Light vs Heat: when to shade greenhouse?

[habitat_gardener]

Originally Posted by Cole_Robbie

There's a few products out for greenhouses that are a "shade paint." They are a white paint that is temporary and easy to wash off.

I have a poly sheeting, aka "greenhouse plastic," over my greenhouse, because it is so much cheaper than the polycarb panels. I tried to make my own shade paint from diluted latex paint, but I couldn't wash it off the following year, even with a pressure washer.

So I use mud. It works just as well...until the rain washes it off. I only need the shade for a few weeks anyway, towards the end of the season. Plus, I get to have the fun of throwing buckets of mud. It feels like vandalism

That sounds like so much fun! I'd be concerned about scratching the plastic (from the sand particles), or having too much fun applying the shade coating and making holes in the plastic. Here, with no summer rain, the mud would stay on the plastic for 6 months. I wonder if that would make the plastic last longer, or degrade it faster.

[aruba1]

Ah, yeah, habitat, I can get lots of ventilation -too much right now, due to the giant rip down the top, but we tend to get more cloudy days than anything, so the greenhouse tends to get too warm very rarely. I still got lots more tomatoes, sugar snaps, shallots and cukes than we could eat, and we gave lots away, too. I think GH gardening works really well.

[Cole_Robbie]

Originally Posted by habitat_gardener

Here, with no summer rain, the mud would stay on the plastic for 6 months. I wonder if that would make the plastic last longer, or degrade it faster.

I actually got the idea from someone talking on a web forum about that being a practice out west. Since it is sunlight that degrades the plastic, I would think the mud would block the sun, and actually make it last longer...just a guess.

[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