Scottish Rock Garden Club Forum
Cultivation => Cultivation Problems => Topic started by: David Nicholson on December 19, 2010, 11:55:27 AM
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If, as the experts! seem to think, we are in for a spell of exceptionally cold winters in the UK I, for one, will have to re-think my current winter greenhouse growing regime. In spite of a bad winter in 2009, although it didn't start as early as this one, and, as far as I can remember, 2008 wasn't great, my winter regime doesn't differ a lot from my summer regime. My current winter regime stems from my Primula and Auricula growing days (I mainly grow bulbs now) and of course they don't mind frost at all. Increasingly I have been leaving my two top vents closed but my two side louvres are open all year round and one glass pane is removed from the door and covered with wire mesh. So, really the only difference between summer and winter is closed top vents and me waving a few sheets of fleece around.
I grow my bulbs mainly in square plastic pots stood on staging (so I can cram as many pots in as possible!) so not having a plunge heating cables are not an option. I'm seriously thinking about an electric heater and keeping the greenhouse as near frost free as possible but given projected increases in electricity costs I would need to consider how to conserve heat and that must include re-placing the glass in my door and closing my side vents. My mind goes back to my earlier white fly farming (Fuschia growing!) when I used to attempt to line out the greenhouse (aluminium)with bubble wrap with varying degrees of failure because it invariably fell off whatever I did.
Others must already have gone through this thinking, and maybe some others, like me, are going through it now. I would welcome some thoughts.
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Good question David.
Mine hasnt changed except now that my species Pelargonium collection is dead I no longer heat the benches. I have four louvres on each side that have remained open and it just occured to me that my Scilla linguata collection is frozen again and I forgot about them. Maybe it's time for me to start heating plunges again.
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I refuse to even consider any heat...it's too far from the house to the greenhouse to lay a cable for any reasonable amount of money. My strategy in Winter is to keep everything as dry as I dare. Anything that doesn't survive is replaced once or twice but after that I give up. At least now I do have everything plunged in sand which helps. Louvres and doors stay open all year, cold wind being preferred to Botrytis.
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I learnt in the previous Winter, that the top of a greenhouse is colder than the bottom. I had various (not very) hardy bulbs growing on the top shelves, and there was the effect of the leaves being OK but the bulbs turned to mush. The same species in contact with the base of the greenhouse were OK. Outside things in pots suffered, things in the ground were OK.
Conclusion bulbs are designed to grow in the warm earth and not in pots in the cold air.
It is also coldest up against the glass, so I've shifted things a couple of inches away from it. Stuffing bubble wrap in the gap between pot and glass would have been good.
This time I've tried, just for interest, to keep the greenhouse frost free. I set off by putting two buckets of hot water in at night, relative to the size of the greenhouse they will absorb a lot of cold. As it got colder, I moved on to a small parafin lamp, the sort of thing that once minded holes in the road, runs for maybe 24 hours. But last night was too cold for that.
I've got the greenhouse completely closed up at night. Rot is a worry, not so sure if it applies when temperatures are below zero and the air is dry.
How much heat depends on how big the greenhouse is, how cold it gets. Here right up to the sea, temperatures aren't supposed to go below -3C. In that situation the cost will be modest.
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I've an 8' x 7'6" greenhouse which I'm trying to keep frost-free (it's full of Nerines, some pelargoniums & echeverias); I've clipped bubble-wrap to all the struts, so the thing is more-or-less insulated on the roof & all sides; I don't have a thermostat, only a max-min thermometer & the Met Office forecasts & I've been using an electric fan heater when the temperature is -3 degrees C or below (down to -10 last night), a paraffin heater if it's -1 or -2 degrees C; electric fan heater is horribly expensive, about £6 per day when it has been running all day, paraffin heater costs about 1/3 of that to run. I might try investing in a second paraffin heater, I'd really like to avoid using electricity, it's so expensive & not an efficient use of energy.
Think next year, I'll certainly get a thermostat as well ~ save me from leaping out of bed at 4 am in a panic to traipse down the garden in my jimjams & wellies just to make sure that the heating is functioning ok.
