabilise the tree against wild weather! Pruning also manages the ratio between the leaves and the woody twigs and branches in the canopy. As the branches get older, the leaves fall off, but aren’t replaced like on a deciduous tree. Over time, the canopy can become mostly composed of old, non-productive twigs, and the tree will not be able to photosynthesise effectively. You can do a heavy, renewal pruning of old olive trees to increase the amount of new growth or to improve the shape of your trees but be aware that the tree will not produce fruit for at least three years afterwards. But if you like the shape of your existing trees, then just give them a regular light prune. This makes it much easier to manage them for fruit production, whether that’s harvesting or managing pests and diseases. For young trees, they need to be pruned to establish an easily managed shape, which is usually a vase shape.

Olive flowers and fruit also have a requirement for even light in order to be effectively fertilised and to ripen consistently across the tree. Pruning the canopy allows light to reach all parts of it.

Olives will survive without water, but produce better if you irrigate them. The first part of the fruit to develop is the pit, then the flesh and finally the oil, so irrigation timing is very important for the best olives! Once the tree has reached “full bloom” (when 70% of the flowers have opened), restrict the amount of water the tree gets as much as possible for the next six-eight weeks. If some rain falls, that’s ok, we can’t control the weather, but don’t water your tree from the hose. Once that initial growing period is past, and you can see the baby olives starting to grow plump around the pits, you can water the trees to ensure that the soil around them out to the edge of the canopy is moist but not soaked. Small, frequent water applications are better for olives than long soaks, which is the opposite of how most fruit trees are irrigated. Mulching around the base of the trees will help conserve the water that you do apply. You should keep watering olives for pickling right up until you are ready to harvest them. If you want to make your olives into oil, you should stop watering them at least six-eight weeks before you intend to harvest as too much water in the fruit makes them difficult to process.

Most olive varieties produce the flowers (and therefore fruits) on mixed shoots which grow both vegetative and flowering twigs. These mixed shoots will only grow fruit once they reach three and four years of age. This means that pruning your olive tree more heavily after a heavy crop can reduce the tendency that olive trees have to grow a heavy crop one year and then a lighter crop the following year. Pruning the tree regularly also makes sure that if you need to apply any fungicide or insecticide sprays, you are using the minimum amount possible because if you have less leaves to cover, you can use less spray.

When you prune your olive trees each year, start by removing any growth that aligns with “The Five Ds of Pruning”. These are branches or twigs that are:

Dead
Diseased
Damaged
Diagonally growing (across other branches, too close together or towards the centre of the tree) or
Downwards growing (towards the ground)

You should assess the canopy in order of these priorities, and never remove more than you need to. It is hard to kill an olive tree by over pruning, but you can seriously affect how much fruit you will get at harvest time for up to three years afterwards if you take off too many branches or twigs. A good rule of thumb is to remove roughly 1/4 of the canopy if you prune each year. If you are undertaking a renewal prune to reshape or reinvigorate your olive tree, you can remove 1/3-1/2 of the canopy.

Start by thinning the upper canopy, reducing the height of the tree to whatever you want to maintain. Eliminate any suckers from the base, then watershoots (vertical branches that don’t produce fruit) and then trim the edges of the canopy to reduce the lateral spread of the tree. This will start the process of removing exhausted shoots, and you should continue to remove those branches that are four year old wood as they won’t produce fruit again. Get rid of any vigorous shoots that are inserted at narrow growing angles, or too closely to other structural branches. Finally, make sure you remove any missed dead, diseased or damaged branches. If you prune your trees regularly, it shouldn’t take more than 5-10 minutes to effectively maintain the canopy after harvest.

The heavy/light cropping habit (known as biennial bearing) that olives are known for is also affected by the tree’s nutrition levels. So, it’s best practice to feed your olive tree some matured compost each Spring as the flowers are developing and again after the harvest in Autumn. You will need to spread the compost around the base of the tree(s) out to the edge of the canopy width, all the way around. Make sure that the compost is not touching the trunk at all as this can cause collar rot in the bark, which can kill the tree. If you want to get a soil analysis done, that can be helpful to pinpoint any potential trace mineral deficiencies. Common ones to watch out for in olives are boron deficiency and potassium deficiency. A lack of boron causes excessive leaf drop after fruiting and a lack of potassium can cause less flower buds to develop. If you choose to apply a commercial fertiliser to your olive trees, please limit nitrogen applications as these will cause your tree to produce vegetative growth at the expense of the mixed flowering shoots.

You may need to apply a preventative fungicide to the trees in Spring if the weather is warm and there is a lot of rain. Some olive fungal diseases can cause the developing fruit to abort (stop growing and fall off the tree) in warm and wet weather. A preventative spray of appropriately measured and mixed copper sulphate or copper hydroxide fungicide (these are approved by organic certification bodies) when the flower buds are developing but before they have opened will help to manage this issue if required due to seasonal growing conditions.

Spring is the best time to apply insecticidal sprays to control scale and Olive Lace Bug populations too. You may need to look into controlling your local ant population if you have a very heavy infestation of scale, as they usually go hand in hand.

