In article ,
Dave Poole wrote:

Clivias are extremely susceptible to exposure from direct sun if they
have been kept shaded. Your plant's symptoms are completely typical
of one that has been severely burnt. ...

This is something that we have discussed before, but I still don't
understand the mechanisms, and so am asking for clarification and
cross-posting. Here is what I understand the situation to be.

The annual peak intensity of the sun in the UK is perhaps 70% of
the daily, clear sky, peak intensity in the tropics, perhaps less.
The ultraviolet levels are a LOT less, but I haven't been able to
find what they are - let's take them as 40%. And, of course, even
those figures apply to (typically) half a dozen days a year in the
UK - the average daily peak in summer is much lower.

My experience is that 'burning' damage is almost always caused by
those few days, and that the sun levels on a 'Phew! What a scorcher'
day might be 50% higher than on a typical 'hot' day in summer. This
is because our sun levels are primarily controlled by atmospheric
absorption, not sun angle. Is that your experience?

I believe that the problems caused by sun through glass are mainly
surface heating, because it is typically associated with slow air
movement, is much more serious close to the glass than a distance
away, and 1/2" air gap K glass double glazing does not seem to cause
the effect much. Can you confirm or deny this?

I don't have a clue what the primary 'burning' effect on plants is
(i.e. ultraviolet or surface heating) and what the 'tanning' effect
is. But they assuredly exist. Can you clarify those at all?


Regards,
Nick Maclaren.