It is important to protect the roots if you want your olive tree to be in the greenhouse all year. You can do this by putting isolating around the pot, or by simply digging a hole in the ground.
Photo: Grøn Kommunikation ©

By Lars Lund

Get advice on how to take care of two of the most popular Mediterranean plants

The climate has changed, and our winters have become mild. This means it can be easier to have tropical plants in your greenhouse than 30 years ago. However, when you choose the tropic or the subtropical, it is essential to consider how they should overwinter, their placement and how you want to shelter them when they go out of the greenhouse to enjoy the summer. Olives and citrus are the two most popular eatable plants in the Mediterranean garden, a tropical greenhouse, and orangery. This article will give tips on taking care of citrus and olive trees. Especially when the trees are moved outside after winter-hibernation in spring can be challenging.

General for citrus and olives

The root is the most exposed part of the plant to frost. Therefore, it needs special protection, especially when the plants are in pots and tubs. Here they are more exposed than if planted directly in the soil. While olives can survive frost, not for a longer period or too hard, it is the fewest citrus trees that can. Therefore, citrus should be kept in a frost-free room during the winter. The more light olives and citrus get, the more heat and water they need. On the other hand, if they stand in a darker room, they will need less water and heat. Neither olive nor citrus can survive if they stand in a completely dark room. The humidity should be around 50 to 70%, so if you keep them in your warm living room, it can become too dry for the plants. Even though citrus can stand in a warm room, it is not ideal. Instead, you could place them in a light room with approx. 10 degrees. In the greenhouse, you can create a smaller room out of bubble wrap with room for all your plants to ensure that the root doesn’t get frost. If the temperature in the greenhouse falls below 10 degrees, you can heat your greenhouse. In my greenhouse, I have a small heater that automatically regulates the temperature when it drops below 5 degrees.

After the winter, the plants should be moved outside. The move can be critical for the plants. The plants need to acclimatise or harden because they will react when they are moved out into the sun. In particular, the light outside in the spring is much stronger. The plants should be covered with non-woven fabric or stand in the shade for some part of the day. Otherwise, the leaves will burn.

Down with the olive

I keep my olive tree in my greenhouse all year. I took up some tiles in my greenhouse to plant my olive tree directly into the soil. The soil is clayey, which olive trees thrive in. I have put down drains in the hole with rocks and Leca to ensure the water can run from the roots. Olive trees planted in a flower bed can fetch water down to 6 meters. Olives can withstand down to minus 10 degrees in a short period, but if there is severe frost over a longer period, which is becoming very rare, I cover around the root with some insulation boards. If there is frost for a longer period, I wrap the olive tree with bubble wrap, but removing it as soon as possible is crucial to avoid the humidity becoming too high. It can be as harmful to the trees as frost because the plant can die of rot. Therefore, some wrap their trees in crop cover cloth, which is more breathable, but the isolation is not as good.

If the tree is in a pot or tub, you need to isolate it and place it on a polystyrene plate. If you take it inside, it must stand in a cooler room, or the plant will begin to lose its leaves. The leaves will usually come back when it is an olive tree, but a citrus tree is not as tolerant.

Olive trees in pots

Olives do not like to be transplanted. If you transplant your tree, it must not stand deeper in the soil than before. You can mix your own soil or buy the Mediterranean soil that consists of clay, sand, and lava.

Blooming and fruits

Olives are easy to care for. If the tree is going to bloom and give fruits, it needs to have a period with cooler temperatures. The shift from the warmth to the cooler temperature sets the flowering in motion. The fruit comes when the tree is between 3 and 5 years. They appear on the outer branches and get pollinated by either a bee or a brush. The vertical shoots give no flowers, and only the horizontal shoots do.

The fruits are ready to harvest when green, but if you want them ripe, they must be black. They only become mature very late in the year.

