How to protect potted vineplants from cold temperatures?

Question

I have two vineplants in 40-litre-pots on my rooftop garden. This will be their first winter outside and I want to protect them from freezing to death. I live in an area where there are a lot of wineyards (southern Germany), but our winters can get as cold as -10 to -15°C.

The pots are made from plastic and are about 50cm high and 30cm wide. They have holes in the bottom so the roots don't sit in water.

• The first pot is a "Jakobsberger" plant with white grapes. It is loacated against a wall facing south with sun from late morning until early afternoon (Position 1 in the image below). Behind the wall is an elevator shaft, so it might be a bit cooler than a normal house wall. The wall should provide some protection from wind. The first pot could easily be moved as it's climbing aid is free-standing.
• The second pot is a "Moldawa" plant with blue grapes. It is located against a metal wall facing west with direct sun somewhat longer than the first plant (Position 2 in the image below). The metal wall is free standing so it provides no protection from temperature changes. As the plant grows along the top side of the wall it has little protection from wind. Moving the plant would be a bit more difficult (due to the same reason of "growing along the top side of the wall").

I though about buying several meters of bubble wrap and wrapping it around the pots multiple times as an isolation. I hope this way I would get a reusable solution that doesn't take up too much storage room during summer.

Do you have other (better) suggestions?

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Remark: Alternative locations for the plants would be:

• Both in the same location (one of the two described above). 1, 2
• Next to the balustrade facing south (Probably quite windy, sun from morning till afternoon) 3
• The elevator shaft wall facing west (Almost no sun in winter). 4
• The house wall facing east (our flat behind the wall, no sun after late morning). 5
• In the entrance area of the roof garden (Our flat behind the wall, not much wind, almost no sun) 6

Answer 1

You're a bit colder than we are in the UK (well, usually) but I'd leave the plants where they are if it's too difficult to move them, stop watering as winter arrives, and use cheap wood (wooden fruit pallets if I can get hold of them) fixed together to create a sort of 'box' around the pots. The 'box' should be a few inches higher than the pots, and about 6 inches away from the outside of the pots. Pack the gap, and over the top of the compost, with straw or shredded newspaper. I usually add a large sheet of plastic cut to fit round the stem of the plant, left draped over the outside of the 'box' which helps to keep out water, but inevitably the wadding you use inside (newspaper or straw) gets wet, so I often find myself replacing it during the course of winter. If the wadding gets very wet, then that's at risk of freezing too, of course. Sheets of glass keep the moisture out better, but we don't all have old sheets of glass or perspex lying about for such a purpose. You can use cardboard instead of wood, but it's likely that will need to be replaced that much more often over winter.

The other option is a roll of heavy duty horticultural fleece and bubblewrap - wrap the pot, and over the top of the compost, with 2 or 3 layers of fleece, then wrap bubblewrap over it, but the bubblewrap should not really be tightly fixed over the compost, up against the stem. The fleece also gets wet though, so again may need removing, drying and replacing, or just replacing during the course of winter.

It's easier and more effective if you can put both plants next to one another and create a single 'box' around both, packing the gap between the two pots with insulation material too. I find this method of overwintering pots more effective if the plants are against a housewall, out of wind, and in any sun that's available during winter. Under an overhang of eaves or a projecting part of the house structure to keep off some of the precipitation during winter is also very helpful, but it seems that may not be an option for you.

Note that temperatures of -15 deg C for a couple of days won't cause freezing of the compost, particularly if the compost is not very wet, but extended periods of those sort of temperatures of, say, 5 days, especially if daytime temperatures don't get above freezing, and your pots are in complete shade, is when the pots are seriously at risk of freezing solid. During milder spells, take the chance to unwrap and dry out, or clear insulation material and replace as it gets colder again. In other words, you can't really just choose a protection method and then ignore it all till spring appears, unfortunately.

Note that a blanket of snow is a good thing - it, too, has insulating properties.

Answer 2

You cannot keep the root ball from reaching outside air temperature but you can reduce wind chill and exposure to fluctuating temperatures. Consider these ideas:

• relocate the plant and pots to an area sheltered from the prevailing winter winds
• avoid locations exposed to point heat sources from the building. (ie not outside the exhaust from a dryer vent)
• avoid sunny southerly locations that could cause a thaw/freeze cycle
• water the plant thoroughly before freeze up
• mulch the base of the roots with peat or similar compost

Bubble wrap will cut the wind but will keep any moisture inside as well. You want to minimize cold drying winds but allow some air circulation. How about a big cardboard box?

Answer 3

For the more rugged grape varieties, keep them pot-to-pot and well-drained. You should mulch them both above the soil and around and between the pots. Do not fertilize. Check the soil moisture and water if required.

Don't introduce electric or other heat to the plants. This can curtail dormancy and trigger premature growth, which will stress the plants.

If temperatures are forecasted to fall below +15 degrees Fahrenheit, place a lightweight plastic or cloth cover over the plants gently. Remove this cover when temperatures rise above 20 degrees or so.