Light vs Heat: when to shade greenhouse?

[sjamesNorway]

Even though I live in Norway, the greenhouse heats up considerably in full sun. I have 50% reflective shade cloth, but am unsure when to use it. At what temperature is shade more important than full light?

Over how long a period and at what temperature will sustained heat lead to blossom drop?

Nights are cool here, but I don't let the greenhouse temperature go below 62*F.

Steve

[sjamesNorway]

Since I haven't gotten a reply to this post, I did some more research. The best answer I found was at the British Royal Horticultural Society web site. I've tried to attach the URL, but keep getting "invalid file". If you copy and paste the wording below onto your search engine, you'll find the link.

Greenhouse: ventilation and shading/RHS Gardening

Steve

[sjamesNorway]

Steve

[KarenO]

Ventilation is more important than shade in keeping a greenhouse a reasonable temperature. Adding a fan and opening up the gh as much as possible will help more than shade cloth. To me temps over 30 c consistently will inhibit pollination. Nighttime temps over 20 also
KarenO

[bower]

Steve,

I did a google for "goldilocks temperature tomato" and found this - see post #7 by FusionPower which gives detailed information about tomatoes and temperature.
http://forums.seedsavers.org/forum/g...p-for-tomatoes
The temperature where blossoms begin to drop is 92F. In my greenhouse, I can only measure temperature in the shade, because direct sunshine makes it read in the hundreds. For me, temperatures above 80 F in the shade are correlated with the temperature in direct sun where blossoms drop.
One very important question is the type of glazing, and the height above the plants. As regards the height, more head space is better, and if ventilation is designed properly the air flow through the higher and hotter part should cool them.
As regards the glazing, plastic is better than glass. When my greenhouse was rebuilt, we picked up some second hand double glazed glass panels and used instead of plexi or plastic. I didn't know then what a difference it would make. The plastic normally used for hoop houses or high tunnels has a big advantage for plants, because it diffuses the light. Best quality greenhouse plastic is also uv resistant to extend its life cycle. I have read UN reports that looked at the effects of higher UV on crops and concluded that the effects are deleterious for fruit crops including tomato. This is consistent with the response of tomato plants in my greenhouse when the UV is high - they wilt in response to the stress regardless of water status, more especially if we've had a run of days without sunshine and then suddenly it's bright and hot with UV 7-8. If the weather is more consistently sunny, they adapt to it after a couple of days, but if they are already close to the glass it's too hot for the blossoms anyway, at least for most varieties, even at shade temperature readings in the 70's.
50% shade sounds like a lot to me, even for a sunny day under glass.
I tried using row cover for shade one year... it was difficult to put it up, but as soon as I did so, the weather changed, and we had the coldest and most unsunny July ever while my plants huddled in the dark.
What is your greenhouse like?

[sjamesNorway]

Hi Karen and Bower, and thanks for your interest. I have a 4 x 12 foot lean-to greenhouse mounted on the south-facing wall of a garage. There are two automatic roof vents, and I've opened up a triangular vent at the top of the side at the end opposite from the door. (I know I need to get a ventilating fan for this opening.) The glazing is 6 mm clear twinwall polycarbonate with UV filter. The shade fabric when needed is under the roof, but outside the 12' wall. I'm growing in containers: regular indeterminates along the high wall, and dwarfs along the short wall.

Steve

[bower]

Steve,

The twinwall polycarbonate is great stuff... I wouldn't use any shade cloth in that case. You can try a fan if it's overheating, or you may need a larger vent in the wall opposite the door if you find the airflow isn't enough to keep temperatures optimal, or even some vents at the top of the lower wall - trial and error I guess, for what turns out to suit the situation best.

[sjamesNorway]

Originally Posted by KarenO

Ventilation is more important than shade in keeping a greenhouse a reasonable temperature. Adding a fan and opening up the gh as much as possible will help more than shade cloth. To me temps over 30 c consistently will inhibit pollination. Nighttime temps over 20 also
KarenO

Karen, as a fellow northerner, I've been following your "True North Tomatoes" thread with keen interest. The varieties you're developing look wonderful!

