I think it has to be considered in conjunction with the temperature. For example, in 60F air temperature even 12 hours of sunlight should not do any harm to vegetables, if they are adequately watered.

So IMO we need to bring the temperatures into the equation. Even In Texas, Florida, Arizona, when the temperatures hover around 100F, 6 -8 hours direct sun can be too much and they provide some shading.

It got a bit confusing for me. 60F in a desert climate? Did you mean 60C???

Yes in Texas, Florida and Arizona, people do provide the shading, but then does this not affect the production because of the reduced number of hours direct sunlight, as a result of shading? Does this mean that higher temperatures reduce the quantity of sunlight required as more photosynthesis can be done?

PS: For clarification's sake, I did bring in the point of temperature in my original post when I said intensity of the sun. Generally temperatures and sun intensity are directly proportional.

To me sun intensity is directly proportional to latitude (or season, if comparing within one latitude outside the tropics). Separate from that is clarity of atmosphere; I would suppose in deserts the air is always or nearly always very clear.

Anyway, I think the 6-hour rule of thumb is for middle-latitude growing seasons. In deserts at lower latitude it would seem likely that plants could thrive on less time with direct sun, or perhaps even no direct sun at all, merely scattered light.

It got a bit confusing for me. 60F in a desert climate? Did you mean 60C???
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Wonder why not too many things will grow in the deserts of Sahara and Arabia ..hehe

I mentioned 60F to demonstrate my point that it is not just sunshine intensity.

In places that they shade the plants(Texas ..), the plants shut down production because of heat anyway. So shading protects them a bit more. They also do STAGGERED PLANTING. one late winter/real early spring and the other for late summer/fall. That is what you are doing, fall/winter planting. This way you don't have to shade the plants. Under 90F should not be a problem with full sun.

Saood, I suggest that you also check with the following forums:

==== Southern Gardening
==== Texas Gardening,
=== Southwest Gardening

They can better understand your conditions that most Northerners.
JUST A SUGGESTION>

When people talk shade for areas of heat and intense sun they aren't talking fully opaque shade neccessarily. Here we use shade clothes of varying tighness of weaves which provide varying degrees or percentages of shade. For example a 30% shade cloth has a relatively loose weave and blocks 30% of the sun. This is good in that it cuts a lot of the heat from direct sunlight while maintaining enough light for fruit and flower production in many vegetables. A 90% shade cloth has a very tight weave and allows almost no light through. We use one of those over the clear roof panels on our chicken coop in the summer to avoid prematurely cooked chickens and eggs. :)

Most vegetables are light saturated at about 30% of full midday sunlight. So in your desert climate, like mine only not as hot, they will do better with as many hours as possible but at reduced intensity. A shade cloth of about 50% would work well. But ideal would be the floating row cover material that transmits about 40-50% of total PAR, photosynthetically active radiation. The floating row cover also diffuses the light and reduces water use which is a huge plus for vegetables.

So ideal is as many hours as possible at 30-50% intensity in a diffused manner.

for me, my gardens get full sun on a northerly aspect, but they only need that from sunrise until about 10am then after 4pm, so some shading would be good, don't have any hardly, so in time will build a poly hoop type cover over the gardens and presently plan on using 50% white shade clothe. in time we will have trees to help.

in full sun with no shade they need more water and heaps of mulch. afternoon sun can be the worst.

len

Here is a link that might be useful: lens garden page

obviously, mulch mulch mulch. I know it will be difficult in Arabia, there is no organic matter to be had anywhere, but where are you exactly?

Fruitnut,

"Most vegetables are light saturated at about 30% of full midday sunlight. "

You mean, if midday sun is approx. 12000 foot candles, plant will only be using 30% i.e. 3600 foot candles? Did I get that right? Does that mean that even at 5000 foot candles (for atleast 6 hours i.e. 30000 foot candle hours) which is generally prescribed for vegetable growing, only 3,600 is actually being used? So by shading by lets say 50% shade, I am reducing the intensity of the sunlight to 6000 foot candles and of which 3600 will be used for photosynthesis and the remaining unused (or worse the plant will actually be using up energy to not use it)

Did I get that right?

And what are those floating row covers? plastic type material which allows only light at reduced intensity but no air? More like that plastic covering that we have on our windows to filter the sunlight?

saood:

What I mean is that a fraction of full sunlight is enough to maximize the rate of photosynthesis for many broad leaf plants. Any more isn't bad except that it is usually accompanied by excessive heat and water use.

We use floating row covers here year around partly to save water but also because it provides diffused light which penetrates the canopy better than direct sunlight and therefore can increase photosynthesis. See the pictures linked below. The material is a spun bonded fabric that allows passage of air, water, and light while improving light quality.

