To shade or not to shade.....

[Thermoman]

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.

I wonder whether the focus should not instead be on temperature. With a greenhouse already near 40 deg C it does not need much more to take a plant in direct sunlight up to cooking levels. Heston Blumenthal, in a fairly recent television series, demonstrated how to 'roast' a chicken at 60 deg C and I well remember that when my father returned from his Second World War service with the Desert Rats, it was with photographs of tank crews frying eggs on the mudguards of their vehicles. A cactus may not be a chicken, or its egg, but I would imagine that it will cook at similar temperatures.

If the Libyan desert is felt to be too far south to be relevant then a recent event much closer to home could be more apposite. Acquaintances of ours, living in London, were forced to summon the fire brigade when a bathroom mirror set light to the curtains. Luckily the problem was spotted before the rest of the house went up in flames. It may be argued that this only occurred because the sun's rays were focussed by the mirror but we have all experienced examples of exposed surfaces, like rocks or sand, being raised to unusually high temperatures in direct British summer sunshine. If you care to place a thermometer in an exposed spot in your greenhouse you might be very surprised by the values it will record.

[IanW]

Yes exactly, it's about heat accumulation as much as anything. As iann has pointed out here numerous times, you can't simply measure air temperature in a greenhouse and assume that's a constant throughout. Surface temperature of some cacti in your greenhouse may exceed this, and will be exacerbated by issues such as sensitivity to it. A long grey British winter with short days acclimatises our plants away from the hot sunny days more typical in many of their habitats year round, and as such it should not be surprising that a burst of sun and warm weather early in the year in say March can really damage our plants.

Things are very different in habitat, as the plants are not surrounded by heat trapping greenhouses- even having a fan on wont guarantee consistent airflow around the greenhouse, ideally you need to be extracting hot air out, to bring the greenhouse down towards the temperature of the outside air, and in turn any plants suffering hotter surface temperatures will see some of that heat pulled away into the cooler air coming in to replace that being extracted. The ideal situation is that you setup a consistent airflow, whereby air of temperature is being pulled in from outside at one end, and being pumped out at the other if you want to prevent scorching without shade and emulate the conditions of habitat where scorch is uncommon more closely.

Doing things like opening doors, having vents, and having a fan blowing air around is sufficient in the UK in most cases, but probably not optimal - our plants will typically still be more stressed than is ideal for optimal growing conditions. Providing shade prevents that buildup of surface temperature that leads to scorch, and that's why it's beneficial - the plants are still getting ample light, but not to the point of being stressed so that they're too busy focussing on water retention and survival to expend much effort growing.

[James Pickering]

A completely unscientific observation:

For the past forty five years or so I have staged the eastern Brazilian cacti and Madagascar euphorbia's that I cultivate here at my town home solely under 30% shade cloth (in small plastic pots) from early May until late September when the high daytime air temperatures are typically in the 100°F+ (approx. 38°C+) range (sometimes much higher) with abundant sunshine. To date I have never had a plant scorch, sunburn or exhibit any heat stress (touch wood).







Added via Edit:

My larger mature Brazilian cacti such as Pilosocereus pachycladus, Micranthocereus doilchospermaticus, Micranthocereus estevesii .......... and so on, are housed in a 30% shade cloth covered "hoop house" during those months at Dan Bach's Cactus nursery -- covered by a film of UV polyfilm in winter.


James

[spinesandrosettes]

I wonder whether the focus should not instead be on temperature. With a greenhouse already near 40 deg C it does not need much more to take a plant in direct sunlight up to cooking levels. Heston Blumenthal, in a fairly recent television series, demonstrated how to 'roast' a chicken at 60 deg C and I well remember that when my father returned from his Second World War service with the Desert Rats, it was with photographs of tank crews frying eggs on the mudguards of their vehicles. A cactus may not be a chicken, or its egg, but I would imagine that it will cook at similar temperatures.