Water and fertilizer

Olives need water but cannot tolerate overwatering. Especially older trees like to dry out, but while producing the fruit, they need water. If you overwater your tree, the tree will drop both leaves and fruits. The same will happen if they don’t get enough water. Too little water makes the leaves curl and become yellow. If the green leaves fall off, it is a sign that the tree needs water. Generally, it is better to let them dry out rather than get too much water. From December to March, the trees are in dormant and don’t need water or fertilizer.

In the growing season, you can moderately fertilize your trees by using a liquid fertilizer, for example, a composition of 4-1-3 or 4-2-4, which is the same composition as citrus fertilizer. This fertilizer is not as strong as other traditional fertilizers. You can fertilize a few times during the season. Some fertilize more frequently, but it does not benefit the trees and can damage the leaves on the trees. How much and how often to fertilize is one of the extensive discussions, and many enthusiasts newer fertilize.

It is possible to prune your olive trees into the shape you want. You can prune your trees until august, but it is rarely necessary. Some branches become very long if your trees are planted directly into the soil. In this case, it could be a good idea to prune it. You can nip the shoots on the branches to make them thicker on the top. On the other hand, it is good for the treetop to get light and air. If the tree outgrows your greenhouse, you can prune it extensively without any damage to the tree. Cut after a leave or a bud. Use the branches for an olive wreath, a beautiful decoration for your greenhouse.

Diseases

Olives trees are healthy plants that rarely get sick, but they can get mealy bugs. They are easy to wash off. Scale insects are, on the hand, much worse to remove from your trees. Find them and scrub them off by using a toothbrush covered in alcohol.

Citrus outdoor and indoor

Citruses are lemons, oranges, kumquats and many other sorts. Citrus is often cultivated in pots or large tubs. The citrus plants which most people choose to cultivate, are also sorts that they want to use to make candied fruit or marmalade. But many people just have the tree to create an exotic atmosphere. An orangery is a warm room for citruses. Although the term orangery has another meaning today. An orangery is now also a large greenhouse, sometimes with decorations like cresting on the roof just like the old orangeries.

Into the pot

Citrus trees can be transplanted directly in the soil in the greenhouse, but it is not practical. A better solution is to be placed outside in the fresh air from mid-May to September. If they are in the greenhouse for too long, they risk getting diseases. A good place would be on the terrace in front of the greenhouse. If you want the tree to make an exotic atmosphere inside your greenhouse, it is important to ventilate. The humidity is the greatest threat.

You can use rhododendron soil mixed with a bit of sand. In contrast to olives, citrus trees like to be transplanted. You can do this every three years. Transplant the citrus tree so it is above the soil, which means that some of the top roots are still visible.

Blooming and fruits

Almost all citruses are self-pollinating. Therefore, you do not have to have two different trees. They will bloom multiple times during the year, and if you want to make sure that the flowers are pollinated, then you can use a small brush. During summer the bees will do it for you. Lemons can be harvested whenever you think it has the right size no matter the colour. Your citrus will thrive if it gets direct sunlight.

Water and fertilizer

The tree should be irrigated regularly during summer after a short while of drying out. The leaves will curl upwards if they need water. It is likely to overwater during winter. The soil must dry out before you irrigate again. Plants that are overwintering at 5-10 degrees should not be irrigated more than 1-2 times a week. Be aware of the pH-value of the soil when you irrigate with tap water. Acid-loving plants cannot absorb iron and manganese when the pH-value is above 6. This is likely if tap water with calcium is used. Therefore, irrigate with rainwater or boiled water if you live in an area where the water has a high level of calcium. Use a fertilizer that has a composition of 4-2-4.

Pruning

Citruses do not tolerate extensive pruning. Try to only remove the dead branches or branches that grows towards the stem, and do it in July.

Diseases

If the citrus tree is placed inside, it is likely that there will come spider mites and scale insects. But lack of magnesium can also cause yellow leaves. Replace the soil and if the plant needs more magnesium, you can add fertilizer mixed with 4 gram Epso salt and 1 liter of water. The mixture can be sprayed on the leaves in the evening and a few times a week apart. Lack of manganese and zinc can also cause yellow leaves. You can use the same method but with liquid fertilizer for orchids and spray that on the leaves.

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