Winter Greenhouse Regime

[JPB]

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

[JPB]

Here they are:

[JPB]

and two more:

[Pascal B]

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.

[David Nicholson]

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?

[gote]

You are all living in the tropics you know 
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
 

[Diane Whitehead]

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

[David Pilling]

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...

[Giles]

Thermal Screens at night might be an option.
Supposedly 50% reduction in heat loss.
http://www.eurekagrowingsystems.co.uk/index_files/Page504.htm

[JPB]

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

 

[Darren]

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.

[Tony Willis]

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.

[gote]

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    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

[David Pilling]

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.

[ArnoldT]

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.