If the Libyan desert is felt to be too far south to be relevant then a recent event much closer to home could be more apposite. Acquaintances of ours, living in London, were forced to summon the fire brigade when a bathroom mirror set light to the curtains. Luckily the problem was spotted before the rest of the house went up in flames. It may be argued that this only occurred because the sun's rays were focussed by the mirror but we have all experienced examples of exposed surfaces, like rocks or sand, being raised to unusually high temperatures in direct British summer sunshine. If you care to place a thermometer in an exposed spot in your greenhouse you might be very surprised by the values it will record.

Thermoman, not to make light, or more appropriately heat, as suggested by your moniker, I would answer your wonderment specified...in a word, no. The thread title already is focused on the correct most important factor, which would be lighting, as suggested by the question to shade or not. While light and heat are not unconnected, I feel strongly that light is the primary and relevant factor, both as a result of my own experience, and my understanding of the concepts involved.
I was under the impression this subject had been thoroughly discussed, and without going back to re-read the entire thread, in light of your comment attempting to direct focus on temperature, I would have to assert, but conditionally, NO. The conditions of course are assuming that we are not talking about extreme temperatures, far beyond even the highs we currently would experience anywhere here naturally on Earth, greenhouse or not. Of course, if anything is hot enough, it will burn, or melt...so only in that context, would temperature be relevant.
Also, I assume we are talking about succulent type plants, that have naturally evolved and are suited to cope with light and heat in natural settings.
Stipulating that temperatures are "normal", even specifically at the high end of "normal" ranges, I believe that the quality of light makes all the difference. The factors most affecting solar radiation are the angle of the sun in relation to a specific point on Earth, and the distance. These factors can be specified as latitude, and the season. Other variables of course are weather and time of day. For that matter, the altitude of the location factors in.

The information from the following link quantifies the effect of several of these factors.
http://www.ccfg.org.uk/conferences/down ... urgess.pdf

Here one can read a discussion regarding the effects of UV radiation on plants.
http://www.pucrs.br/fabio/fisiovegetal/EfeitoUV.pdf


While interesting, the effect of glass on UV (UV-B is blocked), does not at all suggest that plants in glass greenhouses are therefor immune to the effects of UV radiation. UV-A does indeed pass through glass...
http://www.smartskincare.com/skinprotec ... doors.html
"Clear glass allows up to 75% of UVA to pass."

and it is not benign either...
http://www.pucrs.br/fabio/fisiovegetal/EfeitoUV.pdf

also
http://treephys.oxfordjournals.org/cont ... 7/923.full

Here is a link which at least provides the introduction of an abstact, which clearly gives one an overall point of view that altitude is also a factor to consider regarding solar UV radiation being affected by altitude. One would clearly have to make adjustments for a location at sea level, as opposed to a mountainous region.
http://www.sciencedirect.com/science/ar ... 2X98001261

nor can we ignore the effects of Global Warming, which degrades the natural portion of our atmosphere which mitigates UV radiation. For this reason, it is not really useful comparing the past to the present, because the situation is not the same, nor is it stable, and we continue to damage our atmosphere's ability to mitigate solar radiation. Also, as read in the introduction of the above linked "Effects of ultraviolet radiation on plant cells", we see that as ozone levels decrease, radiation reaching the bioshere increases exponentially. 1% reduction in ozone can translate to a 1.8% increase in UV radiation.

Personally, I have plants outdoors and inside greenhouse-type structures. With plants outdoors, I use natural shelter (trees) and position plants so that they receive sun during the cooler earlier hours when the sun is not at it's highest point in the sky, and shaded during the later part of day when the sun's rays are more intense. Location of the individual plants are species dependent, and I'm constantly experimenting on a trial and error basis, with the location of the plants in relation to the shade available at a specific time of day. Plants better suited to withstanding the sun's effects are placed further away from the trees, so that the shade reaches them later. Some plants that seem more or less unaffected by sun, are placed in full sun locations. With indoor plants, I have been using shade cloth additionally to cover structures, and have never had problems with scorching in those conditions. Without shade cloth, I find similar tendencies for plants to scorch as plants located outdoors.

