Information wanted, please

[gardenmad]

Please does anyone know where I can get seeds/plants of cynara hystrix and achillea tomentosum, also while I'm asking does polycarbonate act as good as hort. glass in greenhouses, especially in winter when greenhouse  heated {just} to keep it frost free?   Thanks.

Gardenmad (Bucks, England)

[Anthony Darby]

If the polycarbonate is fitted well, like in a conservatory then is is fine.

[David Shaw]

Glass doesn't have any insulation qualities and I don't see why single skinned polycarbonate would. If it is double/treble skinned then it may have insulation properties but this would be difficult to fit in an aluminium greenhouse?. Currently the outide air temperature is 2C and the greenhouse is also at 2C. I think the main value of polycarbonate would be that it will not smash in a gale.

[gote]

David,
I am afraid that I have to disagree.
Glass has insulating properties. This is not the pane as such but the laminar layer of air that clings to the surface. This is mostly on the inside.
A single pane has an U-value of proximately 5 W/m2°C. The difference towards a single pane of polycarbonate is probably negligible.
The above refers to heat transfer as convection and flow.
Another effect is the effect on radiation. Radiation from the sky (including the sun) is mostly in the visible part of the spectrum and glass is permeable to this.
Radiation back is in the infrared part and to this, glass is impermeable.
In a cold frost night with clear sky, the drop in temperature comes from the infrared radiation that beams the heat out into space.
The glass stops this radiation. However, the glass will radiate by itself so if the glass is heated by radiation from the soil in the greenhouse it re-sends this heat again but at a lower rate.
The whole subject is kind of complicated and your observation can certainly be correct in some situations.
If there is no influx of heat into a greenhouse it will get the same temperature inside and outside whatever the pane insulation - at least in the long run and steady state. However, the thermal inertia will keep the temperature higher inside when the outside temperature is falling. Thus the lowest night temperature will be higher. Also there will be a difference if there is a heater inside the house.
Assuming a house of 3x2m 2m high, It will have a surface around 24 m2. If the temperature difference is to be 10°C then you need 24x5x10=1.2kW Then you probably have draughts as well so 2kW is more likely to be needed.
I hope this is not too confusing
Göte


Göte

[David Shaw]

Thank you for that information, Gote. I am happy to stand corrected. I try to keep our houses very well ventilated so I don't use any heat.

[gote]

If I may add to the complication.
It is a fact that is not universally known that
At room temperature without excessive air flow, about half of the heat exchange goes as radiation.
We seldom feel this effect or rather seldom recognize it for what it is.
Someone sitting close to a single pane window in the winter will feel a "draught". To a great extent this feeling is that the person is radiating heat towards the cold pane and not getting any radiation back. The rest of the room surfaces are warmer so they radiate back and do not cause the chilly feeling.
Similarly when we in a summer morning in the south enter a big stone building we feel pleasantly cool. This is partly because we radiate our surplus heat towards the still chilly stone surfaces that only radiate a fraction back.
This mechanism is very helpful when we want to save our Dahlias in the sudden clear frosty night. A very flimsy cover may stop the Dahlias from radiating all their latent heat into outer space.
It is also the explanation why aluminium foil is a good insulator even if aluminium is a very good conductor. Aluminium is a very poor radiator. It reflects any radiation back and does not radiate its own heat.
 
Göte

 

[Maggi Young]

Well, now, Gardenmad ( June isn't it? ) bet you weren't expecting these answers ? ..... not a lot of help with your seed requests, but some great info about the rest.....what knowledge the Forumists have, and thank goodness they will all share with us 

[gote]

Oops!
Achillea tomentosum used to be a weed here around.
Unless you mean the double form I could probably send you some.
However, that has to wait until summer when I can find them.
If you feel an urge to get it around midsummer next year, please send me a mail.
Göte

[gardenmad]

Thank you all for your very  kind comments - sorry I have not replied before now -I have been held up by other things.   Thank you for your  kind offer of achillea tomentosum, Gote, they are not double, not the ones I saw, anyway, but I do not want to put you to any trouble, however, I would like some.   I will mail later on this, thank you.  Thank you also, David and Anthony, and yes, Maggie, I have been blinded with science!   However, I have been HELPED, and that is the main thing, isn't it?   (Your bulbs were, and are, magnificent!)   
June.  (or am I gardenmad?)

[Maggi Young]

Yes, June, you are certainly gardenmad... but, as Gote told us in another thread, his friends call him a hortomaniac, so I rather think we're all in the same boat..... perhaps we should club together to see if we can get the medicine cheaper by buying in bulk ??!

......................."My name is Margaret and I am a plantoholic".....

[Katherine J]

I hope this is not too confusing
Göte

Göte,
I really admire your ample knowledge. All your comments are so clever.
My physicist brother had explained me a few months ago exactly the same as you wrote about the glass and polycarbonate. But I surely could not explain this so clearly in English.

[gote]

You make me blush Katherine but thanks for the kind words
Göte