How to use "EVER" in the sentence?

All plants I've owned have died.

By the way, "I don't have green fingers." could be changed to "I don't have a green thumb.".

I believe it's known as an intensifier, and it emphasises the meaning.
The sentence means the same without it, but it doesn't sound quite right.

I think "having a green thumb/ green thumbs" is American, while we in the UK talk of having green fingers.

"Ever" means during all of the time period under discussion; that could be all of time, all of recorded time, or some lesser period which will be implied or specified. As Mctag says, it can be an intensifier and can often be omitted but be careful of doing so, as it can also serve as a specifier that no exceptions took place.

All the plants that have ever lived (all of the plants that have ever lived on the earth)

All the plants that I have ever grown (all of the plants which I have grown during my lifetime)

All the plants I have ever grown during my period at Rose Cottage.

I've never heard the plural 'green thumbs' used for this meaning.

Thanks for that.
I will remember it.

I'm not saying that it's impossible to use 'green thumbs'. It may might well be used by someone somewhere.