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Electric warming mats might be an option, with thermostat, and fleece over the top when its particulary cold. At worst, only the leaf tips will get frosted.
(Most mail order places do them)
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Electric warming mats might be an option, with thermostat, and fleece over the top when its particulary cold. At worst, only the leaf tips will get frosted.
(Most mail order places do them)
Malcolm - For the few days that you are likely to be cold I would crank up the electric heater. I don't think the Nerines will like the heating mat and if they get any frost they will go down as Mark found out. We paste buble wrap on the aluminum greenhouse door and use sheets of styro over the northside glass. Propane heater + electric back-up set at 4c, I wouldn't like to think what our small greenhouse costs to heat but probably in excess of £300/yr.
johnw
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I refuse to even consider any heat...it's too far from the house to the greenhouse to lay a cable for any reasonable amount of money. My strategy in Winter is to keep everything as dry as I dare. Anything that doesn't survive is replaced once or twice but after that I give up. At least now I do have everything plunged in sand which helps. Louvres and doors stay open all year, cold wind being preferred to Botrytis.
I have to agree with you Martin. I tried bottom heat and it caused so much drying that corms went dormant before they had regrown. I did not dare water in hard frost. many things are hardier if kept on the dry side and if growth is delayed by this tecnique. One has to watch carefully for signs of premature dormancy though.
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I've an 8' x 7'6" greenhouse which I'm trying to keep frost-free (it's full of Nerines, some pelargoniums & echeverias); I've clipped bubble-wrap to all the struts, so the thing is more-or-less insulated on the roof & all sides; I don't have a thermostat, only a max-min thermometer & the Met Office forecasts & I've been using an electric fan heater when the temperature is -3 degrees C or below (down to -10 last night), a paraffin heater if it's -1 or -2 degrees C; electric fan heater is horribly expensive, about £6 per day when it has been running all day, paraffin heater costs about 1/3 of that to run. I might try investing in a second paraffin heater, I'd really like to avoid using electricity, it's so expensive & not an efficient use of energy.
Quote from: PeterT on Today at 07:52:26 PM
I don't have any heat on, the glass round the sides is only to keep cats away from the sand. I have found a roof alone controls the moisture level which allows me to regulate the growth patterns of the plants and If I get it right Undecided many supposedly tender plants will survive freezing. Much of this green house is oncos so maximum air flow.
A clever Idea to store warm air but it would require a lot of storage to provide enough warmth to mitigate a frosty night.
I read of a tecnique of temperature control where steel drums of water are put in a green house. During the day the glass traps heat and the water warms up, at night the water acts as a radiator delaying the night frost. When the sun heats the green house the next day the drums of cold water slow down the temperature rise. The overall effect is to moderate the temperature fluctuations
Think next year, I'll certainly get a thermostat as well ~ save me from leaping out of bed at 4 am in a panic to traipse down the garden in my jimjams & wellies just to make sure that the heating is functioning ok.
Many Nerines will take some frost and certainly some Echiverias provided they are not damp and trying to grow as though it were spring. In cool temperatures I should worry about the humidity from the parafin heaters for the Echiverias. I had Nerines gibsonii, platypetala, hirsutum, masonorum, filifolia, 'flexuosa alba', and others survive without heat last winter. I lost pudica which I have grown without heat for 6 or 7 years.
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I lost too many potted South African bulbs in a small unheated plastic
house. Five years old, almost old enough to bloom, and mush because
we had a few days of -8C. The weird thing was that large pots outside
on the ground survived, but ones on a wire bench in the greenhouse froze.
Then for a couple of years I put in gallon or larger pots of hellebores so
they could be sold in bloom at a February club sale. I covered them with large
bubble wrap. Oh joy! said the rats - a cozy spot, with delicious food too.
They set up camp under the bubbles and ate the flower buds.
So now anything potted comes in to my large attached greenhouse for
the winter. It is two stories high, 8 by 12 metres. It is large enough that
its temperature remains fairly constant. It can approach freezing, and
might have once when a mango tree died, but generally is a couple of degrees
warmer than outside. I have a wood-burning boiler in an adjacent space, and
a ~500 gallon tank of water to hold the heat, but I only heat it for parties.