[KarenO]

Originally Posted by sjamesNorway

Karen, as a fellow northerner, I've been following your "True North Tomatoes" thread with keen interest. The varieties you're developing look wonderful!

That is so cool that somebody in Norway is interested in my tomato crosses! Tomatoville makes it a small world. Thank you!
KarenO

[bower]

Steve, as Dewayne Mater pointed out, AKMark's thread is well worth reading. He has heated greenhouses and his results are amazing. Mark told me that he found 60 F to be the optimal night temperature for tomatoes. While I don't have control of it as he does, I keep temperature records and he is right, there is a marked difference in the fruit set and growth of the midseason varieties when nights settle around 60 instead of closer to 50F here. I don't often have the problem of night temperatures too high. But when the temperature is above 70F after dark I will sometimes leave the vents open overnight.

There's another document I found useful, by Merle Jensen, which explains how the balance between vegetative and reproductive growth can be tweaked to maximize production - attached pdf. Following this advice to the extent that I can, has made a big difference to my production. I keep the place ventilated as much as possible, even in early spring, aiming for an optimal balance between temperature and relative humidity.

Let me know if you try using the shade cloth and how that works out for you. I'm always interested in greenhouse design issues, and ways to make smaller structures work for tomatoes. You should also check out Tatiana's low tunnel system. There's a thread in this subforum I think... pretty awesome!

[sjamesNorway]

Hi again Bower. I have checked out AKMark's thread, but hadn't picked up on the 60F optimal night temperature. Our nighttime temperatures are generally cool, too. I've read the Jensen paper,and it makes sense. It will be challenging to try to follow his principles. Thanks for your advice. As KarenO says in the above post, it's so cool that there are people who live in other parts of the world who take an interest in each others problems and successes.

I'll let you know about the shade cloth, and I'll be checking out Tatiana's system.

Steve

[Dewayne mater]

Have you taken a look at the Alaska tomato threads? I think you guys are in a similar latitude. I don't think he uses any shade growing in greenhouses.

Practically speaking, I think it is fine to be quite warm in the day, up to 90-92 degrees, so long as the nights fall back into the 60s. What shuts down our summer production is Texas isn't the heat of day, but the fact that the nights cool only to 78-80 degrees, and then only for a very short time, and hour or two at most, before heating right back up.

Good luck!

Dewayne Mater

[sjamesNorway]

My conclusion is that I need to open up the green house more, get some good ventilation going at the peak of the roof, and then only use shade in case of heat buildup over 92*F.
(The fusion_power post at SSE about temperatures was very informative.)

[sjamesNorway]

I thought I'd revisit this thread, because I've gained more experience, and I thought the following table would be of interest.

Illuminance Example

120,000 lux Brightest sunlight
111,000 lux Bright sunlight
20,000 lux Shade illuminated by entire clear blue sky, midday
1,000 - 2,000 lux Typical overcast day, midday
400 lux Sunrise or sunset on a clear day.
<200 lux Extreme of darkest storm clouds, midday
40 lux Fully overcast, sunset/sunrise
<1 lux Extreme of darkest storm clouds, sunset/rise

I've provided more ventilation by converting fixed windows to hinged in the adjoining garage wall (back wall in the greenhouse). I use a fan, and 50% shade cloth. We've had a recent heat wave (for Norway), with temperatures up to 29*C (84*F). So far I've managed to keep the greenhouse temperature under 33*C (92*F).

It's interesting to note that 50% of bright sunlight is still ca 30 times more than the lux value of an overcast day. Taking into account that the days here are now 18 hours long, and that the greenhouse gets ca 10 hours of direct sunlight, I'm sure my plants are getting plenty of light even though I use the shade cloth some of the time.

Steve