Hi oska, and Welcome, sadly there is not much traffic here these days.. I have the same problem as you in my garden so many tall gums trees on the 3/4 acre block masses of roots everywhere so we have to dig rather large holes if just to loosen up the soil and add other soil from down the chook yard plus home made compost you really have to keep working at it and we mulch thickly and buy large rounds of Sugar Cane mulch when we are able.

I have just bought two beehive gingers and a red torch lily and even thought they are under those trees and have only been in the soil for four weeks there is lots of new growth coming up. I also have bromeliads there too some with coloured leaves and flowers.. I too have a heliconia or two and find they do better out of the sun in dappled shade same as the ginger which needs a fair bit of water to get them going.. I am a great believer in Seasol and shower the plants after putting into the garden and every couple of weeks because if the drought.

With no shortage of rain up where you are in the summer and you say you keep up the water I am surprised that your Gingers have died what a shame.. Broms are great for colour here are a few of mine I have no ideas of the names as I buy them from the Sunday Markets down here.

Love the red flower on this one.
{{gwi:2113438}}

This one came with a bonus.

{{gwi:2113439}}

I also have the ones with the bright pink edges Neon's I think they are called.

{{gwi:2113440}}

Elk or stag horns the trees, begonia, orchids, Canna lilies, ferns at the base of trees, cycads are nice even the sago palm looks good under trees..
And don't forget the bougainvillea if you don't want the big ones how about the dwarf or bambino ones in pots for some colour though they like the sun.

Evita.
{{gwi:2113441}}

And the hibiscus for a bit of colour sometime the white gives such a cooling effect.

Swan Lake.

{{gwi:2113442}}

Louisiana Iris are supposed to grow well in wet areas. I hope this is a little help.

And good luck with your garden...Misty.

Thanks for that Misty, and the great photos
I guess I'll just keep trying to improve the soil, I think the garden has been pretty neglected for some time - since I started fertilising I've had all sorts of things start popping their heads up (Alocasia, Caladiums, native lilies) that have never shown their heads before. I love bromeliads but they're so expensive here. I only have about 2 spots in the garden when bougainvillea or hibiscus would flower, I keep pots in those areas so I can move as needed.

It's a shame there's not much traffic on this forum anymore - some of the earlier posts were great to trawl through.
cheers
oska