There is a complete hollow in the main trunk which you can see through from one side to the other. Is this ok or could it cause problems?
The tree may have problems getting water and nutrients from the ground up through the trunk and network of branches and leaves. But if the tree looks healthy, then it has probably devised a way to get around it. Keep checking to make sure it is structurally sound.
https://www.gardeningknowhow.com/ornamental/trees/tgen/patching-tree-hole.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/patching-tree-hole.htm
I understand that the root system of the olive tree does not like to be in water so this self watering pot seems to be ideal. The watering is done through a funnel which goes down to a separated water reservoir which is separated from the actual plant and the plant gets it's water in a more natural way. I hope you can help me with some expert advice.
I would opt for a standard container. Watering when needed is better than standing water, as standing water can have a tendency to facilitate infection. You're container will need to contain drainage holes. This will allow all extra water to leave the container, which will mitigate overly saturated soils.
Just make sure that the soil is COMPLETELY DRY down to about 3 inches between waterings. Below this should be slightly moist.
The top of the soil needs to alternate between moist and dry in order to prevent infections, and to promote beneficial microbe growth. If you are not able to determine moisture at that soil depth, then a meter can help. These articles will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/edible/fruits/olive/growing-olive-trees-in-containers.htm
We have two potted olive trees that we purchased a few years ago because they were advertised as being “hardy” and that we wouldn’t have to do anything for the wet Seattle winters. It’s always looked a bit sgraggly and sparse and the branches seem thin. I’m curious if it is healthy or if this is how it’s “supposed” to look.
They look like they are struggling. Even if they are hardy, keeping them in containers lowers their winter hardiness. I would bring them inside for the winter or plant them in the ground. Here are more care tips:
https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-8/zone-8-olive-trees.htm
I’ve recently bought an olive tree which I want to repot from its plastic container into a larger terracotta pot. When I took the olive tree out of the plastic pot, I noticed a smaller plastic ‘mesh’ pot around the root ball. The roots are growing through the mesh and I don’t want to damage the root ball trying to get it off. Should I leave the plastic mesh pot alone to repot as the roots are so tangled up in it or should I try to cut the mesh away? I’m worried about damaging the root ball if I cut away, but I also don’t want the mesh to restrict the roots. I’d be grateful for your advice. Thank you.
It will be much better to cut the mesh. It will do more damage, long term, to leave it. This article will help you with potted olive care:
https://www.gardeningknowhow.com/edible/fruits/olive/growing-olive-trees-in-containers.htm
This collection of articles will give you much more information on the subject of olive trees:
Hello, I planted an Olive Seed for my best friend's daughter's birth in late march. The seed was in a small pot, partially in the sun indoors, in a well heated room. I germinated after a few weeks, and grew a little sap. I then left it on the window sil throughout the second half of summer. It grew up to 9 sets of leaves, and as the tenth was sprouting, it seemed to stop growing. I did transplant it, at which time I broke part of the tap root. But it still had plenty of root, and it seemed healthy for a few weeks thereafter. Then in mid-september, it started looking ill, the green leaves turned a blueish purple color. I moved it in when it started looking frail, it's now in a room in a heated building and it gets a few hours of direct sunlight every day. Is the sap dying? Can I do anything to make it healthier?
Unfortunately, this is likely due to the main root breaking. This can introduce pathogens that will kill the young sapling.
From here, I would make sure to use a fungicide, and make sure that the soil has plenty of time to dry out between waterings. They will appreciate drier soils.
These articles will help:
https://www.gardeningknowhow.com/edible/fruits/olive/olive-tree-care-information.htm
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
I recently purchased a property in Greece with 2 dozen olive trees. All the trees are over 10 years old some 20-30 years old. However only 7 or 8 produce olives, though I hard that decades ago all the trees produced. Is it possible to get olive trees to grow olives again?
Unfortunately, your photos did not come through. Olives stop producing, mainly, due to disease. Since they are adapted to dry, poor soils, feeding them should be limited. You may have some result with adding phosphorus and potassium rich fertilizers, but don't add much nitrogen.
Here is an article that will help you with Olive care:
https://www.gardeningknowhow.com/edible/fruits/olive/olive-tree-care-information.htm
Answer question
They grow in, naturally, poor and dry soils. I would not fertilize them.
Likely, the potting soil that you plant them into will contain more than enough fertilizer to last them for many years.
Should a soil test show extreme deficiency, then you can top dress with compost.
Here are some articles that will help:
https://www.gardeningknowhow.com/edible/fruits/olive/olive-tree-care-information.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/making-sure-soil-drains-well.htm