In article ,
Dave Poole writes:
|
| No, my experience is that the leaves will burn, regardless of whether
| it is a 'scorcher'. I find that it tends to coincide with direct
| overhead sun and is far worse on plants that have not been very
| gradually 'hardened' to increased light levels. Clivias can cope with
| full sun in the U.K. but they need very gradual hardening and placing
| them suddenly out of doors as indicated by the O.P. is a recipe for
| the bleached out look. I hasten to add, that here in the far south at
| least, non-burnt, fully exposed plants do not grow as well as when
| given some shade and typically have short, broad leaves with a slight
| yellow caste. I've experimented with 3 year old seedlings many times
| and there's no doubt that growth rates at full exposure are as much a
| 50% lower than those given light to medium (20 - 40%) shade.

Thanks again. That could well account for my C. nobilis - while
the leaves are very dark green, the other symptoms are similar.

| My own mature plants get some full sun later in the day - ironically
| when heat levels are at their highest which is usually from
| mid-afternoon onwards. There is a difference between exposure to sun
| at a high angle (noon) and at a much lower angle (late afternoon).
| Quite a few shade loving plants can cope with the latter, but not the
| former.

I am surprised that your maximum heat is so late, but that effect
does not surprise me. The light path through the atmosphere gets
rapidly longer after mid-afternoon. That is the main reason that
the ultraviolet levels are dangerously low in the UK winter.

| 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?
|
| Well having grown them in very large glasshouses where they were never
| closer than 3 feet and often as much as 10 feet away from the glass
| and still got burnt, I do not think proximity to glass is the complete
| answer.

Oh, no, but I have certainly seen it be heavily dependent on the
distance - such as 6" away burning badly and 2' away not burning.
My other posting explains what I think that is happening, and it
could equally occur in large buildings (after all, the area of the
'mirror' goes up as the distances do).

| 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?
|
| Well, it is a bit of a poser, I'll agree. If surface heating was a
| contributory factor, there would be little or no damage at low
| temperatures. Unfortunately burning can occur when air temperatures
| are only 15 or 16C. The burning appears to be a bleaching of plant
| cells with cell contents being oxidized. I think suddenly increased
| UV exposure may be part of the problem and if I were to 'hang my hat'
| on a reason, I suspect it would be that.

As I explained there, the first conclusion doesn't necessarily
follow. You could be right that it is ultraviolet rather than
pure heating in the 'glass' effect, but I hold another view.
What is certain is that both ultraviolet and surface heating
cause cell damage, in plants just as much as in humans.


Regards,
Nick Maclaren.