"John" wrote
My lemon tree (in a conservatory) loses its leaves each year about this
time.
Does anyone know why?


They are evergreen plants so shouldn't lose all their leaves at once, so it
must be the conditions they are in or your care that is causing this.

As others have mentioned, our citrus trees have been outside against a S.
facing white wall since April with me keeping an eye out for frost forecasts
so as to bring them back under cover for the night, a wheeled trolley helps
if you aren't built like me. :-)
They cope with anything above freezing ( and a bit below but I don't take
the chance) and they certainly prefer it outside despite the cold nights.
The difference after a couple of days out is noticeable.
All now in full flower together with well growing fruit from a winter
flowering on one of the Lemons and the Orange.

Get it outside in the sun asap, keep it watered but never let it remain wet,
never use a water tray. Water with rainwater if possible, feed every other
watering and flush through on the other watering so as not to get a salts
build up in the pot.
I'm watering (flooding) mine twice a week now.
Bring it back in at the first forecast of frost in your area.

--
Regards
Bob Hobden
17mls W. of London.UK