Common Pests and Diseases of Olives in Victoria

You should not spray any fungicide or insecticide in your garden on windy days, when the temperature is over 25ºC or the UV index is moderate-high, or if rain is forecast within 48 hours. Doing so can either cook your olive trees, or is a wasted effort as the rain will wash it off before it can be effective. Please make sure that you also don’t spray anything on your olives for at least two weeks before you plan to harvest them as the sprays can be ingested from the skin of the fruit even after pickling or turning them into olive oil.

Scale is an insect that can grow on both olives and citrus trees, so if you have both of these trees in your backyard and one variety is affected, it’s good to check the other for symptoms. Keep an eye out in the Spring for any orange or pinkish dots of the underside of the leaves of your olive tree as these are the baby scale. They don’t fly, so won’t travel far, and can be controlled using white oil spray, or a targeted insecticide that makes them sterile so they can’t breed future generations. It’s good practice to spray again about six-eight weeks after the initial application to get any eggs that have hatched since. Two applications per season will usually be enough to manage the population effectively.

Olive Lace Bug is a flying insect that’s native to south eastern Australia and arrives in olive groves that are close to native vegetation. It is usually detected on the edges of the grove or garden first, and will spread rapidly to all olive trees in the area, causing defoliation and tree death. Again, this insect is best controlled in early Spring if you see small brown or black dots on the underside of the leaves, or tiny yellow pinpricks on the surface of the leaves. As for scale, you can use horticultural oil spray, or there are targeted hormonal insecticides for use and the same spraying regime and maximum of two applications per growing season apply.

You will often find that a heavy scale infestation comes with Sooty Mould, where the leaves and bark of the trees turns black from a fungal infection. This is caused from the sweet honeydew that is excreted by the scale insects. This honeydew can also attract ants, which protect the scale population so that they can farm them for their honeydew. The ants track the fungus spores up and down the tree, spreading the infection. So to deal with sooty mould, you will need to start by putting a couple of rounds of masking tape around the trunk of your olive tree (sticky side out) and covering that with horticultural glue. This will trap the ants and stop them spreading the sooty mould and protecting the scale. Now any predatory insects in your garden will be able to attack the scale, or you can effectively spray them. The sooty mould can also be washed off with a mild detergent spray, and adding eucalyptus and/or tea tree and/or clove bud oil to the spray can help to prevent more spores from growing. Sooty Mould is prevalent at any time of year.

Peacock Spot is another fungal disease of olives in Victoria, but this one is most active during cold and wet conditions. It causes the olive fruit flesh to become soft and rotten in spots, which affects the fermentation process of pickled olives, and affects the oil quality. The disease with also show up on the leaves of the tree and can cause defoliation if left unchecked. This fungus is best managed using copper sulphate or copper hydroxide sprays at the first sign of its presence, and at least two weeks before the fruit shows signs of beginning to ripen. Do not spray the fruit less than two weeks before harvest begins.

All pests and diseases can be mitigated by making sure that your olive trees are in excellent health throughout the growing season by managing their water to keep the surrounding soil moist but not wet (olives are susceptible to root rot if they get wet feet), and feeding them with a balanced and well matured compost every six months.

Anthracnose is most often seen in warm and wet conditions. It is most active in Spring and late Summer/early Autumn, around the time of olive fruit harvest. This fungus has two distinct presentations, one that causes spotted ripening on the fruit skin and the second that causes the tip of the fruit to become mouldy and shrivelled. You should assume that the spores are always present on your olive trees, and if the Spring growing season is warm and wet, a preventative application of copper fungicide as mentioned earlier will help to manage this issue.

When to harvest your olives

The timing of your harvest will vary a lot based on the variety of trees you have, and the microclimate in which they grow. Some table varieties do not turn black, they only become “yellow ripe” where the shade of green on the skin goes from bright green to yellowish green. Any table variety can be picked for pickling at this yellow ripe phase if you prefer green olives for eating. The only difference between green and black olives in the jar is how ripe they are! If you are intending to turn your olives into oil, wait until at least 60-70% of the fruit on the tree has turned, or is turning, purple on the skin. Oil from the early harvest (greener fruit) is bigger in flavour, but there’s less of it, while oil from fully ripe fruit is very mild in taste but you will get a better yield from the fruit.

Another way to test if the fruit is ready to harvest is to pick one and squeeze it between your fingers. Any liquid that comes out should be white in colour and feel slippery (from the oil) when you rub your fingers together. If the liquid is clear and feels watery, the olive isn’t ripe yet.

Are my olives better for pickling or for making oil?

Both table and oil varieties of olives can be either pickled for eating or processed into olive oil, but traditionally they were grown for different purposes because the bigger olives make for a more satisfying snack for less effort! So the bigger the olive, the more likely it is to be a table variety and the smaller olives tend to yield a higher volume of oil by weight. If you didn’t plant your olive trees and don’t know what variety they are, most backyard olive trees are table varieties because until fairly recently it was very difficult for small scale growers to get access to an olive oil processor who can handle small amounts of fruit. Olive oil production on a domestic scale is a difficult, messy and time consuming activity.