Here is a link that might be useful: floating row cover

When you read a gardening reference about how much light vegetables need, you have to take into account that whoever wrote that is basing it upon conditions available where they garden. So you have likely been reading about gardening done someplace with much less intense sun. I live only halfway down the coast of the US from my parents, but the difference in sun intensity is enough that my tomato plants fruit a bit better with shade in the afternoonsthan they do in full sun, whereas at my parents' house the tomato plants in the same type of shade would just languish.

I think fruitnut explained it better than I did, which makes sense since he is likely the one I first learned about shade cloth from over on the fruit and orchard forum. :) Plus his sun makes mine look like it isn't even trying. :)

Fruitnut mentioned light saturation which I'd like to expand upon. At a certain intensity of light, which is different than quality or quantity of light, light availability ceases to be the limiting factor for photosynthesis. Instead the limiting reactant generally shifts to CO2 or water. This is why greenhouse growers can introduce CO2 to the houses and see boosted growth. Quality of light refers to the wavelengths of the spectrum which reach the plant and is largely irrelevant to gardening using the sun as a light source. Quantity of light is a function of both intensity and duration. To answer your question in the original post, intensity of light will reduce the duration required up to the point of light saturation.

In other words for your scenario the intensity of light is too high and and the energy not utilized for photosynthesis is essentially cooking the plants. Assuming your plants have all day to receive light (no big trees or structures to cast shade), the duration is a fixed number and intensity is the factor that needs to be adjusted to bring total light quantity in line with the plants ability to use this energy in photosynthesis.

To make a long story short Fruitnut is right. What you want is all day less intense sun, which is best accomplished by a suitable shade cloth. The amount of light transmission required will depend not only on where you are growing but what you are growing as well.

If your soil is sandy/salty that will compound the moisture issues. High salinity makes it more difficult for roots to absorb water and sandy soil won't hold water. Both of these can be mitigated with liberal additions of organic matter as Len and Glib mentioned.

Thanks Fruitnut and sunnibel.
and thank you Gurnoegardens for the elaboration. It certainly made things more easier for me to understand. I am the type of guy who wants to make things right in the first place more than later after experimenting :)

I am actually a container vegetable gardener growing on roof top of my apartments and using potting soil.

Can I therefore conclude that because of light saturation the number of hours of higher intensity sunlight will not play any role to reduce the number of hours required for vegetable plants? The 6 hours of sunlight (full sun) is like at optimum light intensity. Would that be correct conclusion?

As for the shade cloth, I have it placed over my containers. But I was just thinking that the 50% shade cloth is a woven one and therefore there are 50% empty spaces and 50% fabric. So theoretically there is 50% complete shade and 50% full sunlight - so 50% of the plant is actually exposed to 100% intensity and 50% with no direct light? How does then shade cloth work?

saood:

Did you even look at the row cover fabric? That reduces light and heat without casting any areas of full shade. In fact for the lower leaves it increases light intensity by diffusing the light. Diffuse light enters the leaf canopy from all directions. Thus the lower leaves are less shaded by the upper leaves.

The shade fabric doesn't cast 50% full shade. The fabric is small diameter and the sun isn't a point source of light. So if you hang the shade cloth a couple ft above the plants, there is little or no full shade.

"Can I therefore conclude that because of light saturation the number of hours of higher intensity sunlight will not play any role to reduce the number of hours required for vegetable plants? The 6 hours of sunlight (full sun) is like at optimum light intensity. Would that be correct conclusion? "

Yes, it sounds like you got the gist of it. 6 hours of direct light is a very general guideline for what is "full sun" in a moderate climate. According to these guidelines 4 hours of direct sun exposure is not "full sun" although we know that in your case it may be enough to harm the plants. Discussing this in terms of light quantity is more accurate than saying full vs. part sun. Pnbrown referred to this phenomena when he described plants thriving in "perhaps even no direct sun at all, merely scattered light." which is similar to what you've set up with your shade cloth. Plants under a shade cloth don't ever technically see 6 hours of direct sun which is not to say they will not thrive. Perhaps it'd be best if we just dropped this generalization of "at least 6 hours=full sun."

Tomato or squash plants for instance will do best with as much sun as they can get as long as the other conditions are not prohibitive. If you could cool the area and pump in CO2 then the amount of sun you have would be a boon to the plants. Given that you can't and that heat and dryness become limiting factors in your real full sun, "full sun" plants are actually likely to do best with long duration mitigated light (a.k.a. dappled sun). I can't be sure as to how much mitigation is necessary and how much is too much. To determine that you may have to talk with other growers locally or do some of your own trial and error. The right % of shading(to clearly differentiate from time in shade) for lettuces might be too much for zucchini and vice versa. Another consideration is that sun intensity is not consistent throughout the day. There may be plants that like to be out in your full morning sun but can't handle the blaze of the afternoon sun. Clear as mud?