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I refuse to even consider any heat...it's too far from the house to the greenhouse to lay a cable for any reasonable amount of money. My strategy in Winter is to keep everything as dry as I dare. Anything that doesn't survive is replaced once or twice but after that I give up. At least now I do have everything plunged in sand which helps. Louvres and doors stay open all year, cold wind being preferred to Botrytis.
I'm with Martin, for much the same reasons and adopt a similar approach although most of mine are in square plastic pots unplunged. The watering balance is more of a challenge with those bulbs which are in leaf at this time of year. I don't grow many South Africans though.
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I have a lean to greenhouse attached to the house on two sides. I have used a small dairy barn heater set to go on at 45F.
I put eight black 5 gallon pails with lids on the bench on the east facing side.
I have a thermometer probe in one of the pails.
During daytime the water in the pails heats up to about 50 F and the greenhouse heats up to around 45 F with outside temperatures currently running at around 25 to 30 F.
Each 5 gallon pail can store about 41 BTU's per degree F rise
With 8 pails full of water each degree rise results in a gain of 330 BTU's.
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Sorry should be 5 gallon pails with lids on the under bench on the east facing side.
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I was inspired by an interesting book, Solviva, by Anna Edey.
She ran a large greenhouse, partly heated by the sun and partly by
chickens living in it. (She tried to get her sheep to stay inside too
but they preferred the cold outdoors.) She was even growing enough
tomatoes to sell in midwinter.
Then I started to think more about it. I used to keep chickens and
I'd have an allergic reaction whenever I cleaned out the coop. Not
a good idea to fill my greenhouse with feathers and dust.
And the solar heat. Right! Before I designed my house back in the
60s, I read all about that. I faced all the windows south, expecting
solar gain. My calculations did not take into account how high all
those conifers were and how low the winter sun's trajectory is. Or
our maritime cloudiness. On the few winter days when we actually
had sun, I would have it for about an hour when it managed to shine
from the unobscured sky above the road.
So I get NO solar warmth, but the infrequent winter sun is cheering.
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Hi, I have greenhouses in remote places (no electricity). I have for years now lined them with large bubble plastic inside AND out. That saves me a lot of heating costs. Heating is by gas bottles, a pair at each greenhouse with an auto changeover device.
To fix the bubble plastic in place, I use elasticated curtain wire with small hooks screwed into the ends. On my aluminium greenhouses, I have drilled small holes into the frame to anchor the hooks. On the wooden greenhouse I screw in larger hooks to which I attach the curtain hooks. It works really well. The double layer keeps out the windchill.
The bubble insulation has to be three layered, two smooth outer layers and the bubble wedged in between. It's a bit more expensive than the two layered bubble, which is smooth on one side and bubbly on the other. The latter is rather flimsy I find and it only serves for packaging.
I have a greenhouse on an allotment where I grow Cymbidiums. It is on a hill exposed to fierce easterly winds, which used to rip off the bubble in no time. I have overcome this by putting shade netting over the external bubble layer. This is easier to fix to the structure and I haven't lost my insulation yet, fingers crossed.
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Yes, I can use that curtain wire trick. Thanks.
It must be dim inside when you add the shade cloth to the double bubbles
Do the cymbidiums not need much light?
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Given our low angle sunlight in winter you can safely line the north side (and possibly the east?) of a greenhouse permanently with 5cm thick polystryrene sheet sold for insulation. In fact if this is then either left white or covered with reflective film or foil (or old mirrors in my case) this will actually increase the light reaching your plants by reflecting back the sunlight coming in from the south. My crocus now grow straight up rather than leaning to the south. This reduces the area for heat loss by a considerable amount, and it is on the sides which get the worst of the cold winds in the UK too.
I use an electric fan heater with an external thermostat which is set to maintain at 1 to 2 C. Bubble plastic is applied to all sides and roof when temps below -3 are forecast, to prevent the heater working too hard. The glass and plastic alone will keep out -2.
The lowest external temp I've measured here so far this winter is -4.4C which I know is milder than most of you have had.
