UK winters can be brutal. Winter 2022 was one of the worst winters I have had in the UK and we lost a lot of plants. I don’t know of anyone that did not take a hit, both heated and unheated. From a silver lining perspective, hard lessons were learnt and will hopefully not be repeated.
When it comes to getting your babies through the winter, night temps are not much of a concern. It is the day temps that should be monitored, and actions made based on prolonged cold day periods 0c or below. If the day temps remain cold, the plants do not have enough time to thaw out and stay frozen, this is when things go pear shaped. A few very cold days will be fine for most Trichocereus and Lophophora.
There are 3 main things you need to protect your cacti from: extreme cold, frost and fungal spread (lack of airflow).
Keep an eye out for any frost forecasts, frost will turn trichos and lophophora into mush at worst or burn the plants at best.
So, here is my take on overwintering to try and help you avoid any of the same mistakes I have made in the past?.
There is nothing wrong with growing all year round, we just do not have the weather for it and running artificial lighting will get expensive.
For the UK climate, it is better to get the plants cold enough, so they stop growing all together, this is around the 10c mark. Anything below that and the plants will shut down and stop growing all together. The main advantage of this would be to avoid etiolation (skinny top growth).
Because the sun is not strong enough and it is not light for long enough, the plants will start stretching for resources (sun and heat). This will result in the skinny top growth and a weaker plant.
As I primarily focus on Trichocereus and Lophophora, this content is geared around them specifically. These concepts will apply to most cacti, but it is always best to research the temp tolerance levels and if the plant is not cold hardy, alternative arrangements will need to be made (more on this later).
Trichos can handle the cold, more than most. I have had plants survive -5c temps before, albeit for short periods.
The trick is to try and keep temps above 0c to stop the plants from ever freezing in the first place.
Lophophora are less hardy than trichos but can still handle the cold for shorter periods.
Ok, winter is coming (yup, I did ?). The key to a successful overwinter is to adequately prepare as being reactive will generally be too late.
If you have some high value plants (I realise they are all valuable), I will always recommend bringing them indoors for the winter, this applies to Lophophora mainly. If you have a garage, stick them in there for the winter (while ensuring adequate airflow).
If you do not have a garage, any cold spot in the house will do the trick. Try and keep temps below 10c mark for effective dormancy. If these are not an option, a south facing window will do the trick but the plants will unlikely go into dormancy due to central heating. In the case, you would need to lightly water the plants a couple times over winter, just make sure they can dry out completely or you risk the roots rotting.
When it comes to greenhouses, the most effective way to keep your plants safe would be to use a heater (duh). Obviously, there are costs associated to heaters.
As an example, using the energy price cap (34p / kWh), running a 2kw heater will cost around 70p per hour of usage. Obviously, with a thermostat, the heater won’t be on all the time. The costs could add up quickly if it is a particularly cold winter. This is still the best and most sure way of protecting your plants over the winter.
A slightly more energy efficient heating would be those low energy pipe heaters. They will not heat a full greenhouse but would do the trick next to seedlings to keep them from freezing.
The other option for standard heating would be propane heaters. I am not a fan of gas heaters at all, the fumes really stink up the place and are harmful to the plants. But, as a last resort, will do the trick.
For most of us, conventional heating is not really an option (thanks energy crisis / living crisis) so we are forced to get more creative with protecting our babies over winter.
Unheated Greenhouses (Low Tech)
Horticultural Bubble Wrap
The first thing I would recommend doing is bubble wrapping your greenhouse, this creates a double-glazing effect and traps warm air between the bubble wrap and the side of the greenhouse. This is an effective way of keeping the temps steady and trapping warmer air inside the greenhouse for longer.
The key is to properly seal the bubble wrap and have as little gaps as possible for warm air to escape. I use a clear polyethene tape to take the pieces of bubble wrap together.
One thing to note and I will be reiterating this a few times over the article is to ensure adequate airflow, fresh air exchange is super important to keep fungal infections at bay. Open the doors during the day and close in afternoon.
Fleece For Frost Protection
Fleece is good to protect against frost. Fleece creates a barrier between your plants and the icy frost and can save them from the worst of it. Please make sure that the fleece is not touching any of the plants as that will nullify and protection as the moisture will absorb onto the plant through the fleece.
If you do not have fleece at hand, you can use blankets or even newspaper (untested so cannot confirm effectiveness).
A bit unconventional but very effective is to use cups for the tips of the plants, this will protect them from frost damage and the tips aborting. I used them last winter and it proved to be very effective.
Water Tanks / Bottles
Water tanks or bottles can be filled with water and left in the greenhouse, the water will warm during the day and slowly release heat over night. They will not make a huge difference but is good for a couple degrees. Remember, we are not wanting anything more than to keep the plants from freezing. Painting them black will help the absorption of heat.
Composting generates energy (heat) when the plant matter breaks down, this can be very effective at generating a fair amount of heating for the greenhouse.
You can go the traditional route using straw and manure, but I find the smell to be extremely off putting. Another option would be to use a pile of leaves in a bucket and let the composting do its thing.
A word of warning here, as we are working with organic matter, bugs and pests become more relevant. This is mitigated in largely by the fact that it is winter and most bugs etc. go dormant themselves.
We have covered ways to protect your plants against the extreme cold and frost. The next thing you need to be wary of is still air. Still air allows for fungal infections to run rampant. If left unchecked, you could lose a whole greenhouse in the matter of days.
Most cacti have fungus present on them all the time, it is airflow that keeps the fungus from spreading and causing hassles.
Whatever you do, please ensure you are providing adequate airflow to your plants regardless of where you are keeping them.
I have mentioned airflow a few times, I really hope this is sinking in. Protecting against the cold / frost only could result in a still air environment. Fungal infections thrive in still air environments and if left unchecked, it will plough through your plants and will result in crying and feet stomping.
If in a greenhouse (heated or unheated), it is best to open the doors daily to allow for fresh air exchange, you can close the doors in the afternoon to allow for some heat to build up.
Inside (heated or unheated)
If by a windowsill, prop the window slightly open for a bit during the day. if in something closed off like a garage, open the door daily for a few minutes to swap out the stale air. You can also have a fan in there to circulate air around the room.
Greenhouses that are too overcrowded can also present some challenges with airflow, try and provide adequate space between plants to allow for air to circulate. Unfortunately, this is not a realty for me so I ensure loads of airflow to mitigate that as much as possible.
Overwintering can be a stressful period and is repeated yearly to make matters worse. If you have the budget, traditional heating is always the best option. Outside of that, your main goal is to stop the plants from freezing and from frost using some or all the above suggestions (shotgun approach).
To end, most winters are quite tempered in the UK from my experience. If we have a winter like 2022, low tech and unheated routes generally result in one or two casualties so please just be aware of that and prepare yourself for some losses. All we can do is try and minimise as much as possible.
Let’s just pray we have a not so extreme winter this year.
I would love to hear about any other options available. If you have suggestions, please feel free to discuss in the comments.