Yes I saw the row cover but since I have not seen it selling over here, I therefore did not ask about it and only referred to the 50% shade cloth.

The reason why I asked the question as above was based on the fact that I got one light meter today and I was using it. Under full sun at around 1pm it gave me a reading of around 124,000 lux. However, when I put it under that shade cloth at some points it was giving me the same reading and some other points a significantly reduced reading and the only explanation that I could make out was as given in my question.

Btw, I found some row cover options at amazon.com. But don't know which ones to buy. As I see the description, they speak a lot about frost protection and that is not a concern over here. Can you recommend somethings from there?

Also since we were talking about PAR, I read this in one book on vegetable growing by the name of 'vegetable growing handbook'. From that I understood that upto 1200 foot candles, plants absorb 100% sunlight for photosynthesis and from then onward it is either not required or the usage reduces very sharply. The link is provided below

Here is a link that might be useful: Vegetable Gardening Handbook by W.E. Splittstoesser

Yes, it is very much clear as mud :) I can tell you that what I learnt from this thread will go a long way in making me a better gardener.

Thanks again G/G. While everyone contributed to my learning, you and fruitnut put all that puzzling material together as one understandable whole.

Still missed some of the most important information needed.

Most C3 type photosynthesis plants saturate at about 1/4 full sunlight. C4 type plants saturate a bit higher at about 1/3 full sunlight. Saturation means that any more light than this and the plant not only can't use it, it actually consumes energy because the plant has to translocate water to evaporate away excess heat from the leaves. C4 plants are almost all grasses such as maize. Virtually all other garden plants are C3 types. Plants use about 1% of the sunlight that falls on the leaves.

The most significant missing info above is about ultraviolet light. UV damages and kills plant cells if they get too much. At higher altitudes, more UV penetrates because the atmosphere is thinner. Less moisture also means more UV gets through. This means some high desert locations are especially vulnerable because the air is very thin and dry. There is also a latitude dependency. The higher the latitude above 23.3 degrees North or South of the equator, the less UV gets through. Just think of the angle sunlight is striking the earth and it will make sense. The more direct overhead the sun is, the less atmosphere there is to absorb UV. This effect causes more UV to get through closer to the equator and significantly reduces UV above 40 degrees latitude with a special warning that near the poles, the ozone layer may be too thin which dramatically increases UV exposure in those regions. If you happen to be in a high desert climate near the equator, you have the absolute worst situtaion for UV damage. Most palm tree species have a thick waxy coating on the leaves to protect from excess UV.

So to add to your information, appropriate use of shading should take into account the type plant being grown, the amount of UV, and the climate.

Tomatoes use C3 photosynthesis so need at least 1/4 full sunlight. If grown in high desert conditions near the equator, they would benefit from at least 50% shading to reduce both UV levels and total sunlight levels. Here where I live at 36 degrees North Latitude, they benefit from 30% shading in mid-summer.

I learned from it as well. If I ever move to a desert I'll have some idea what to do..

in arabis, do they grow grain crops?

harvesting grain creates a bi-product straw. use news paper or cardboard with a light cover of soil/sand.

len

To take this one step further you need to compare the light saturation of a single leaf to that of the whole canopy of leaves which can commonly have a leaf area index of 4 or more. An LAI = 4 means that the surface area of leaves in the canopy is 4 times the area of soil that it covers. So while a single leaf many be light saturated at 25% of full sunlight, the canopy of leaves responds with higher photosynthesis at higher light levels.

This is where the diffusing properties of thin clouds or floating row cover can be beneficial. Diffused light, since it comes from all directions, penetrates deeper into the plant canopy. More light on normally shaded lower leaves means higher canopy photosynthesis. That can translate into more yield.

Interesting. I am looking at a retirement place in Zone 9b (retirement is 14 yrs away but trees need to be planted, house remodeled, soil nourished, etc.), and probably it would be worthy to make permanent installations to stretch row covers April through September. 3 crops a year? I am thinking posts at the beds corners, with supporting frames on top. Opinions?

I was trying my light meter with several type of plastics bags and light colored bed sheets that I had.

50% filter was provided by a white translucent plastic bag. Bed sheet reduced almost 70-80% light intensity whereas clear plastic had almost no impact.