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Hello Diane,
there is a reduction of light, you are quite right. for that reason I use the lightest shade netting available. I found one that gives just 20%, the cymbidiums seem OK with that. I am also considering using pea netting over the bubble instead of shade netting. that's a bit harder to fix and may be less effective due to its flimsy nature.
But I'll try it next summer. At present I can't get out of the house, snowed in solid. I anticipated this and stocked up with food, but I miss my daily newspaper. :) :( :)
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A very interesting thread with some very ingenious ideas. My old greenhouse was a very old (40 years) and small Alton in cedarwood now residing at the allotment in pieces waiting to be erected. This had lots of gaps to let out the heat brought about by my miserly maintenance. Previously I had little use for a greenhouse as I always believed that the best place for plants was in the garden where I had less chance of killing them and they looked more natural. So my greenhouse prior to this was used mainly for propagation and overwintering young and very tender plants and was therefore fine. Joining this Forum and having a little more time on my hands made me get more interested in bulbs and realising that many winter bulbs are best enjoyed in the comfort of somewhere protected from the weather was good for the both of us. Having decided to invest the kids inheritance I bought a new 12 x 8 custom jobbie on a half brick base with windows to all panels (auto in the roof) the thought being to allow good ventilation hoping that I could leave these open all year round. Well after the current bout of weather it seems this falls into the category of "the best laid plans of mice and men". I had also decided that I would buy any accessories I might need at the time so as to get over the pain of parting with my money. As a result I also bought plunge staging, a thermostatically controlled fan heater and since I needed a light a mercury vapour lamp.
I am growing mostly Cyclamen, Crocus, Narcissus and a few other things mostly bulbs. The problem faced here was that many of the plants will take quite low temperatures whereas Cyclamen rolfsianum and persicum and a few other tender plants do not like cold at all. What I have done so far is to set the fan heater to come on at 0 centigrade (32F) and God only knows what it is costing- but there is at least some peace of mind well at least until the meter is read.
In terms of soil warming cables I also have a few cuttings which I have been trying to root so I have used a soil warming cable on these but like previous comments this dries out the sand plunge very quickly- I only noticed this morning ::)
My greenhouse is unfortunately in a semi shaded position (my neighbours trees) and does not get enough light at critical times meaning the the plants have a tendency to lean toward what light there is and as they quickly root into the plunge I do not want to damage the roots by moving these- well the cyclame are OK but not the others. As an experiment I am using a mercury vapour lamp a timer switch which comes on for a couple of hours before dawn giving the plants a little more daylight
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Ian's comments about spending the inheritance reminds me of a bumper sticker I saw on a fancy sports car.
"My heirs will walk"
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Ian's comments about spending the inheritance reminds me of a bumper sticker I saw on a fancy sports car.
"My heirs will walk"
;D ;D ;D
I believe the term used now is I'm off SKIing
Spending the Kids Inheritance ;)
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The one thing that surprises me is the number of people who don't heat their greenhouses then complain there whatever has died.
I keep my 11 x 8 greenhouse at 5'C all through the winter with an electric heater, there not that expensive when you consider the cost of the plants.
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Well, I have read a lot of false information concerning the supposed hardiness of South African bulbs. No wonder one would lose them by following it.
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The one thing that surprises me is the number of people who don't heat their greenhouses then complain there whatever has died.
I keep my 11 x 8 greenhouse at 5'C all through the winter with an electric heater, there not that expensive when you consider the cost of the plants.
Do you line it out or take any other action to keep costs down?
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Well, I have read a lot of false information concerning the supposed hardiness of South African bulbs. No wonder one would lose them by following it.
Alberto, it would be helpful if you indicated examples of where you disagree with what has been written so far please
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I keep my 11 x 8 greenhouse at 5'C all through the winter with an electric heater, there not that expensive when you consider the cost of the plants.
[/quote]
Do you line it out or take any other action to keep costs down?
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I don't line it as it only reduces what little light is available this time of year.
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The one thing that surprises me is the number of people who don't heat their greenhouses then complain there whatever has died.
I keep my 11 x 8 greenhouse at 5'C all through the winter with an electric heater, there not that expensive when you consider the cost of the plants.
I do not complain I just replace them with hardier subjects.
