In article ,
"ned" writes:
|
| I have a few niggles, too! ;-)
| I am not a Californian, nor was the orchid specialist who gave me the
| advice - which has served me well over the last 20 years. I am sorry
| if it does 'NOT map' directly to Cambridgeshire. It has proved
| satisfactory for Buckinghamshire and Lincolnshire.

You are now changing the topic. My point is that it is a myth
(and a dogma) that most such tropical plants will scorch in the UK,
DUE TO THE SUNLIGHT, even in direct sun. They WILL scorch if
immediately behind glass, but that is a different effect. See
below.

I have been unable to find measurements of the ultraviolet levels
in the UK, but the average light levels in the UK in June are
about a half of those in the tropics. Because of the much longer
light path and water vapour content of the atmosphere, which
absorbs ultraviolet preferentially, the ultraviolet level in the
UK will be WELL below a half of that in the tropics EVEN IN
MIDSUMMER.

Using myself as a guinea-pig confirms this. I have started to
burn on moderately tanned skin in 45 minutes in the tropics; I
have NEVER burnt, not even on untanned skin, inland in the UK
(salt and wind burn is another matter).

In winter, of course, ultraviolet levels are as near as dammit
nil.

| I have only a
| Phalaenopsis and Cymbidium,
|
| ....... Oh. Definitely an expert then. ;-)

It is you who are claiming to be the expert, not me. What I can
tell you is that I have NEVER seen sun scorch in the UK due to
light levels as such on any of the standard "keep out of direct
sunlight" plants. And I have kept a fair number in direct sun,
both in the open and in a south facing conservatory.

95% (perhaps all) of the claims of "sun scorch" in the UK are
nothing more than localised overheating, either because the plant
is in a position that its leaves get very hot or because its
capacity for cooling is overloaded. You often get "sun scorch"
of plants like primulas in full shade; the light level has nothing
to do with the matter.

Both of those plants have been left in full sun in the recent "hot"
summers without a SIGN of scorch. I have seen them happy with full
sun elsewhere in the UK, too.

| but the following remarks apply to most
| tropical house plants (including my orchids).
|
| In terms of direct sun, even our summer sun is rarely strong enough
| to worry them - yes, I really DO mean that it is comparable to
| the light levels in a tropical forest!
|
| A tropical forest can be a very dark and oppressive place and some
| orchids will never see direct sunlight..

Sigh. (a) think contrast - such places LOOK very dark, because
they contrast with the bright light in the open. (b) sun scorch
is caused by ultraviolet which is a much higher proportion of the
visible light in the tropics.

| Similarly, most DON'T want high humidity, ESPECIALLY in cool, dark
| conditions. UK relative humidities can be sky-high,
|
| .... rarely, and certainly not .......

NO, NOT RARELY. Reaching 100% pretty well every night, even in the
driest parts of the UK. Even condensing atmospheres are common.
The purpose of heating in much of the UK is to reduce humidity, not
raise the temperature. That is WHY we have so much trouble with
tender plants that can take -20 Celcius in other countries.

| In a relatively
| cool, naturally ventilated room in most of the UK, (where) the
| humidities
| ARE what (desert) Californians (might) call high!
|
| Perhaps Kew don't know any better but their 'high humidity' tropical
| environments are positively dripping with water. Few domestic
| environments come anywhere near close to those conditions. My
| suggestion of a bowl of half submerged pebbles is unlikely to create
| 'high' humidity.

Perhaps you failed to observe the glass roofs. Most plants can
handle a lot more humidity when in reasonably active growth. I
quite agree that your bowl of half submerged pebbles is neither
here nor there. Please also reread my comment about "cool,
naturally ventilated".

One of the most common ways to kill tropical plants in a UK winter
is to keep them merely protected from frost. It is very common
for the relative humidities in greenhouses etc. to get to 100%
and stay there; this causes very rapid fungal, bacterial and algal
growth. No, I don't know how sensitive tropical orchids are to
such attack at 5 Celcius and 100% humidity, with effectively no
ultraviolet to kill the surface fungi etc. But a HELL of a lot
of "no direct light" tropicals ARE sensitive.

| My advice was stated to be 'general'. For me, it has successfully
| stood the test of time. If Mr MacLaren's orchids survive a different
| regime, then that serves to prove just how tolerant orchids can be and
| should lessen Gea's fears.
|
| Gea, as with most things in life, approach them with moderation in
| mind, orchids grow in many homes across the UK. They can in yours,
| too. ;-)

I agree with that.

What I am arguing against is the myth/dogma of "no direct sun",
as generally applied. It just isn't true, and is positively
harmful for many tropical plants in winter. I am not saying which
orchids will thrive on very low light levels, because I don't know.
But I am saying that most of the ones standardly sold in garden
centres in the UK can handle direct UK sun perfectly well.


Regards,
Nick Maclaren.