To shade or not to shade.....

[Thermoman]

About five years ago I set out to prove that shading is not necessary in this country. This year I admitted defeat but I still find it difficult to understand how plants which are exposed to the blazing tropical sun in their natural habitats (i.e. near the tropics of Cancer and Capricorn, where most species of cacti appear to reside) can be scorched by the comparatively feeble summer sunshine in this country. OK, in the wild many cacti live in the shade of other plants and the exposure of Copiapoas is mitigated by the famous fogs but there are habitat photographs showing these plants literally 'out in the midday sun', completely unprotected. How can an hour or two of British sunbathing cause so much damage to such 'extremophiles'?

I have taken the usual precautions, viz:–
1. Ventilation
Doors and vents all wide open. In my relatively small greenhouses this, aided by 12” fans, ensures a healthy – even bracing – air flow whenever needed. No stagnant corners.
2. Acclimatization
For several years my plants were exposed, unshaded, all year-round. No bubble-wrap, no sudden changes.

There is little doubt that these measures were effective. It also appears that older, better established plants are at less risk than seedlings, or cacti that are not in the best of health, particularly those with poor root systems, for whatever reason. Results have nevertheless been unpredictable. In some years I got away with it. In others a few plants got scorched but on occasions significantly more damage was done. Admittedly I have been able to establish that scorching rarely occurs on other but 'scorching' days. We don't get too many of those but neither do we get much warning of their arrival. Clear blue skies with wall-to-wall sunshine, light winds and greenhouse temperatures over 40ºC, despite 'flat-out' ventilation, are the warning signs but they are difficult to anticipate and it takes a while to splash on the whitewash or affix the shades or netting. Few growers are prepared to raise and lower their defences repeatedly throughout the season, as the weather changes. Most put it up, or paint it on, as soon as conditions appear appropriate and then leave it in place until the end of the season. It can look pretty silly in a wet July.

Some cacti appear to be significantly more prone to sunburn than others. The list of susceptible species holds some surprises. Several contributors, beside myself, have mentioned Copiapoas. I have lost more than a few from over-exposure to the sun. Eriosyce are also relatively easy to damage, together with Turbincarpi, Coryphanthas and Gymnocalyciums, but Parodias, Rebutias and Echinocerei seem to be more resilient. Echinopsis is, however, easily damaged. Perhaps unsurprisingly, Mammillarias appear to be almost immune. I do not shade my 'Mammillaria House' and have yet to see a scorch – one or two 'red faces' now and then but no burning. I have a lot of mams., many of which are covered in hair or a network of spines, which appear to act as very effective sun screens.

A really odd feature is that plants that scorch in one year may not do so in the next, whilst neighbours that suffered no damage in the previous year emerge with hefty scars. Even adjacent plants of the same species can react quite differently.

The result of my experiments is that I have now, reluctantly, returned to netting but I do wish that some clever manufacturer would come up with shading that could be put up and taken down in minutes, rather than hours. I am not, however, optimistic.

[Phil_SK]

I do wish that some clever manufacturer would come up with shading that could be put up and taken down in minutes, rather than hours. I am not, however, optimistic.

Drape it over the outside of the greenhouse?

[Thermoman]

Drape it over the outside of the greenhouse?

That is precisely what I do but I am a bit slower up and down ladders these days and, with three greenhouses, I consider myself lucky if I am back indoors within 3 hours. (More often it takes 3 days!) Don't forget that, if the netting is external, it has to be secured against the wind, otherwise you might soon find it shading next door's greenhouse instead of your own.

If I ever come into a large sum of money, I will order photochromic glass for my greenhouses. I believe that it is at least at the research stage but, at current prices, I suspect that it would be cheaper for me to employ a man to pop in a couple of times a day in order to put up the netting, or take it down, as appropriate.

[IainS]

Hi Thermoman: Thank you for your extremely interesting post about about your attempts not to shade. I'm also someone who questions the logic and necessity of blanket shading and so far haven't suffered any losses (apart from one temperamental Echeveria that got partly damaged) - although that may likely change with time. We only have three Copiapoas, so they will be positioned to benefit from some shelter from neighbouring plants next summer. It's especially good to hear that no Mammillarias have suffered.

I would like to have responded in greater detail, but we're getting ready to return to our nature reserves in South Africa on Wednesday morning. Again, thank you for a well-written and thoughtful post.

[JoZoo]

About five years ago I set out to prove that shading is not necessary in this country. This year I admitted defeat but I still find it difficult to understand how plants which are exposed to the blazing tropical sun in their natural habitats (i.e. near the tropics of Cancer and Capricorn, where most species of cacti appear to reside) can be scorched by the comparatively feeble summer sunshine in this country. OK, in the wild many cacti live in the shade of other plants and the exposure of Copiapoas is mitigated by the famous fogs but there are habitat photographs showing these plants literally 'out in the midday sun', completely unprotected. How can an hour or two of British sunbathing cause so much damage to such 'extremophiles'?

