I have two Joshua trees, grown from seeds indoors from the great state of Arizona, approximately 5 years old, now about 5 inches high! (Wow).
They have recently started to go yellow at the tips and was worried about their health.

Now I known I'm in the wrong latitude but wondered if you have any tips.

Hi Nigel1956,
I don't know a thing about the desert type Yucca but hopefully someone knowledgeable about Joshua Tree (Yucca brevifolia) will be along soon with suggestions and advice.

Joshua trees are very picky plants. There is a reason they are found in only one small area on the entire planet.

I think growing them for 5 years indoors is a feat in itself. I have seen large ones in gardens only twice growing outside their normal range but they are the exception.

I have gotten Joshua trees to grow for a couple years but eventually, they always fail.

BTW, average growth rate is 12 inches a year.