Winter greenhouses: overwintering plants in an unheated greenhouse
A greenhouse is a real asset to a garden. It can in fact even be used in winter to overwinter plants, a bit like you would use a conservatory.
A greenhouse is not only useful for creating ideal growing conditions but also for getting non-hardy plants through the cold season. However, there are various aspects to consider. Read on to find out which plants can be overwintered in a greenhouse and the best way to go about it.
Using a winter greenhouse: requirements
If you want to use your unheated greenhouse in winter, insulation is essential. Bubble wrap is ideal for this because it is translucent and easy to attach. It can be attached in various ways. There are commercially available adhesive tapes and fastening systems for mounting hooks on the greenhouse to hang the bubble wrap when needed. There are differing viewpoints as to whether you should attach the bubble wrap to the inside or outside of the greenhouse. While the layer of bubble wrap is more exposed to the weather on the outside, reducing its durability, condensation often forms between the bubble wrap attached to the inside and the greenhouse. Overall, it is often recommended that smaller greenhouses be insulated on the outside and larger ones on the inside. This is because a larger greenhouse has more surface area exposed to environmental factors, as well as the added cost of renewing the insulation. It is to be expected that bubble wrap attached on the inside will remain intact for longer than when attached on the outside.
Polystyrene is only suitable for insulating to a limited extent because it is not transparent. However, it can be placed under planters to provide insulation from below. You should also make sure that the greenhouse is well sealed. Small gaps can be sealed with silicone.
The temperature inside a greenhouse depends on various factors such as the material, the insulation and the amount of sunlight. Plastic greenhouses are usually better insulated due to their lower thermal conductivity. Although the temperature in an unheated greenhouse is usually only a few degrees Celsius higher than outside, this can make a significant difference and prevent plants from freezing. If it is very cold outside and below roughly -5 °C, you can protect your plants with horticultural fleece. As such, a greenhouse in winter does not provide good growing conditions for most crops, but can be a great place to overwinter plants. A bubble wrap greenhouse is not so suitable for overwintering cold-sensitive plants. However, it can be used to protect hardy plants from waterlogging.
If your region typically has very cold temperatures in winter, it may be worthwhile to invest in a frost monitor. This is a temperature-controlled heater that turns on as soon as the temperature is about to drop below zero. However, this method is not particularly environmentally friendly. Storing the plants in the cellar or garage is more sustainable.
If you urgently need to protect your greenhouse from frost and cold temperatures, you can also use a homemade heater. To do this, place a large candle under an upside-down terracotta pot with a hole. Before doing so, make sure that there is no flammable material nearby. This heating method is unlikely to cause a considerable increase in the greenhouse temperature, but even the slightest increase can be enough to prevent plants from succumbing to the cold. Keep in mind that one candle can only heat up a maximum of 2m². While it is possible to set up several heating candles, you should consider the sustainability of this approach. We only recommend using this method to bridge short periods of frost.
Generally speaking, you should be giving your greenhouse a regular clean, but it is especially important if you want to use your greenhouse over winter. Clean it thoroughly in autumn and spring to prevent wear and tear and to ensure that as much light as possible can enter through the clean panes during the darker months. Also ensure that your plants have a sufficient water supply in winter; they usually require less water in winter than the rest of the year. Lukewarm water is best for watering. If you place your water storage tank in the greenhouse, it can also act as a heat buffer. Make sure to ventilate on frost-free days, as excessive humidity encourages plant diseases.
Overwintering plants in an unheated greenhouse: points to bear in mind
- Insulation: sufficient insulation is crucial for using the greenhouse in winter — bubble wrap is a great insulator because it is transparent and easy to install.
- Greenhouse material: plastic greenhouses have better insulation due to their lower thermal conductivity, offering more protection against the cold.
- Additional insulation or heating: in very cold temperatures, it may be necessary to use horticultural fleece or a self-made heater. Keep an eye on the greenhouse’s temperature.
- Cleaning: clean regularly to prevent wear and tear and increase light transmission.
- Water supply and ventilation: adjust watering to the plants’ needs in winter and ventilate on frost-free days.
Most plants have lower nutrient requirements in winter and should not be fertilised. You can start fertilising again as soon as spring arrives. Our Plantura Liquid Citrus Food is ideal for many plants that overwinter well in unheated greenhouses. Our primarily organic fertiliser is safe for pets and garden animals and can be easily applied when watering.
- Perfect for all citrus plants & Mediterranean plants
- Liquid fertiliser for healthy plants & aromatic fruit
- Quick & easy application - child & pet friendly
What to grow in a winter greenhouse?
A winter greenhouse can be used to grow Mediterranean plants such as oleanders (Nerium oleander), lemon trees (Citrus x limon), olive trees (Olea europea), herbs such as rosemary (Rosmarinus officinalis) and thyme (Thymus vulgaris), as well as small fruit trees. It is easy to keep pot plants in a winter greenhouse, just make sure to move them in good time. If you bring them in too early, they will get used to the warm temperatures and will find it difficult to acclimatise later when the temperature drops. If, on the other hand, you are too late, you risk frost damage as the plants will get too cold outside. It usually makes sense to move your plants into the greenhouse in around September or October. You can also keep vegetables that store well such as kohlrabi (Brassica oleracea var. gongylodes), carrots (Daucus carota), leek (Allium porrum), winter radishes (Raphanus sativus var. niger) and celeriac (Apium graveolens var. rapaceum) in the greenhouse until you are ready to consume them. Refrain from washing any vegetables you wish to store. Root vegetables especially can be stored very well in soil, such as in a container. Make sure this soil and the soil of any plants overwintering in the greenhouse is free of snails and not waterlogged.
Did you know that you can even grow some vegetables in a greenhouse in winter? Check out our article on what to grow in a greenhouse in winter to find out more.