Thank you for sharing the results of your experimenting over the last few years it was really interesting to read about what you have experienced. I have managed to go all summer this year with no shading and I'll admit I have lost a number of Turbinicarpus but I don't know really whether that's due to not enough water, too much sun or blasted mealy bugs. I mainly have all of the plants you mentioned, Copiapoa, Turbinicarpus, Gymnocalycium, Mammillaria and others Melocactus, Matucana, Ariocarpus and all sorts of other bits and pieces. British weather is a curious thing. In our oldest green house we have shading that is on a pully system so can be opened and closed easily. I can take some pictures if you want me too as I'm sure it would be fairly easy to rig up and once in place no more need for ladders.

[Tina]

I treated myself to some of these pull down blinds that go on the outside, they fix on the top ridge, they aren't cheap & I only got 3 of them so that I can shade a set group of plants if I want to I may get more next year if I think they are useful.
I did get them cheaper as I was ordering some other items, it's always good to get discount.
http://www.harrodhorticultural.com/addi ... d9236.html

[James Pickering]

.......... Growing under permanent bubble wrap does not seem to have inhibited the flowering of Rebutia albipilosa or affected it's spination.

R.-albipilosa.jpg

I would say not, Dave That is one stunning picture

James

[DaveW]

It's always puzzled me why plants burn (rather than overheat due to greenhouse being too warm) under glass since some claim you cannot get sunburned under glass, therefore I see no reason why plants should be different in that respect to humans. However I found this reply to whether you could sunburn behind glass:-

"You can, actually; it depends on the glass.
We know that electrons in any molecule can only absorb radiation at certain frequencies. It turns out that the electrons attached to molecules in typical glass (like the glass in your windows at home, or the safety glass in car windows) can absorb radiation at UV wavelengths, but not at visible light wavelengths – therefore, visible light passes through glass as if it weren’t there, but UV radiation is absorbed. It depends on the glass exactly how much UV radiation is absorbed, though. UVB rays are shorter than UVA rays -- that means they're more energetic, and they're usually the ones responsible for a sunburn. UVA rays are closer to the visible part of the spectrum, so it makes sense that some UVA radiation can make it through the glass. There is a difference of opinion on exactly how much damage UVA radiation does to you, but we do know that since it has a longer wavelength, it goes deeper into your skin. While UVB rays seem to be the primary culprit in skin cancer, UVA rays have been implicated as well. And according to NASA, most tanning salons use UVA radiation, probably because it burns less but still causes the pigmentation in skin that manifests as a tan.

This quote is also American so reverse left for right as UK steering wheels are on the opposite side:-

"Reality: Glass filters out only one kind of radiation -- UVB rays. But UVA rays, which penetrate deeper, can still get through. That's why many adults have more freckles on their left side than their right -- it's from UV exposure on that side through the car window when driving."

See:-

http://www.ncbi.nlm.nih.gov/pubmed/19614895

The thing I cannot find out is if there is any difference in UV penetration into the greenhouse through UV stabilised and non UV stabilised bubble wrap?

[Phil_SK]

This is a guess but...
The process you describe (glass molecules absorbing some wavelengths) will also be true for polythene. However, polythene is more susceptible to damage by this absorbed energy than glass and it perishes because of the long-term effects of absorbing uv. It follows that glass + polythene cuts out more wavelengths than glass alone (and that glass doesn't cut out everything totally otherwise polythene would last forever).
Anything that is uv stabilised will have something added to it, or coating it, that absorbs so much of the uv that there is very little left to damage the polythene in the same way that the ingredients of sunscreen absorb the uv that would otherwise reach your skin. It follows that uv stabilised polythene will let through less uv still.

I think.

[Thermoman]

I treated myself to some of these pull down blinds that go on the outside, they fix on the top ridge, they aren't cheap & I only got 3 of them so that I can shade a set group of plants if I want to I may get more next year if I think they are useful.
I did get them cheaper as I was ordering some other items, it's always good to get discount.
http://www.harrodhorticultural.com/addi ... d9236.html

Thanks for the link, Tina. What appears at first to be 'just the job' does, however, present a few snags in my case. I have one octagonal and two decagonal greenhouses. Fitting roller blinds to the vertical sides could be awkward because, at least with the Harrod pattern, they only appear to come in fixed widths, none of which exactly match the size of my glass panels. The resulting gaps could still leave some plants exposed.

A much bigger problem is presented by the roofs. I can see no way of fitting roller blinds to these quasi-circular structures. My multi-sided greenhouses do look so much nicer that the traditional pattern but there is clearly a price to pay - and, talking of price, a quick estimate of the cost of shading just the vertical surfaces, by this method, suggests that I would have to lay out roughly £1,000.

When I return in another life, I am determined that it will be as an aristocrat with a large staff to attend to all my needs, amongst which will be greenhouse shading and, as I get older, lugging around all the really heave pots. One can but dream.