Greenhouse Vegetable Gardening (2015)

Not all plants make it through a Swedish winter; they need protection to survive the cold. There are several ways to do this, and the amount of insulation and shelter plants need depends on the types of plants you’re growing, and where you live. The biggest hurdle for most plants isn’t the cold itself, but abrupt changes in temperature. Many plants survive just fine in cold weather and snow; it’s an early spring thaw that melts away the protective snow cover from the plants, followed by a cold snap or freezing, that kills the plants. They need protection against direct cold, and also against moisture. For insulation to work at its best, it needs to stay dry.

You must also remember that roots are more sensitive to cold than parts of the plant that are typically aboveground. The soil acts like a large storage facility with heat, as it will never be as cold as the air and the wind, even if the soil freezes. Potted roots freeze faster than roots in the ground, so even the potted plants need insulation.

A wheelbarrow full of potted plants.

Protecting your plants against the elements

The main concern in overwintering plants is that in the fall, they must all be moved into the greenhouse. Keep the soil in the pots slightly moist, and prepare insulation according to each plant’s level of sensitivity. Once spring arrives, their covers must be removed and the plants put outside to stop them from sprouting leaves and flower buds prematurely. For early flowers, remove the insulation but leave the plant in the greenhouse, and start watering. Keep in mind, however, that there’s always an inherent risk in letting plants flower early in the greenhouse: if the temperature dips low overnight, the early growth might freeze, so be prepared to cover the plants with a row cover if the weather forecast predicts frost, or keep a thermo cube frost sensor activated.

The right time to move the plants into the greenhouse in the fall also depends on where you live, but more often than not it will be when leaves start falling and plants go dormant. Chill-sensitive plants, like dahlias, need to be moved inside a little before the first frost. Many woody plants, on the other hand, actually need a touch of frost to help them go dormant, so they should be left outside for a few weeks after the first frost. Perennials should not be fertilized in the fall, as this will entice them into start growing again.

The advantages of a greenhouse

The greenhouse is a dry place where no rain and snow can seep through. Dry soil doesn’t freeze as hard as wet soil, and there’s also better shelter from bitterly cold winter winds. You can shield your plants against harsh outside conditions further by insulating your containers, wrapping up plant stems and branches, and adding extra layers of insulation around the whole plant.

A greenhouse with overwintering plants.

You can also avoid the hazards of repeated dips in temperature by sequestering several plants in a separate, slightly heated, area of the greenhouse.

Individual plants are more or less sensitive to cold, and this needs to be taken into account when you set up your overwintering plan. The hardiest plants can be set close together, which guards them against the cold. You can take it a step further and bury the plant roots in the ground—with or without their pot—and cover the ground with leaves. Thanks to the dry atmosphere of the greenhouse, the leaves offer great insulation against the cold.

Similar measures can be taken by placing potted plants close together in boxes, and packing the boxes with mineral wool, packing peanuts, straw, peat moss, or other types of insulation—which all work well in a dry greenhouse. Covering plants with peat moss outside would be catastrophic: the peat moss absorbs large amounts of water that would then freeze into a thick layer of ice; if the ice were to slowly melt, it could suffocate the plant. Wrap the plants with plastic-covered mineral wool mats or pack them in straw, wrapping the lot in netting to keep the straw in place. Pliable ground cover mats offer good insulation too, and can be shaped into stiff, protective cones around plants. Thin fiber row covers wrapped loosely in many layers also work well, and are excellent for layering over groups of plants in boxes. Dry leaves, fir tree branches, burlap, shading cloths/sails, bark chips, and sawdust are all useful materials for insulation. If the weather turns unusually cold, stash the plants in a box with a sheet of polystyrene at the bottom; the mat will prevent the cold from making its way up from the ground through the bottom of the box to freeze the roots.

Different kinds of small trees are also highly sensitive to cold. Plants made up of a trunk and a crown of bushy foliage, like standard roses, standard black and red currants, and the like, need extra protection around their crown. The grafting area—the point at which the trunk meets crown—is more cold-sensitive than the rest of the plant. Even a hardy standard growing in the ground needs its crown wrapped in burlap, textile cover, shadow cloth, or similar type of blanket.

Which plants you decide to bundle up will depend on your geographical location. In the south of Sweden a fig tree overwinters outside, whereas in the middle of the country it needs to be wrapped up. Rosemary will survive outside in some—but not all—areas of Skåne, the southernmost province of Sweden. Garden thyme tends to die both in the south and north of Sweden. Ultimately, the best way to proceed depends both on your type of plant and your place on the map, so ask your neighbors, members of a gardening club, the employees of your local garden center, or plant nursery for advice. While there are no sure-fire guarantees, and there will always be wild cards like the weather, it’s always valuable to glean from other people’s experience, and beneficial to share what works and what doesn’t with others.

Greenhouse winter insulation

You can insulate the greenhouse itself to overwinter your plants. You can place a smaller, bubble wrap-like plastic greenhouse (that can be heated) in the larger greenhouse. All non-evergreens can be overwintered, spaced close together for protection against the cold. Evergreens, on the other hand, need more space—they require a proper amount of light year round and thus cannot be crowded.

There are many ways to protect non-hardy plants from the winter cold. It is of utmost importance that all insulation material stays dry.

The low winter sun brightens up the greenhouse.

The ideal temperature at which to overwinter flora varies according to the plant. Even if it were easily available, most plants—with the exception of orchids and other tropical plants—don’t do well in tropical heat and light during the winter season. Most of the non-hardy plants we cultivate prefer cooler winters, albeit not as cold as Swedish winters. This goes to show that warmer isn’t always better, and that you’ll need to find the point at which most of your plants will survive. Plants that tolerate a variety of conditions give us leeway to choose the overwintering method that’s most convenient for our space. Geraniums can be kept in well-lit and cool environments, but also do fine in cool and dark sites. If they’re overwintered in the light they will grow and may even flower; if they’re kept in the dark they will go dormant, which means the temperature will need to be lowered to nearly 0°C (32°F).

The list on page 115 shows recommended overwintering temperatures for an array of plants.

Lily of the Nile—African lily

Recommended temperatures (however, not guaranteed) for overwintering less hardy plants

Lawn daisy.