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Wintering of Plants, Emphasis on Cold Frames

For a home gardener there are several alternatives for wintering rhododendron seed plants and other tender plants in very cold climates.

  • Outdoors, covered with Picea branches or snow. This is the most risky alternative in cold areas (down to -20°C -30°C) close to sea without dependable snow coverage. The method has worked pretty well for me over on two year old seedlings. The most tender ones have died during winter and this is what I have hoped for as a natural screening method.
    On the other hand I have also lost some plants because of too high moisture level on the leaves of the plants when it has been warm and humid, temperature above +5°C for long periods during winter. Picea branches is not a good idea when it is not cold enough.

    Rhododendron plants covered with Picea branches   Rhododendron plants covered with Picea branches
    (Click on the pictures to see them larger.)

  • On a cool porch or in a heated shed. The plants need shaded daylight or artificial lightning and scarce watering when temperature is above +5°C.

  • In a heated greenhouse if such is available. As an extra advantage you get prolonged growth season but energy consumption can be large without added insulation.

    Green house divided to insulated area    Insulated area in the green house    Green house temperature and energy consumption
    (Click on the pictures to see them larger.)

  • Indoors at normal room temperature, in pots covered with transparent plastic bags or in plastic bottles. Some ventilation is required not to have too moist air. As an advantage the Rhododendron plants get a couple of extra growth cycles before summer.
    My own unorthodox growth conditions that I've found to give good results: Temperature +23 ... +26ºC, humidity 90 ... 95 %, artificial light from fluorescence tubes 18 hours/day. Potential growth of mould can be prevented with sphagnum moss.

    R. campanulatum inside during winter
    (Click on the picture to see it larger.)

  • In a cold frame. This method is suitable even for young under one year old rhododendron plants. The temperature can be kept in the cold frame well above -10°C with good insulation during cold winter temperatures down to -30°C. The plants don't need light or watering during the winter.

Cold frame is excellent for wintering of rhododendrons, azaleas and other woody plants. It is probably the most cost effective and safest way in cold climates to get small seed plants survive the winter without any damages.

Kylmälava edestä

Cold frames are ideal for wintering seed plants that are not yet hardy enough to survive the freezing low temperatures. The wooden walls have a 50 mm thick foam insulation and the glass windows are covered with 50 + 50 mm thick foam plane insulation during winter.

I built my first cold frame in 2001 based on drawings from Osmo Jussila. Already the first winter gave proof of benefits of the cold frame. The temperature was down only to -3°C when the minimum outdoor temperature had been -21°C. Even the leaves of the azalea seed plants had stayed green during the winter in the closed dark cold frame! After that I built two more cold frames as the needs seemed to grow.

The cold frame should have an uninsulated base so that warmth is transferred from the ground even when it is close to freezing. The wooden walls are insulated with 50 mm thick Expanded Polystyrene (EPS) foam panels. The roof has glass windows and they are covered during winter with a double layer of 50 mm EPS foam panels. Where snow is available it can give good additional insulation on the walls and on the roof.

Kylmälava sivulta

Side view of the cold frames.
The glass windows can be lifted off from the wooden supports. The size of the cold frame is optimized for large plant boxes that can stay in the frame the whole summer.

I have put large stones inside the cold frame to control the temperature. Energy is accumulated in the stones during the day and it is released gradually into the air during cold nights. The role of the stones is emphasized in autumn and spring frosts when the cold frame is not covered with foam panels. To protect the plants from moles and mice it is advisory to add thick plastic sheets and/or dense metal net on the walls deep into ground.

I have not dared to dig my cold frames very deep into ground in fear of to much moisture. The melting snow in spring time might result in bad flooding in cold frames as the ground is still frozen and the drain pipes are not yet functioning. The ground in the cold frames is at same level as the outside ground.

Kylmälavan taimet

I once made an experiment on the effect of large stones in the cold frame during a two week period in autumn frosts.

Case 1.
Small stones in the cold frame, glass windows ajar by 3 cm.
- Minimum outside temperature -5.5°C
- Minimum temperature in cold frame -2.0°C
- Temperature difference -3.5°C

Case 2.
Three large stones added, glass windows ajar by 3 cm.
- Minimum outside temperature -6.0°C
- Minimum temperature in cold frame 0.0°C
- Temperature difference -6.0°C!

The benefit of using large stones to level the temperature is not extremely large but still significant.

Kylmälava talvella

Snow covers the cold frame during winter. However, there is not much snow every winter in the archipelago and I cannot trust only on insulation that snow can provide.

My cold frames are in the garden of my summer cottage and in practice I cannot open and close the frames depending on daily temperatures as I'm not there. I close the glass windows of the cold frames in October before night temperatures drop below 0°C or depending on the forecasted weather I may leave them slightly ajar (a 3 cm gap). On weekends I have ventilated the cold frames. No until November when colder nights (-10°C) are threatening I have placed the insulating foam panels (EPS) over the glass windows. The foam panels freeze tightly into wood and to avoid that I've put plastic bags between the panels and the wooden frames.

I check the status of the cold frames in March or April when the ice under the foam panels has melted. I remove the panels but still leave the glass windows closed or slightly ajar until end of May because of potential cold frost nights. On weekends I have ventilated the cold frames and watered if needed.

Kylmälavan isot taimet

One of my cold frames is higher and meant for larger tender plants that could not survive winters in the garden. This is a picture that I took just after opening the cold frame in April. Looks good!

Kylmälavan atsaleat

The decidious azaleas can keep their leaves during winter! I took this picture in February while peeking into the cold frame to check that everything is OK.

I don't claim that I have the optimum temperature in my cold frames for wintering the plants. I think it would be better if the average temperature would be a bit colder. However, I have not noticed any major problems in wintering rhododendrons, magnolias and other woody plants in the cold frames. As an exception are rhododendrons from the Taliensia -subsection (for example R. bureavii ) that have thick indumentum on the leaves. They seem to suffer in cold frames and some of the new leaves don't survive over the winter.

I have noticed slightly some mould but only on dead leaves. If needed I have sprayed in April into the cold frames with a mild solution of fungicide Euparen M. This has effectively prevented further growth of mould.

Cold frame temperature statistics

The temperature in the cold frame has been as an average slightly above 0°C, minimum temperature being +0.5°C -6.0°C depending on winter (from -11°C to -26°C) and maximum temperature being +3.0°C +7.0°C.

I have been very pleased with my cold frames. They have kept the temperatures well controlled during winter. As an extra benefit has been the usage of the cold frames during summer for growing seed plants. The cold frames warm up early in the spring and the growth starts earlier than on open land and the plants are protected from spring frosts. During summer the moisture level is suitably high in the cold frames and the warm microclimate results in good plant growth. In autumn the cold frames offer shelter from early autumn frost and the growth season is prolonged.

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