Guide written by:
John, Passionate gardener, Cambridge
While some plants can happily spend the winter outdoors, others will only tolerate brief periods of frost. Non-hardy plants, on the other hand, must be kept indoors. Whether you have a heated or unheated greenhouse, follow our guide to keeping your cold-sensitive plants safe over the winter.
- Plant storage
- Watering and ventilation
- Plant health
Wintertime: when to take in your plants
Non-hardy plants can be split into three categories.
Tropical plants are the most sensitive to the cold. Hibiscus, dipladenia, bougainvillea and orchids must all be taken indoors as soon as the temperature hits 10°C which generally happens around the start of October. These types of plants enjoy full light with consistent temperatures, whether this be provided by a conservatory or a greenhouse. Set up a greenhouse heating system for these plants.
Cacti prefer to be taken indoors before the start of the autumn rain which they dislike as much as the drop in temperature. You can leave them in an unheated greenhouse.
Mediterranean plants should be the last of your plants to go indoors and this should be done around the end of October to mid-November, depending on local climate. Frost-free conservatories or greenhouses kept at temperatures of 3 to 5°C are ideal for most species including citrus plants (lemon and orange trees), oleander, birds of paradise, mimosa, canary island date palms, and so on.
How to store your plants in a greenhouse
Firstly, you should only bring healthy plants into the greenhouse as, in confined spaces, diseases or pests can easily be spread to neighbouring plants. Inspect all above-ground portions of each plant, including stems and leaves, making sure to check the undersides as this is where pests often hide.
Remove any diseased leaves and treat plants as needed using an organic insecticide before taking them indoors.Space is often limited in greenhouses. However, you can work around this by following a few recommendations.
- Make space by pruning plants. Some, such as fuchsia, geranium or brugmansia, will tolerate a good cutting back.
- Do not position your plants too close together to allow the air to circulate between them.
- Be careful not to place your plants too close to glass or plastic walls which can get very cold at night or, conversely, may lead to sunburnt leaves.
Be sure to monitor and care for your plants
Monitoring plant health
Greenhouses are confined spaces making them the ideal environment for disease and pests. As such, you should monitor the health of your plants.
Assess them from time to time, making sure to remove any dry or mouldy leaves.
Monitoring the temperature
So-called cold greenhouses will not provide enough protection for the coldest periods. You should therefore insulate your plants using either a frost fleece or bubble wrap. If you go for the latter option, be sure to avoid leaving the wrap on for too long as – unlike frost fleeces – this material does not allow for air circulation. You can also protect your plants by wrapping them with straw or other natural fibres. You should always have a heating system on hand.
Heated greenhouses can be a godsend for gardeners, but remember that, at the end of winter, the sun alone can provide considerable heat through glass. Some light shade should be provided during this time.
Hang up your garden hose for healthy plants
Do not fertilise your plants over the winter and remember to only water your evergreen plants sparingly.
Always let the soil or compost dry out between waterings. Citrus plants will require more regular watering as they do not lie dormant over the winter. On the other hand, succulents and cacti should be kept dry.
The importance of greenhouse ventilation
Over the winter, the level of humidity inside the greenhouse will always be higher than that outdoors. Ventilation is therefore essential for preventing the outbreak of disease.
Make the most of milder days to air out your greenhouse. Otherwise, try to wait until temperatures are above zero. Just a few minutes of ventilation will do. If your greenhouse is heated, it can be tricky to find the right balance between heat and ventilation when outdoor temperatures are particularly low.
Heated or unheated, you should always open up your greenhouse as soon as the outdoor temperature exceeds the temperature inside the greenhouse.
When to take your plants out of the greenhouse
The right time to put your plants back outdoors will depend on the climate where you live.
Follow the opposite instructions for putting your plants into storage: meaning the last plants to go in will be the first ones to come out following the last frost. Tropical plants can wait until well into the spring to return to their spot in the garden.
Guide written by:
John, Passionate gardener, Cambridge, 57 guides
When I was young, I was already working in the family garden. Perhaps that is where my interest in plants and gardening came from. So, it was logical for me to study both plant biology and agronomy. At the request of various publishers I have, over twenty-five years, written many books on the subject of plants and mushrooms (a subject that is close to my heart).They were mostly identification guides at first, but shortly after they were about gardening, thus renewing the first passion of my childhood. I have also regularly collaborated with several magazines specializing in the field of gardening or more generally in nature. There is no gardener without a garden, I have cultivated mine in a small corner of Cambridge for the last thirty years and this is where I put into practice the methods of cultivation that will I advise you in as well.