How much does good ventilation make up for less scorching?

[newstart]

Since removing panes in my new greenhouse it occurred to me that the plants may naturally scorch less if held in 9cm pots up to 4 months or more. Does the extra venilation naturally cause less scorch but shading still become essential. I guess I need not ask this but I find these things very interesting. Of course by trial error I'd find this out any way eventually but thought i'd ask.

Thanks David.

 

[Maggi Young]

Good ventilation, provided by removing panes of glass will help a little against scorh, and forcedventilation by means of a fan will be even better... but if the weather is very sunny and hot and the glasshouse is greatly exposed to the sun then scorch may  still prove a problem .
I cannot see what impact the size of the pots the plants are grown in would have on scorch? I can see that a small plant suffering scorch may be more likely to be completely destroyed than a biger plant , which might be able to recover from a  small degree of scorch/sun damage that would wipe out a little plant....

[Ezeiza]

Hi:

    To obtain a substantial effect panes to be removed must be those at the top of the greenhouse as this is the part of it where hot and overheated air will collect. Removing panels from the sides have no effect on the grenhouse temperature. Escaping hot top air will produce an ascending flow that will ventilate and move big volumes of air.

    Of course black plastic pots will overheat more easily than terracotta and while colors.

[Maggi Young]

Alberto is correct, of course.... if the main effect in removing glass is to reduce heat but in the UK where the object of removing the side panes is more to create a through air flow while preserving overhead cover from rain, the removal of upper panes is not really feasible. Louvred roof vents are the answer for that .

[David Nicholson]

Scorch hasn't been a problem this summer in this part of England

[Ezeiza]

Of course that is right, Maggi, I was thinking of a Mediterranean summer!. I was wondering if there is in the UK a device one can install that lets you open one or several panes without actually removing them. Something that could let hot air escape but at the same time giving overhead protection, like a cloche. Perhaps it already comes with the greenhouse when one buys it but I have not seen one so far. Summers in the future will be hotter it seems and the scorching temperatures are here to stay. In this Hemisphere we are awaiting a spring and summer with unexpectedly hot weather as it happened in yours. It has killed lots of plants last summer. At the same time the winter has been really cold, therefore the obvious option of trying more tropical plants is not feasible.

[iann]

A device that lets hot air escape through the roof without removing a pane?  You mean like a roof vent?  Got them

Scorch is a term used loosely for several different ailments that tend to occur in strong sunlight.  One is UV damage caused directly by sunlight.  This tends to occur in the open and shouldn't be all that common for alpines that are generally adapted to high UV levels.  Symptoms can be reddening or sometimes a pale yellow and even white colour from chlorophyll destruction, localised on exposed parts of the plant.  The only solution is shading, either until the plant adapts to the light levels, or permanently.  Although greenhouse glass stops a high proportion of UV, plants can still be damaged by the UV levels, perhaps also simply by visible light levels.

There is also severe dehydration caused when transpiration far exceeds the ability of the roots to supply more water.  It can also be caused by salts in the soil or sprays on the leaves, both of which increase the dehydration by osmosis.  Symptoms are brown crispy edges to leaves.  The solutions are obvious.

Then there is simple overheating.  Since most plant tissues are not killed by brief high temperatures below about 50C, or 60C for some plants, heat scorch is usually caused by strong sun heating the plants without air movement to cool them.  Symptoms are local bleached areas, or sometimes death of the whole plant.  It is a big problem behind unshaded glass because the heating effect of the sun is increased.  The air temperature almost becomes irrelevant, although cool air is obviously better for cooling the plants if you can get it moving.  You can measure the temperature of the plant surface directly with an infra-red thermometer.  This problem is very common with stem succulents which cannot cool themselves effectively by transpiration or ventilation.  Long term problems can be caused by quite moderate temperatures that exceed the range where the plant can metabolise effectively, but these aren't usually described as scorching.

A few roof vents will never provide sufficient ventilation.  Hot air rises and all that but it just isn't enough.  Plants behind clear glass heat up so fast that they will still scorch.  I've seriously scorched plants in cold frames completely open to the sky, and even plants in the open air can be scorched.  You can provide cooling by removing the side glass if there is a breeze, otherwise not very effective.  A fan provides a breeze on days when there isn't one naturally.  Watch out for plants in the corners and against glass where the air doesn't move so much.  Pot colour makes surprisingly little difference.  Dark coloured pots seem to heat up more quickly, but they also cool down more quickly, overall I haven't noticed any consistent difference.  The pot material can make a big difference.  In particular, porous pots containing moist soil stay very cool, although they can get very hot if they are completely dry.

[newstart]

Thanks everyone for answers that's very helpful . I will put the shading on as well as having ventilation. To be honest it was always unlikely that shading could be avoided but it does show that ventilation is important to help with scorching and provide air to the plants.

[ArnoldT]

I've had Boophone disticha's scorched in a greenhouse with roof vents wide open and a fan running 24/7.  Although  they were located in the corner of the greenhouse which lends truth to Iann's statement about corners being hotter.  Shading is a must here in Northern New Jersey in summer.

[angie]

I have had plants scorched in my greenhouse this summer and in Aberdeen, Scotland I didn't think this would be possible  . I have now fitted shading.

Angie