Hello, Bradley. I am not sure where this tree is growing at, what your mum's USDA Zone or what type of camellia the tree is but, being 30 years old, I would start doing a soil test looking for mineral deficiencies (for example, lack of nitrogen can produce yellowing in leaves and typically shows up first on the older leaves, etc). I would also try to determine if water drains well in that area or if the soil is compacted. For example, after it rains, does water collect and drain slowly? Government Agriculture Extension Offices, university's Agriculture Departments and some plant nurseries offer soil test services.

When the problem starts (not sure if you might be able to catch that now), I would also look for pests/scale infestation... although it will require someone to go up a ladder to check for signs.

http://homeguides.sfgate.com/causes-yellowing-leaves-camellias-45976.html

Thank you for that, and for such a speedy response!

I'm in Australia actually, on the west coast in a city called Perth. From what I can tell, the equivalent USDA zone here would be 10 for you - temperate to sub-tropical. We have hot, dry summers and cool, wet winters (no frost). Low humidity and temperatures range from around 4c-25c in winter and 18c-40c in summer.

I'll look into getting the soil tested, and also there are a lot of plants around the base of this tree and I wonder if they are choking it out. They were always there, but are denser now than ever before.

I sometimes wonder if it would be possible to remove the tree from the ground and place into a large pot - the soils here on the coastal plain are very sandy and generally a nightmare. I wonder if it would do better in a purpose built soil... if it would survive the shock.

Maybe in winter?

Yes, plants in the base may be chocking/competing if allowed to grow large. I typically try to prevent growth near the base and up to the drip line by covering the area with organic mulch. If it is weeds that are growing there, I would pull them in the winter. If they are plants that I want to keep then I would have to review where the roots are mixing and plan what to do more carefully.

As for moving in winter, they will suffer less of transplant shock in the winter but, as you try to transplant a larger and larger plant, the risks of problems also increase. Perhaps it may be best to propagate the plant from cuttings, ground layering, etc., test the soil (add minerals that the soil may be deficient in) and propagate the shrub elsewhere (in the ground elsewhere or growing in containers). There is an El Salvadoran restaurant that I sometimes frequent which has camellia sasanquas growing in pots year-around so, know it can be done. I am just not sure how well the tree may take to being moved.

On the link below, click on propagation if you are interested.

https://www.americancamellias.com/care-culture-resources

Any time that you have a plant with some sentimental value, consider propagating first and transplanting it second.

Great advice, thankyou!!

If it were me, I'd try to keep the plant in place, but also try to dig around carefully to amend the nearby soil, and to check to see if the roots are checked by nearby plants.

A soil test can give you an idea of specific deficiencies, but as Luis points out, a 30-year-old plant is likely to have some soil compaction, so just digging around a bit and adding some organic matter may be enough.

I can't see the base of the camellia, but adding some sort of organic mulch that will break down gradually may help with plant nutrition, and also keep the plant more evenly moist and perhaps help control nearby weeds.

Good luck,

Virginia