Janet is quite right of course. Although Cordyline australis is
tolerant of poor soils and fairly dry conditions, it produces faster,
more healthy growth if given adequate moisture and an annual top-
dressing of well-rotted manure or compost. The coloured leaf forms
tend to fade or dull down if not given a regimen of regular feeding
and watering, so don't force them to linger in thin poor soils, they
deserve better.

The ordinary green form is normally cold resilient to around -8C. but
with -10C and below being quite common this winter, many have
suffered. They have thickened, woody, almost tuberous rhizomes below
ground and most will be able to regenerate from these. Especially
those that have been established for a few years. In their native
habitat, many lose all top growth due to forest fires and are
perfectly capable of producing replacement growth. Depending upon the
weather, new shoots can develop at any time from spring to late
summer, so don't give up hope if nothing appears immediately.
Naturally, new shoots will need some protection during the first
winter or two - especially if there's a recurrence of the recent very
cold weather.

Which leads to an earlier point, there is no need to thin out the
number of shoots unless you specifically want a single 'trunker' or 3
trunked clump. It's purely down to personal preference and you can
allow every shoot to develop without any detriment to the plant.
There's a magnificent specimen in the front garden of a neighbour that
has 8 or 9 trunks having been chopped to ground level 10 years ago. It
is now a spectacular fountain of foliage some 5 or 6m high.

It has probably been pointed out before, but these plants are not
'Dracaenas', which are only remotely related and closer to Aspidistra
and 'Solomon's Seal' than Cordyline. Nor are they palms in any sense
of the word.