Untill I get my hands on the row cover, is it advisable to use this white plastic instead of the 50% shade cloth. Unlike the 50% shade, it wont allow air to pass through.

glib:

What you are talking sounds a lot like a high tunnel. The covering could change depending on the season and climate.

In winter there are poly materials that diffuse the sunlight to varying degrees. The IR greenhouse poly gives a fair degree of diffusion. I have a double layer inflated on mine and that gives nearly 100% diffusion. Total PAR may only be 40-50% of outdoors but the crop yields are wonderful and water use 1/2 of outdoors.

In summer covering could be changed to 40-50% shade cloth or possibly row cover in low wind areas. That would allow air circulation. Any shade cloth will diffuse the light some. You might find shade cloth, maybe white or translucent or aluminet that diffuses light to some degree.

In an open sunny area diffusion isn't really necessary. But some reduction in light and summer heat can still be beneficial in a 9b climate.

Thanks. For 9b, I thought, no tunnel (in Michigan I have winter hoop houses) and I will just grow winter crops (peas, carrots, radicchio, chard, etc.). But in summer, indeed, a little shade cloth would be of help.

Florida, Glib?

Southern Italy, close to Ancel Keys retirement place. I am looking only for the time being.

Wow, that would a significant change, in more ways than climate.

They are very friendly, and I speak the language (I am also fluent in French). But the really welcoming part is that EVERYONE in the village has a large garden. That is what made me think about it. Easy to make friends this way, and they are so passionate about it, and it is so woven in the social fabric (which is strong). My connection is that one of my sisters married a guy whose father is from there.

I know you are a vegetarian, but there are enough people around that will get you homemade goat cheese (from their goats), or raise you a pig (I would only raise rabbits and chickens, large animals are too much trouble). If you help them with the olive harvest you will get your oil, as I did 2 years ago. Bartering is common. Beautiful mountainous land, too.

I am functional in Brazilian Port, which as I understand it is close enough for basic communication with most Italian-speakers.

Sounds great, especially the olive oil part. I think a lacto-vegetarian could fit into the food way pretty well.

Thanks Fusion Power.

Over here whenever I see the weather website, it mentions the UV index as the highest i.e. 10 :-(

How do I take care of it when selecting a shade cloth? I dont know if there are many option available locally? Is there any substitute material for UV filter if I cannot find any purpose made UV filtering shade over here?

I'm kind of late to this forum, so pardon me if this has been pointed out, but the sun is NOT any brighter in the desert. As noted, the intensity of the Sun is the angle with which it is shining on a surface, to a small degree the height above the horizon, and well, whether it's a cloudy day. That's just physics. No, you don't get sunburned faster in the desert than you do anywhere else, though sunburning may also depend on temperature, which is a different matter.

My plants in HZ10 seem to like less sunlight than they would further up north, but that's just a matter of heat. When it's 100F outside, full sun isn't fun to be in, for me or a plant. Because whatever that sun is shining on is hotter than that.

So the bottom line is that for sun loving veggies, more sun is ALWAYS better, unless it overheats them and kills them. It's really just that simple.

I will add, partly from reviewing some good earlier comments, that altitude (above sea level) is also an important consideration in solar irradiation, but mainly for UV. UV burns leaves and human tissues, but I think isn't otherwise relevant to the well-being of plants.

Of course, the reflectivity of the ground makes a big difference. That (and altitude) is why you can get horrendously sunburned while freezing in the bright white snow on a mountain. I suppose in the desert, sandy ground is a lot more reflective than, say, grass or asphalt would be.

But the desert gets as much sunlight as anywhere else at the same latitude and elevation. In the sea-level parts of the Sahara, out on the sand, you get about as much sunlight hitting you as you do on the beach in Florida.

I agree with DAN. But there are few factors beside latitude (incident angle) that can make a diference and Those are, I think, humidity/haze and air pollution. Due to those factors the air can reduce the intensity of sunshine on the ground.

Another factor in the desert area is reflection of sun's ray AND heat wave coming from the heated ground and objects like walls, sand. ..So then you have two heat sources, One is the sun an the other is the surrounding.

deleted. double post

This post was edited by seysonn on Sun, Oct 27, 13 at 4:16

Thanks Seysonn and Dan.

My light meter shows that the the intensity of the sunlight is 12000 foot candles at around noon, but the temps at that time are around 90F. At the moment I have armenian cucumber, tomatoes and pepper growing in containers. Are you both telling me that I don't need to shade it? Shading reduces the reading to 5000 fc. But then whether I shade it or not - as fruitnut said earlier - that the plant has already reached light saturation and practically it does not matter much whether it is shaded or not. The one plus of shading is however, the UV rays may get filtered out and better for the leaves.