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A tip from the building trade:
To prevent roots freezing and to increase the temperature.
A method I used before going over to electric heating.
1. Thermal boards used in building either during the construction to fill the wall caverties or as secondary insulation. The boards range in price from £5-£30 and you lay them on the floor and staging. They require holes making to accommodate supports the same thickness as the boards so heavy items can be placed resting on the supports.
2. A more professional job can be done with planks across the supports.
3. WARNING remove the insulation once the weather warms up or you can Cook your plants.
Similar to lagging outside pipes to prevent freezing.
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Forgot to mention the boards are available from builders merchants or B & Q.
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Well, I have read a lot of false information concerning the supposed hardiness of South African bulbs. No wonder one would lose them by following it.
Well, that year I was awakened by strong winds and slashing sleet and heard the greenhouse door blow open. I just snuggled down in my warm bed and didn't go out to fix it. So it wasn't bad advice that did in my South African bulbs, just my cursed need for comfort.
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I've found that larger bubble plastic is much better than the usual smaller plastic. It is quite tough and can be used again and again. Rolls are 2m wide so I put them on the sides of my greenhouse and they stand by themselves and only need a little support (clips or tape).
I use an electric heater and a large fan. The fan is absolutely necessary as I do not ventilate during cold spells. Also it prevents that the corners of my greenhouse freeze. I keep the temperature at 6C with a thermostate and it costs me 0.5-1 Euro/day during frost, which is acceptable for me...given the quite large and special collection I have.
The cooling box with pleiones is yellow and I keep the temperature in it at 2C using another thermostate.
So far so good while it was -10C last night, but the greenhouse is under trees so -5C is the surrounding temperature. Still extremely cold for December here..
See uploaded pics in next post
Hans
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Here they are:
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and two more:
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I use the same "big bubble" plastic but because of the light only have it upto 1.5 m/5 ft. and used snappers so there is an extra layer of air to insulate the glasshouse. The condensation on the glass is frozen but the temp near the plants is 4 C. This is my first winter with this glasshouse but I am glad I chose to have the roof and back of the glasshouse with 8 mm polycarbonate as it really helps and was not so much more expensive than 4 mm glass. As visible on the picture I only use a single fanheater (Parasene Frost Shield 3000 watt) to keep the glasshouse frost free. The glasshouse itself is 6 x 3 meter with the highest point at 2.70 and uptill now, despite temps dropping to -8C I only had to put on "2" with the dual heatrings on. I bought a second one as back-up in case it was not enough to heat it bit it is still in the box I bought it in.
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Given our low angle sunlight in winter you can safely line the north side (and possibly the east?) of a greenhouse permanently with 5cm thick polystryrene sheet sold for insulation. In fact if this is then either left white or covered with reflective film or foil (or old mirrors in my case) this will actually increase the light reaching your plants by reflecting back the sunlight coming in from the south. My crocus now grow straight up rather than leaning to the south. This reduces the area for heat loss by a considerable amount, and it is on the sides which get the worst of the cold winds in the UK too.
I use an electric fan heater with an external thermostat which is set to maintain at 1 to 2 C. Bubble plastic is applied to all sides and roof when temps below -3 are forecast, to prevent the heater working too hard. The glass and plastic alone will keep out -2.
The lowest external temp I've measured here so far this winter is -4.4C which I know is milder than most of you have had.
Darren, when it is cold enough for you to use bubble plastic and heat do you ventilate the greenhouse at all please?
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You are all living in the tropics you know ;D
Today we have -9 yesterday it was -20 Last year´s record was .26.
Today sunrise is 9AM and sunset 3PM.
This is the reason why I do not consider growing winter bulbs in a heated greenhouse a very practical proposition in my garden. Even if I were to heat it, the lack of light would cause etiolation.
A comment. Whatever natural heat there is in cold spells comes from the Thermal inertia of the soil below. Thus there is a great difference between pots standing on a staging and pots standing directly on the floor. Some insulation like a sheet of bubble plastic over square pots standing very close on the floor will make a significant difference in a cold spell of limited duration. The same insulation applied around pots on a staging will only work an hour or two - unless there are heating cables or similar source of heat available.
