Full sun for as long as possible is the key to making Erythrina
happy. It can behave almost as an herbaceous perennial in cooler
areas, but given good growing conditions in summer, it can easily make
enough growth to flower well in July/August each year. In coastal
regions with little frost, it can be grown in the open border where it
will become quite large - sizeable shrub to almost a small tree. In
colder regions, the protection of a sunny, south facing wall will
ensure its survival even if it is cut to the ground each year.

Erythrina develops a very thick, woody, somewhat tuberous root-stock
with numerous 'eyes' on the subterranean portions of the stem that
will sprout if the top growth is knocked back by frost. A thick mulch
can be provided as extra insurance, but I remember several plants
growing in the south borders at Edgebaston Botanical Gardens that were
never mulched. In those days of 'proper winters', they never failed
to put on a good show.

Sacha, I would avoid planting under trees, even if there is a fair
degree of light and a very high canopy. You are more likely to get
straggling stems that don't die back nor many flowers. The borders
against the house walls are far more suitable. The reflected heat
from the walls will encourage strong, fast growth culminating in large
clusters of flower.