Also, I would like to note that acclimation to the sun's rays is vital to the plant's ability to withstand solar radiation.
I have experimented with agave of the same exact cultivar, to quantify the effect of the sun on color and form. Plants accustomed to protected conditions suddenly switched to outdoor sun will burn, in as little as a day. This year, with several plants of the exact same cultivar, I placed one in a location receiving full sun until about noon early in the Spring, so that it could adjust as the days got longer and the sun more intense. This plant did change in color, but did not burn. Another plant relocated later in the season burned in a matter of a day. Other plants left in shade-cloth protected greenhouse space never burned, and retained their accustomed coloration. One plant unfortunately subjected to an ill-timed move to a brand new greenhouse situation that hadn't yet been covered by shade cloth also burned, even though the covering of the unit supposedly offered some level of UV protection. Adding the shade cloth corrected the problem.

Not to minimize the value of air circulation, which has already been discussed by others, other than the use of openings which can be closed or opened, I do not use fans with the exception of the first greenhouse structure erected here, of wood and fiberglass materials. That has an A-Frame type of roof, and uses solar powered small attic fans to discharge the hot air which rises to the apex, and a standing oscillating fan stationed at one entrance, utilized on warm days to move the air across the inside. There's an entrance at either end of the 20 x 20 structure, one always opened unless it's rainy or cold, and the other only opened up on hot days to allow cross-ventilation. Also, there's a tan shade-cloth stretched across inside which mitigates the light which penetrates through fiberglass panels which comprise the roof. All 4 sides are also windowed with fiberglass panels. It's bright inside, and it does heat up on average a good ten degrees over the ambient outdoor temperature. It often breeches the 100 degree F mark inside, but as alluded to earlier, it is not the heat which causes damage, but solar radiation, which is kept in check with the use of the shade cloth.

Earlier this year, I had the opportunity to visit the premise of a cacti grower, who grows all of his cacti in cold-frame type of units filling his yard. They are built with legs, so that the tops are at about waist-level, and the door or top of the units are hinged, and can be propped open to various degrees, and the large glass windows built into the opening tops are white-washed with thinned white paint. Light gets through, but it's thus filtered. Clearly, that's an experienced person who has figured out a good system that works for him. For comparison value, his location is on the inland outskirts of the S.F. Bay Area, at 130 feet elevation, and the area is known for relatively warm and sunny weather (circa 80 degrees F May-Oct with record highs over 100 for each of those months) of a " warm summer Mediterranean climate" typical of California's interior valleys.

Back to the original premise - shade or not to shade, yes...shade - species dependent, location dependent, even subject to the particular individual plant's acclimation to the specific conditions experienced.

[IanW]

Whilst there's no doubt that UV can cause scarring on plants, I think it's a bit silly to argue that it's the only factor in scorching in UK greenhouses and heat plays no effect:

- Scorching only occurs here on hot days, and yet UV levels can be high on cooler days. A clear prevailing siberian wind, will lead to a cooler day with no scorching, than a warm African wind where fine Saharan sand particles increase pollution and hence reduce UV levels where scorching occurs.

- Improving air flow has a clear impact on reducing scorch in the UK, but has no impact on the amount of UV reaching the plants. You covered ambient temperature, but still glossed over the issue of surface temperature - whilst it may be 100f in the greenhouse, it will be hotter on the surface of the plants where the sun is directly hitting it.

- On a hot day, in a greenhouse with no ventilation, I can take two clones of a plant, and put one inside near the glass, with no shading, and poor airflow, and one outside next to it, and I can guarantee you that the one inside will scorch, and the one outside will not. This is despite the fact that the one inside is at least seeing some reduction in UV, whilst the one outside sees no such reduction. The one outside might indeed "red up" more, but I don't think we ever get enough UV here in the UK this far North to see purely UV induced scorching so that's as far as it'll get. The one inside with poor airflow, near the glass.

