Can a tomato plant get too much light?

growneatJuly 14, 2014

I have read where in most locations during the time of the year when the sun is nearly overhead that tomato plants get far more sunlight than they need or actually use, in some places using only around 50% of the light available. Does excess light do any damage to the plants or tomatoes other than the effect of the light on temperature, ie heating the tomato and plant up such as sunburn?

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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Damage from light? No. From heat? Yes.


    Bookmark   July 14, 2014 at 6:42PM
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seysonn(8a WA/HZ 1)

Like any plant, tomato can benefit from sun (direct exposure) to some limit. In the midst of summer with 15 hours of daylight, tomatoes do not need 15 hours of sun. Logically there must be an optimum amount that it needs. When they say "full sun" that does not imply 12 to 15 hours of direct light. Six hours is often considered full sun (Lower limit) Perhaps 8 hours should be enough.

One thing to remember is that tomato can get quite a lot of light indirectly for its photosynthesis need. It has bee said that light intensity in a day with overcast skies is almost equal to what plants get under fluorescent light. Or very close to it.

With direct sun also comes heat energy. Again, plants need certain amount of heat. That can be found in indirect light exposure too. In general most plants would rather have temperatures in 70 to 86F.

I am growing tomatoes with 4.5 to 5.5 hours of direct sun. I am doing ok but I think 7-8 hours would have been ideal even in our cool PNW.

That is my take according to my own experience. I have also gardened in GA with about 6-7 hours direct sun. In the dog days of July and August that seemed to be more than necessary.

    Bookmark   July 14, 2014 at 8:18PM
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seysonn(8a WA/HZ 1)

Solar altitude (angle) is a measure in sun rays intensity.

When the sun rises, the solar altitude start from "0" angle and rapidly rises and in mid day reaches its maximum. In any given latitude in the Northern hemisphere that altitude angle is the highest on around June 22nd. Down south in TX it can be like 80 degrees (more or less). Here in Northwest WA it gets as high as ~ 65 deg. Not as high as south Texas.

So, in south Texas there is more power to sun than in North Washington state. That is what is also responsible for hot temperatures . So when you combine high temperatures and high solar intensity, it is not something desirable for tomato and many other plants. In other words a 12 hr direct sun might be fine in NW WA but 8 hrs can be too much in south Texas.
Then in cooler climates direct sun is desirable, not for photosynthesis alone but also as a heat source for growth and fruits ripening.

    Bookmark   November 3, 2014 at 1:13AM
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PupillaCharites(FL 9a)

Too much light can damage leaves. For the same reason, we "harden off" transplants/seedlings gradually.

The plant can handle any higher light intensity as long as changes are gradual and the plant has the chance to adapt ... though it can't adapt to too-high temperatures as already mentioned. Same for low light flux ... needs a chance to adjust its efficiency.


    Bookmark   November 3, 2014 at 6:39AM
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Tomato fruits can suffer the effects of sunscald or overheating but I doubt you are referring to either of those issues. I experienced the overheating firsthand this summer when I forgot to open a high tunnel until midmorning and I actully had "Cooked" green tomatoes on some plants and many split ripened tomatoes. It was especially frustrating since I was harvesting produce to submit for our county fair that day and the red tomatoes that I was intending to enter were damaged severely, the cost of the 15 blue ribbons I received in other produce categories.

But otherwise you want all of the sun you can get IMO. I'm convinced that the fruit that ripens fastest on any given plant will have the best flavor. People say that heat is all that is necessary to ripen tomatoes but a fruit that is buried under dense leaf canopy will remain pink far longer and will lack flavor of exposed fruits.

    Bookmark   November 3, 2014 at 9:12AM
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Tomatoes are C3 photosynthesizers that saturate at about 500 mols. Full direct sun gives about 2000 mols of light at sea level on a normal day during the 10:00 a.m to 3:00 pm time frame. The atmosphere filters out roughly 70% to 90% of the harmful UV under these conditions. Go up to Denver CO and the increase in altitude means less atmosphere to filter out the UV. Denver can get up to 3 times more harmful UV than a sea level location in Texas from a combination of higher altitude and less moisture in the air. The more air sunlight passes through, the less intense. Any time the sun is within 15 degrees of the horizon, only about 25% of the sun's rays hit the earth, the rest is absorbed in the atmosphere. This is why you can look at the sun when it is near the horizon but not when it is high in the sky.

Tomatoes originated in a tropical highland area of the Andes mountains within 10 degrees of the equator. They developed a waxy coating on the leaves plus specialized cells on leaf surfaces to protect from the intense sunlight in their native environment. The plants can still be stressed from a combination of high temperatures with high light intensity. If insufficient water is available, the plants will suffer though the effect is usually to make the leaves more susceptible to fungal diseases. So by a combination of high solar intensity, high temperatures, low water availability, and attack by diseases, tomato plants can have their lives shortened and production reduced.

Some tomato growers use shade cloth to reduce plant stress. Shade cloth can both extend productive life and increase total production if correctly used. 40% shade cloth is available from greenhouse suppliers and is appropriate for this use.

    Bookmark   November 4, 2014 at 2:10AM
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PupillaCharites(FL 9a)


The question of light intensity alone is not harmful as long as the plant has time to adapt, unless you have some info showing it is which would be very interesting. (In reasonable environments ... not the Gobi Desert). At low light levels, leaves develop with a different composition. Adapting means giving it time to change its structure to accomodate a high flux.

Up to 70% shade cloth can be necessary in Florida to cut the temperatures (and even then not work). I did fine with standard 30% material and the work is in keeping the plant watered. It is a lot of work, too. A big plant can easily transpire a gallon a day here and if its watering needs are met it appears to produce very well with no signs of damage. I have to say appears because my night temperatures are too high and cause problems with fruit set, so it is more of an assertion than a statement.

Photosynthesis is a photochemical process. When the flux is above the capacity of the leaf to absorb, light is simply not utilized and not damaged, which I think the OP is asking.

Whether UV can cut life expectancy or invite disease is another question. Whether the OP in the mid-Atlantic is wondering about supplementary light which does not have high UV content is another question. You really need to look at total yield IMO, at least that is what I am interested in. For example, if I got 25 pounds out of a plant in 3-4 months, I would consider that plant more successful than one that lived to a ripe old transplant age of 6 months and gave only 15 pounds.

Temperature is important, but doesn't sound like the OP was asking about it either, like many things that can contribute to an unhealthy environment if the plant doesn't get at least 6-8 cool hours to do processes it does best in cool temperatures, and probably in darkness.


    Bookmark   November 4, 2014 at 8:59AM
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