"Spider" wrote...

I assume you're growing them on a balcony since you're living in a flat,
otherwise you wouldn't be worrying about frost. Because they've sprouted,
I think they will need some protection, even though they're normally hardy
perennial bulbs. Put them as close to the building as possible and, when
frost is forecast, put some horticultural fleece (or old net curtains)
over them. It you put three or four canes around the edge of the pot to
drape the fleece over, it will give slightly better protection. If your
flat is high up and the balcony is at all windy, you will need to tie the
fleece in place.

In the summer, you will need to watch out for Red Lily Beetles, which will
(together with their larvae) decimate your plant. Worse still, if the
adult has recently chomped on a virussed lily, that virus will pass into
your plant ... then it's a throw-away job :~(. I suggest you google on
Red Lily Beetle so you know what you're looking for. Slugs and snails
should be less of a problem, but if you know they can reach your balcony,
protect your lillies with pellets or similar remedy.

Last winter I was given some lillies that had already sprouted. They
grew, if a little weakly, but certainly didn't flower. Because yours are
more robust, you may be lucky. Whatever they manage during growth, there
will come a time when they naturally die back. All the energy and
foodstuffs in the green part of the plant should be allowed to die back
naturally, as this feeds the bulb ready for the next season's growth.
Only when that growth is yellow should you cut away the stem. One last
tip: don't over-water. Lilies *hate* sitting in wet soil; it rots their
basal plate, which is where the roots grow.

Excellent advice and I might add that they prefer acid soils, which is why I
can grow them very well out on our allotment, so Ericaceous compost would be
best. They can stay in the same compost for a few years IME but do feed them
well in the growing season to build up the strength for flowering the
following year.
As Spider says, those Lily Beetles are a pain but at least they are bright
red, the larvae cover themselves with their own excreta so look for lumps of
dark brown bird droppings under the leaves. You have to look daily or more
they work fast. I must admit having lost a lot of plants in the past despite
being vigilant I now spray as a matter of course. Still can't grow Madonna's
though.

--
Regards
Bob Hobden
just W. of London