What is the best general greenhouse temperature for alpines?

[newstart]

I am growing starter alpines , nothing special. Delosperma, veronica small, sedum, thyme, saxifraga etc. I looked at my thermometer today in some pleasant october sun and it showed 18 degrees centigrade. I don't understand Fahrenheit by the way. Is this a good even temperature for plants which are not tender. Or should I lower it with the outdoor true conditions but not being quite so cold as outside. Its 13 degrees outside so maybe 15-16 would be best say. On a dull day I expect it would be a similar procedure of ventilation to a temperature a few degrees above the outdooor temperature eg 10 degrees after ventilation on an outdoor day of about 8 degrees. We have sort of gone into this before but not talked about temperatures as such which I find an excellent indicator. I am still fairly new to alpine growing as you can see so it would be excellent to see the standard way people tend to do it.

I am having a look at some of the varieties suggested with this link from an AGS alpine enthusiast. I absolutely love this write up but the person who wrote this has not offered a name. You can write to the person concerned though. Thought I'd share this if folks have not already got it.  http://portraitsofalpineplants.com/Portraits%20of%20Alpine%20Plants.htm

Best regards David.

[Diane Clement]

I am growing starter alpines , nothing special. Delosperma, veronica small, sedum, thyme, saxifraga etc. I looked at my thermometer today in some pleasant october sun and it showed 18 degrees centigrade. I don't understand Fahrenheit by the way. Is this a good even temperature for plants which are not tender. Or should I lower it with the outdoor true conditions but not being quite so cold as outside. Its 13 degrees outside so maybe 15-16 would be best say. On a dull day I expect it would be a similar procedure of ventilation to a temperature a few degrees above the outdooor temperature eg 10 degrees after ventilation on an outdoor day of about 8 degrees. We have sort of gone into this before but not talked about temperatures as such which I find an excellent indicator. I am still fairly new to alpine growing as you can see so it would be excellent to see the standard way people tend to do it.  

David, I don't think that most people growing the plants you list would be heating a greenhouse at any time of year.  Those plants are hardy and would survive outside. One of the main reason for growing alpines in a greenhouse is to protect from winter wet, hence the roof.  Cold is not usually a problem and overheating is more likely to be an issue.  You will find that the temperature inside an unheated greenhouse is often a couple of degrees above the outside temperature.  If the sun is on it, the temperature could be considerably above the outside temperature.  Warmth can cause overheating and if the air is damp, it can also encourage botrytis and other problems.  It is therefore best to have as much ventilation at all times.  I have no heating, louvres open along all sides of my greenhouses all year, the door and roof vents are left open.  My small greenhouse has two panes of glass permanently removed.  The only time I might shut the louvres, roof vents and door is if the temperature goes below -5C.  I would certainly advise you to turn off all your heating and let your plants start to cool down for the winter.  

[Lesley Cox]

I agree with every word Diane has said. All the plants you mention would probably be better planted out in the garden and will be quite rain/frost/snow hardy. Keep your alpine house for the slightly tender species or those whose winter flowers or woolly foliage need protection from rain splash and the like. Even then, only the slightest heating should be enough. Heat applied to alpine plants generally produces etoliation and weakness and plants which look very out of character.

The gentleman in your link is, I think, John Husband, a skilled gower and owner of a most beautiful dog. His advice (John's, but probably the dog's too) would be very worthwhile for the plants in his excellent website.

[Lesley Cox]

Returning to Diane's comments, I would say that air movement is vital. I remember visiting an alpine house in Huddersfield way back in the early 80's. There was an April blizzard and snow was banked up against the sides of the house but the owner still had the doors at each end of the house wide open, and a fan blowing cold air along the centre. I froze nearly to death but the plants were very happy.

[newstart]

Thanks I appreciate your answers I shall not worry about exact temperatures then. Ventilation I am aware of from previous posts I have made but its useful to hear it again. I had not intended heating my greenhouse and I really hope that I did not come across as having said that. It would be a case of really soft growth and certainly disease I agree. Thanks for the warning though just in case I had thought along those lines. The question really was more a inquiry about an optimum temperature for non-sensitive alpines. I can see now that this is not really a question in the sense the temperature changes with the seasons anyway. Thanks all !

David.

[Maggi Young]

I have the following note on the LINKS  page of this Forum (http://www.srgc.org.uk/smf/index.php?action=links;cat=2  ) to te website you mention, David....which is indeed owned by Forumist John Husband...