- Acclimatisation in terms of UV makes little difference here, you can take a plant grown in Southern Italy outside where it's acclimatised to much higher levels of UV, and shove it in a non-ventilated, non-shaded greenhouse on a hot day and it'll still scorch.

I don't know the exact interaction between heat, and UV, and it's possible that higher temperatures leave plants more prone to UV damage, but you can't simply discount the impact of heat as irrelevant, as it's a fundamental factor in scorch here in the UK. On a 30c day in the UK, we have tarmac melt, and that melts at about 50c, so you can see that the surface temperature is substantially higher than the air temperature. In a greenhouse where air temperature can easily hit 40c - 50c (I've actually measured air temperature of 56.4c before in my old greenhouse), it's not unrealistic that our plants could be seeing surface temperature on their epidermis well in excess of 60c. It'd be rather naive to think that this could have no impact even though this could happen on a sunny March day, where amount of UV exposure is low.

So long story short, scorch in the UK correlates more closely with temperature, not UV exposure. For it to be all about the UV there needs to be some compelling explanation as to how higher temperatures but lower UV result in scorch whereas lower temperatures and higher UV do not.

FWIW I'm not even sure that all plants behave the same in this respect - tolerance to UV scorching seems to differ between plants. I find Orchids such as Vanda and Phaleonopsis for example are much more prone to UV scorching, whereas I do not believe I've ever had a cactus suffer purely from UV damage in the UK - certainly nothing I grow outside has ever scorched, and I grow plenty of cacti outside both year round, spring - autumn only, and some summer only. The only stuff that's ever scorched is that in the greenhouse during periods of excessive temperature, especially when I've failed to provide airflow with a fan.

[Thermoman]

...So long story short, scorch in the UK correlates more closely with temperature, not UV exposure. For it to be all about the UV there needs to be some compelling explanation as to how higher temperatures but lower UV result in scorch whereas lower temperatures and higher UV do not...

We may have our disagreements but I found this latest post of yours to be thoroughly convincing.

[spinesandrosettes]

With all due respect to your comments, to which I am not really very interested in arguing, I would like to respond, since you took some liberties interpreting what I said.

I'm not one who much likes my words twisted, or reinterpreted with incorrect alternate meaning. If I'm not mistaken, I believe I said quite early on: " I feel strongly that light is the primary and relevant factor".

Note that I did not say that light was the only factor, and all else is irrelevant, which has an entirely different meaning to what I said. I do tend to choose my words with some level of care, in efforts to avoid misunderstanding, but it would appear that even using care and being precise in one's language is no guarantee that people will necessarily read what was actually said. To summarize my comments such: "a bit silly to argue that it's the only factor in scorching in UK greenhouses and heat plays no effect" is quite inaccurate, especially considering I did in no way make such an argument.

What I know for fact is that the use of shade cloth is my most reliable method of eliminating sun scorch on my plants. This had been proven in all 3 of my indoor plant structures, and 2 of them use no fans whatsoever. We just today "enjoyed" another 100 degree cooker, and my newest greenhouse structure was over 110 inside, vents and door open. No fans or artificial air circulation. It's just covered with shade cloth, otherwise under the full force of the sun's rays, and zero plants got cooked, or scorched. Take that shade cloth off, and I'm looking at absolute disaster. That's a fact.

You said: "...glossed over the issue of surface temperature - whilst it may be 100f in the greenhouse, it will be hotter on the surface of the plants where the sun is directly hitting it." Bingo. The use of shade cloth has a direct impact on the direct sun hitting my plants. In fact, such an impact that none of my shade cloth protected plants have scorched, ever - air circulation or no air circulation. That's an absolute fact, and if it wasn't, I'd be doing something else, or additionally. I did not need to gloss over any issue, because the use of shade cloth prevents that issue from being an issue to gloss over.