In some cases it is possible to make use of heat leaking from houses bu putting frames or insulation on the outside of pots or beds in close proximity of the wall to the heated area. I have seen gardening advisers claiming that the insulation should also be all around the pots and this of course defeats the whole exercise. The idea is to utilize the heat losses from the building in a sensible way.
I have an old greenhouse that is partly dug into the ground. only the upper half is above soil level. I have made a kind of cold frame inside and in that I have a thermostat and a 500W heating element. I have used this off and on for overwintering tender subjects and it works. The fact that the frame is "indoors" makes it easier to tend to the plants. If I were to have it outside snow would melt on the glass and freeze to ice in cold spells. There is some light of course unlike he situation in a garage and that helps in some cases.
If someone is VERY interested i will try to post a sketch but I cannot do it before January.
Merry Christmas everyone and a happy New Year
Göte
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Göte's greenhouse is similar to ones described in a 1941 book Winter
Flowers in the Sun-Heated Pit Including the Lean-to Greenhouse as a
Complement to the Pit by Kathryn S. Taylor and Edith W. Gregg.
I own a copy, but have not put it to use - they lived near Boston in
Eastern U.S.A., which has enough winter sunshine to make it feasible.
They describe how to make three kinds of pit greenhouses: free-standing,
next to a building, and attached to a building so one enters from that
building. They excavate four feet.
Here are a few quotations:
In December the first impression on entering the pit may be a wave of
fragrance from the mounds of purple violets. The misty blue of forget-me-nots
...... paper-white narcissus...late chrysanthemums seem to have caught
and held the light of all the sunny weather.
in February .. malacoides primroses whose delicate perfume one notices
at once on entering ... winter-flowering pansies ....... butterfly flowers of
sweet peas
Later still ... feathery yellow acacia, heaths, camellias and alpines
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There was a recent BBC programme in which Major Dick Strawbridge MBE constructed a green greenhouse. The idea was to dig a hole under the greenhouse and fill it with heat retaining material (broken glass), and then have a fan which in the day circulated warm air from the greenhouse into the heat retaining material, at night the fan would circulate heat in the opposite direction.
I read this week that the Eden project was investigating geothermal heating for their biomes. The answer lies in the ground...
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Thermal Screens at night might be an option.
Supposedly 50% reduction in heat loss.
http://www.eurekagrowingsystems.co.uk/index_files/Page504.htm
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I find additional lighting with fluorescent daylight tubes works fine. My greenhouse is in the woods, so I have to supplement light during winter for winter-growing plants anyway. In this way I can provide around 100 micro-Einstein.m-2.s-1 of PAR (photosynthetic active radiation 400-700 nm). But during winter, at out latitude (53 degrees North) we do not have PAR levels higher than a few hunderd micro-Einstein in the open field. Even less at cloudy days.
Additional lighting will cost energy, but I can reduce the heating costs considerably as i can shield without having to worry too much about incoming daylight.
Hans
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David (N),
Ventilation has not been possible for a few days as I leave for work very early and it has still been several degrees below freezing with no sun on the greenhouse to raise the internal temperature. When I am at home (today for example) I will open up the greenhouse when the sun is on it - approx 9am to 3pm. I have had no problems with botrytis - in fact less than usual on fading flowers etc. I reckon that this is because the humidity is extremely low and temps are also staying too low for much fungal growth. The bubble plastic has been on for the last week permanently.
I have noticed that things like Massonia are lasting ages because of the cold. But also some autumn flowerers such as Lachenalia rubida are still not open! The low humidity is causing pots to dry quite quickly - things in full growth may need watering despite the cold. I am pretty convinced that I was too cautious last winter and some pots dried out to much (leading to aborted flowers). Obviously this only applies to pots kept frost-free. Little use trying to water those frozen solid in frames outside!
Giles' suggestion of a thermal screen is a valuable one. I have experimented with a horizontal layer of bubble plastic at eaves height when very cold last winter and it works well. I discontinued the experiment as I am not confident of my engineering skills and am nervous about it falling onto the fan heater with potentially disastrous consequences. However for someone more skilled than I would certainly recommend giving it a go.