A super site with photos of great alpines in an English garden, well photographed and described, along with sections on what is an alpine; growing in the garden or troughs ; and plant nomenclature. Site owned by forumist "Lampwick"

 We don't heat our glasshouse now they are full mostly of bulbs, nor did we when they were doing duty as 'alpine houses' proper.... it is more about cooling them, with ventilation, natural and forced, by fans, if needed, to keep the air moving and the temperature down.

Finn Haugli  of Tromso in far Northern Norway, tells of how they can lose plants very quickly, even outside, when the temperatures hit 30 degrees C.  Knud Lunde of Stavangar says the same thing.... that they  can incur losses when the temps. are in the high 20s.
Hope that helps.

[newstart]

Hi yes Maggie that was the kind of gauge of things I needed. A thanks for telling me who did the website.

Bye for now!

[Lesley Cox]

Maybe it could be said that for plants which have no special needs and are cold or heat hardy, ANY temperature, i.e. the temperature at that moment, is the optimum temperature.

[newstart]

Yes very true thanks again Lesley.

[ArnoldT]

I plan to keep my greenhouse at 40 - 45 F this winter.  Heating is a must.  Our outdoor temps can go to 8 - 10 F in the dead of winter.   Keep a fan on most of the time for ventilation.

[newstart]

Thanks.

[newstart]

Hi can I just ask one final thing on temperature. I think from what I remember you are meant to place the thermometer in the warmest part but not in direct sun. The real air temperature heats the thermometer plastic if left in full sun giving a higher reading than what it really is... ??  If put in the warm area of the greenhouse out of the sun it gives the true air temperature ??. Hope I am right otherwise I could look very foolish, still you live and learn hey!

Thanks 20 degrees C is a useful pointer for summer not that you should worry about this in autumn and winter as this is just a time for protection from freezing only. We are still getting some good sunny days where I am though so its worth knowing about the thermometer positioning.

Many thanks for all the superb info this is a great forum! -mind u I have been on it over a year now and never said that.

David.

[ArnoldT]

I place my thermostat at the bench level as close to the growing area as possible.  Also a circulating fan runs almost 24/7 to keep the air moving.  Growing SA bulbs too warm can lead to etiolated foliage.  We are having a warm spell at the moment here in NE USA

[gote]

Hi can I just ask one final thing on temperature. I think from what I remember you are meant to place the thermometer in the warmest part but not in direct sun. The real air temperature heats the thermometer plastic if left in full sun giving a higher reading than what it really is... ??  If put in the warm area of the greenhouse out of the sun it gives the true air temperature ??. Hope I am right otherwise I could look very foolish, still you live and learn hey!
David.

Unless you are using an infrared thermometer the stark truth is that a thermometer does not measure any other temperature than its own.

At room temperature heat transfer to or from a thermometer or any body is about half as radiation and half as convection. This means that if the air temperature is 25 and all walls floor and roof are at 15 it will feel as 20. This is the reason to why we feel cold when sitting close to a big window in the winter.  We radiate our heat into the colder pane. This is also the explanation why we can get frost on a car window even if the air temperature is not below freezing. For plants and humans, the evaporation of moisture comes on top of these effects.

The opposite effect is of course when we, on a cold day, have coffee in the street café in the radiation of a gas burner. Or - when the sun directly radiates some plants in an otherwise cool greenhouse. 

The conclusion is that one should not need to measure the "true" air temperature since the plants are subject to this mixture of influences. The thermometer should be as close to the plants as is practical but "seeing" as much of the ceiling and walls as they do. It should probably be grayish but that is less important as long as it is not covered in aluminium foil.

I fear that I have created confusion 

Cheers

Göte

 

[newstart]

Thanks for your answers it makes interesting reading. Given I cannot get electricity to my greenhouse I don't plan using fans which is probably not essential luckily, but thanks for the info. I can see what Gote is saying in terms of temperature not being that important given the other factors too. I still felt it might be useful to use a thermometer given its a rough gauge of temperature at least. Going back to my question though- if its at bench level is it meant to be in direct sun, the thermometer, or will this give an adverse temperature. In full sun it gives 27 degrees C and its clearly not that hot . This is why I thought the plant bench in the shade should be considered as normal air temperature. Once its sorted it sorted, thanks folks!

David.