In article ,
Dave Poole wrote:

Regarding hardiness, the problem is that even with a Poncirus
rootstock, the relentless cold-wet combination in a good many parts of
the UK *without* compensatory day-time temperature rises plus early,
sustained, warm spring weather means that few varieties can survive
out of doors all year round. Poncirus does impart some hardiness, but
not quite enough for UK conditions. I have a 'Valencia' orange, which
has overwintered in a large container out of doors for 5 years and
both lemons and Nagami Kumquats do well locally.

Yes, that is the point. Interestingly, the same applies in some
parts of the tropics, though the problem there is saturation combined
with continual warmth!

If the precipitation is heavy enough, and the evaporation low enough,
even pure sand will remain largely saturated. Plants that are very
sensitive to that (whether in combination with cold, warmth or other
factors) can suffer. As the evaporation in the UK is essentially
nil all winter, even light continual rain will keep most soils at
least semi-saturated. The fact that the water is just passing through
doesn't help.

And, of course, the frequent light frosts cause damage, and the cold
and dark conditions mean that recovery by regrowth isn't possible.
In the tropics (the high rainfall areas), the problem is that the
fungi and bacteria grow faster than the plant can.

I have no idea of the physiological and related reasons why some
plants can handle this and some can't, but it is very obvious that
is the case.