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First picture is how the greenhouses look at the moment. The temperature in the far one is -7c as I type. They do not get any sun at this time as they are on the north side of the house. The middle and end ones both have a closed sand plunge frame inside.
This is constructed of wood and is lined for the first time this year on the bottom and sides with 2cm polystyrene sheets. each has a 75 watt soil warming cable buried in the sand set to 5c. This is the second picture.
I keep orchids gesneriads and sundry bits of tender things in the middle house and the far one has cyclamen graecum,other tender cyclamen and biarums in it.
I open them on frost free days and nights and they have been closed for over a week now and there is no sign of them being opened with -16c forecast tonight.
Condensation is bad but not causing a problem and I should have made the tops more sloping so it would run of. Low light is not a problem as the plants are only just above freezing and there is no growth happening. I do not grow winter growing bulbs except for a few which are clogging up the windowsills in the house.
So far this has worked for me and is cheap and easy to construct and operate.
Most of my crocus are in a greenhouse at the bottom of the garden and this is frozen shut and covered in frost and so I have no idea of either the temperature or state of the bulbs.
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It all depends upon the situation as Diane points out. If I have excess heat in the day I can store it in something and get it out again but If I have a heat deficit in the daytime there is zero help.
We all know that snow helps outside especially if it is light weight and stays that way. The plants and wildlife (mice and voles and other varmints) will have a decent temperature below the snow because the thermal inertia in the underlying ground is so great. The summer heat remains for a very long time.
In the highly interesting papers on germination of Erythronium japonicum, that someone pointed me to, there are temperature figures. It is stated that the temperature in Sapporo at ground level rarely drops significantly below zero C under the snow cover. Light snow that never has ben subject to thaw is as good insulator as mineral wool and similar substances.
It is not practical to emulate 20cm of snow using bubble plastic. However, a layer of that stuff increases insulation in a greenhouse by a factor 2-4. For someone living in the tropical climate of the Britis Isles ;D that will make a significant difference - for me it does not.
I must heat if I want to be frost free. If I want to be able to pay for the heat I cannot heat a greenhouse but I can heat a frame inside it.
We now have a heat wave outside. It is only -6°C in daytime.
Göte
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I wonder if the snow blanket theory works with greenhouses. What seems to happen to me is an inch of snow freezes to the top of the greenhouse and is unmoveable, as a result the sun which often accompanies cold weather doesn't get in, and the greenhouse gets colder and colder.
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I guess I was lucky. The sun melted all the snow on the roof of the greenhouse and it just slid off. It was much darker inside when it was snow covered. I did increase the temp a bit which may have helped in the melting.
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I wonder if the snow blanket theory works with greenhouses. What seems to happen to me is an inch of snow freezes to the top of the greenhouse and is unmoveable, as a result the sun which often accompanies cold weather doesn't get in, and the greenhouse gets colder and colder.
It certainy does not apply. A snow blanket on top of the ground will insultate so that the heat stored in the ground does not disappear quickly. The ground is a bad insulator but has a great thermal capacity. This means that there is a lot of low temperature heat available in lower layers that is conducted up to the bottom of the snow. Here the opposite happens. The snow has very little thermal inertia so it does not "steal" much heat as long as the temperature is below thawing. It further inslutes so the heat does not dissipate quickly.
The result is that the temperature difference between deeper layers of the ground and the bottom of the snow is small whereas the temperature difference between he snow bottom and top can be considerable.
The situation on top of the greenhouse is different since there is no soil in contact with the snow bottom. There is only cold air. The snow will of course stop warming from the sun just as described.
The problems in my climate lies in ice formation. If the ground is insulated by early snow and the snow starts melting from heat from below, the soil is obviously not frozen and the resulting water is drained away. (And the temperature will be 0C as described from Sapporo) A heated frame or on a greenhouse, will be unevenly heated and we get ice formation on the colder parts. The same problem can occur in a mild spell if the top layer of the soil is frozen and the snow starts to melt. This situation is worse in pots standing outside since they are less well heated from below and the rim will keep the water in.
I have found it next to impossible to have heated frames outside because of this ice formation. This is why i keep my heated frames inside the greenhouse where there is no snow.