Do you agree to the above?

As I understand it , plant can benefit from sunlight for any length of time but only can utilize 30% of it at any given time. But excess of that is not vital to the process of photo synthesis. And at high temperatures excess intensity can have negative effects.
isn't that why with artificial light they provide up to 16 hours to growing seedlings ?

But on the other hand, at moderate temperatures 12 hours of full sun should be fine. So It is the TOTAL AMOUNT OF ENERGY PER UNIT OF TIME that is critical. That is why shading is done in conjunction with high temperature. For that reason, at temperatures under 90F shading is not necessary.

JUST MY UNDERSTANDING>

Under 90F shading is not necessary, but even if it is done, it wont be any disadvantage. It may help out with filtering harmful UV light? Right?

First of all, unless you're at really high altitude (I mean like, 10,000 feet or above), forget about UV. It's irrelevant.

I don't think that too much light is an issue either. As I said, it's mostly about temperature. If it's hot, and the plant is in full sunlight, the plant will feel hotter! Again, remember, the AIR temperature in full sun is lower than the SURFACE temperature for anything dark, like a leaf, or asphalt.

My plants get 100+F air temperature in full sun for four weeks here in Central Texas. How do I know when to shade them? Well, if they droop even when the soil is moist, they're too hot. They're not pumping water fast enough to keep up with the evaporation by the heat. I can't do much about the air temperature, but I can keep the sun off of them if I need to. I'd like them to have as much sunlight as possible (as Seysonn points out), but not if it means they're going to get overheated.

I guess that means that the way the plants look is more important than what your light meter says.

Actually, I should say that the only time I've ever shaded any plants was when I was planting a fall crop of beans in August. Those seedlings were delicate enough, with shallow enough roots, that they needed some protection. I think I also once put shade over some squash, but they were on their last legs anyway.

Cukes might get stressed by the heat. Tomatoes, probably not, unless they are leggy, with very long stems. Peppers LOVE heat and sun.

Now, I don't live in a dry desert climate, but I can presume that evaporation rates will be higher there, meaning that plants could get stressed with somewhat lower air temperature. But again, just look at the plants. They'll tell you if they're unhappy.

Hope that helps.

Thanks DAN. It certainly helped me understand.

While we discuss this thing, I had shared a link to a paragraph in book on vegetable growing and looking for comments. No comments on that yet in this thread. Perhaps you can have a look at that part about light and give your expert views.

Basically it says 1200 foot candles is the max the green plants utilize. Fruitnut and seysonn and a few others seem to agree on a light saturation point but at a higher %age then just 1200 fc over 10000 fc at midday full sun (i.e. 12%).

Here is a link that might be useful: Vegetable Growing Handbook.

"Basically it says 1200 foot candles is the max the green plants utilize."

I'm not familiar with that number, but I'm not sure what it has to do with shade cloth. Full unobstructed sunlight, from straight overhead, is about 10,000 fc. If plants don't mind being in brighter light, why bother making it dimmer? Now, 1200 fc is about what you get on an overcast day, and making every day an overcast day for your plants doesn't seem that smart. Of course, it's only the leaves that are fully exposed to the sun that are getting that full intensity. Most of the leaves are not fully exposed. So those leaves could well be getting their 1200 fc only on a bright sunny day.

Now as I see in this thread, there are two opposing views - well not so very opposing - but still some difference of opinion:

1/ You opinion is that the intensity does not matter as long as the temps are under control. Higher intensity may actually be a good thing. UV is not a problem in low areas

2/ The other opinion is that higher intensity sunlight is actually not of any major use because of the concept of light saturation. Generally (and exceptions may exist) higher light intensity brings with it higher temperatures and therefore shading to reduce light intensity (and therefore temperatures) is not detrimental to the plant growth as the shade cover is only reducing the light intensity which is above saturation point (and therefore of not much use anyways).

Do you think I have summarized the two views properly?

I think these two views are pretty consistent.

Though as I said, it's simplistic to use a light meter to determine whether the lighting is above or below the saturation point. In all of my plants, there are few leaves that are both face-on to the sun and unshadowed by other leaves. In full sunlight, I doubt if the rest of the leaves are saturated with sunlight. So if you're going to shadow the plant to keep those few well-exposed leaves at the sunlight saturation level, all the other leaves are going to be WAY below the saturation level.

That being the case, just put the light meter away, and look at how stressed the plants are. If you think they might be stressed because of overheating, try some shading. But if you're shading when they aren't heat stressed, you're probably losing out.

This isn't about conserving sunlight. If a plant gets more than it needs, as long as it isn't being stressed, so what?