Your proposed experiment: "On a hot day, in a greenhouse with no ventilation, I can take two clones of a plant, and put one inside near the glass, with no shading, and poor airflow, and one outside next to it, and I can guarantee you that the one inside will scorch, and the one outside will not." While I am certain that experiment is valid, and your proposed result accurate, I do not recall recommending "no shading". Let's modify the same experiment, this time incorporating the use of shade cloth, as I propone. Want to guess what happens, or better yet, what won't happen?

Bottom line, one's gotta do what they gotta do. Whatever works for you, is right for you. Shade cloth works for me, so that's right for me, and I don't have to bother fussing over fans and electricity.

I do not argue your success in doing what it is you do, and since it is successful, you should keep doing it.

My solution is simple, cheap, and effective. Works for me, which is why I will continue doing what I do.

[Phil_SK]

I don't think anybody's words are being twisted: one of you feels that light levels are the main driver of scorching and one feels that heat build up is the main driver. Shading affects both of these.

[IanW]

What I know for fact is that the use of shade cloth is my most reliable method of eliminating sun scorch on my plants. This had been proven in all 3 of my indoor plant structures, and 2 of them use no fans whatsoever. We just today "enjoyed" another 100 degree cooker, and my newest greenhouse structure was over 110 inside, vents and door open. No fans or artificial air circulation. It's just covered with shade cloth, otherwise under the full force of the sun's rays, and zero plants got cooked, or scorched. Take that shade cloth off, and I'm looking at absolute disaster. That's a fact.

Right, but you're assuming that the shading is working to prevent scorch largely because it's eliminating UV, but it's also reducing the surface temperature of your plants because the sun is not heating up their epidermis so much directly, because the sun's rays are instead mostly hitting your shading material and heating that up instead. Any subsequent heating up of the air temperature is a result of the law of thermodynamic entropy such that heat will move from areas of greater heat, to areas of lower heat.

This is why you don't really need airflow, because you're not allowing as much heat to reach the surface of your plants as you would if you had no shading. If you had no shading you could just as easily grow your plants without scorch if you can move away the heat from the surface of your plants efficiently. That's why I suggested the optimal solution for growing without shading as being a system where you're pulling in cooler outside air and blowing it through the greenhouse with a fan, and then extracting it out the other end with an extractor fan. If you can achieve this kind of airflow, thermodynamic entropy ensures that the cooler air coming in passes over the surface of your plants, heat is exchange from the hotter surface to the cooler air, and that now heated air is move on and out the other end of the greenhouse.

The problem we have is that we rarely achieve optimal airflow, people typically just shove a fan on and hope for the best. This probably works for a while, but if the air that's removing heat from the surface of the plants can't escape quickly enough, the overall air temperature will build up, and as rate of heat transfer is proportional to the difference in temperature, you will see a reduction in the ability of the circulated air to move heat away from the surface of the plant. That's why simply shoving a fan in wont always be sufficient to allow growth of plants without shade. Often it wont be an issue, because it will reach sunset before the efficiency of heat removal is reduced to levels where it can no longer remove heat from the surface of the plants quickly enough to prevent them heating to a level where they become damaged, but on a very hot day that might not be the case.

[Peter]

Mrs T and I are off to John Lewis today to buy voile curtains because the light through the patio doors (double glazed, e-glass) has bleached a costly rug and dried to the point of cracking, a nice leather sofa. We have been told that the voile will reduce the UV effect.

I shall continue on hot days, to put my very large air mover fan at the entrance to my 40ft greenhouse. The draught (gale force) this creates in the greenhouse as it drags air in from outside then chucks it warmed up, out of the door, disrupts Mrs T's expensive coiffure on the rare occasions that she enters my haven, however in the past Copiapoas still scorched, especially haseltonianas, until I left bubble insulation in situ all year. End of problem.