Bright sunlight = 100,000 lux
Completely overcast = ~20,000 lux

10 hours of bright light: 66 moles/day
10 hours of completely overcast: 13 moles/day

Moles needed to grow veggies: 18 per day

These are generalities. Some veggies can produce decently with 15 moles/day, plus a few cloudy days do not seem to affect their growth, as long as the temps are not high and the soil moist. The latter will lead to leggy plants.

Mike

In general the broadleaf vegetables are light saturated at lower light levels than grasses like corn. But even corn does pretty well with some cloudy days. Usually it is cooler on cloudy days and that compensates some for the lower light levels. On things like greens, cloudy weather has very little effect.

The Fruitnut

I think that, unless you live in an exceptionally cloudy place, like Forks, Washington (in the temperate rainforest of the PNW, receives somthing like 144 inches of rain a year and has like 300 plus rain days a year), the cloud factor is sort of "figured in" to the equation of light requirements.

It will vary from year to year. Overall, I think that temperature and rainfall, or lack thereof, are a bigger issue than light levels for plant growth. There is plenty of light in the middle latitudes of the nothern hemisphere in the summer for plants to perform as long as they are sited appropriately for their species, ie, don't plant the hostas in the full sun and the pumpkins in the deep shade.

On the flip side, I have noticed that I can grow lots of things here in partial shade that everyone says requires full sun. We have very long days and very rarely a cloud in the sky. I find that a lot of them actually do better with some shade.

Nicely put Mike.

I think, but am not certain, C3 plants do better in the shady situations than the C4 plants in general.

Ask and you shall recieve!

Great Answers - Thanks!

What are C3 and C4 plants?

mauirose: I think the link below gibes a good basic description of the answer to our question.

Michael

Here is a link that might be useful: C3/C4 description

backyardener,

A really good lighting area to study is Daily Light Interval. It centers around the quantity of lights plants receive. Sort of like a rain gauge measures the amount of rain. Whether it is a downpour (bright light) or a steady drizzle (overcast skies), an inch of rain is an inch of rain. A mole is a defined amount of accumulated light, specifically PAR light. If a plant needs 15 moles a day, it doesn't make a lot of difference whether they get it over a 12 hour or six hour period. (As an aside, I presume there is an analogy to rainfall, - after a certain point, too much light is not going to have any effect, sort of like water running off soil if it rains too hard.)

Greenhouse growers use the DLI as a basic guide. Unfortunately, every guide I have seen lists bunches of flowers and similar plants but clumps GH veggies into one category.

Mike

We moved to Wisconsin from Minnesota 3 years ago after gardening in Minnesota for 15 years. We were blessed with wonderful sandy loam soil and grew great crops of veggies every year. We used to live in a zone 7 and I must say our garden in Minnesota did just as well as in the warmer zone that had warmer less cloudy days.

Here is a link that might be useful: Vegetable Gardening

gardener56,

The question is a little silly I admit. It isn't like the veggies don't grow well. Even tomatos, peppers and amazingly I grew some watermelons. It peeked my curiosily since you always hear, "Pick a spot with full sun". With liberal cloudiness and the fact that my 1 acre property does not have one spot that does not get some shade during the day (unless my neighbors wake to sound of a chainsaw someday). Ha, ha...

Go about 498 miles west of Anchorage where the sky is overcast all day every day and one seldom sees cabbages over 30 pounds. Near Anchorage 60-90 pounds are seen most years (A record near 105 pounds I think.)

I assume most of us are not growing for competition but these numbers show the difference between sunshine and clouds at the same latitude, hence the same length of daylight.

Aztomn: Mike makes more very good points again. Speaking of greenhouses, location is everything partially based on light intensity. For example, carnations for cut flowers do extremely well in CO, a state with very intense winter light. If you went to an area like northern Ohio with far fewer days of intense sunlight, you'd find no carnation greenhouse ranges there due in large part to the light.

A good example of your original question is growing a seedling in low light vs full sun, depending on the plant and difference between the 2 lighting levels, you may find the low light plant getting taller and spindly than the high light one.

Different plant species are better suited to different lighting conditions, all respond according to what they are growing in. If you are interested in day length (actually night length) effects, look up the term photoperiodism.

Thanks everyone! I did learn quite a few things. Knowing these won't change my backyard, but increasing your knowledge always helps one to better understand what is happening and why.