Lars Lund

15 Jan 2021 09:48

Heat for the delicate ones

This article gives you tips on how to protect your delicate plants. You can make your greenhouse smaller by building a small house of laths and bubble wrap. You can also buy a ready-made tent of bubble wrap that you can put up in the greenhouse, build a plant hotel or use different kinds of additional heat.


By Lars Lund


A wood stove in the greenhouse is nice, but as actual heating for the greenhouse's more exotic plants that have to survive through the winter, a wood stove is not the best solution. It provides warmth and cosiness on a cold autumn day, or if the greenhouse is used to make Christmas decorations. But if it is to serve as heating around the clock, you have to get up at night.


Although the climate has gotten a little warmer, one thing is for sure. Sooner or later the frost will come, and many of our non-frost-tolerant plants need to be protected. In the case of some plants, keeping them frost-free does not require a lot. Dahlia and fuchsia, for example, do not mind the dark, and most people have a garage or an insulated shed to use for the plants that are not depending on the light. Fortunately, when the plants are in dormant, the requirement for light is less, but there are also solutions for plants that need a little light. Your greenhouse is ideal as a room for overwintering plants. Most likely you do not have so many exotic plants they require the entire greenhouse to be packed. But some of the house must be wrapped. In other words, the greenhouse glass needs a layer of bubble wrap.


Large bubbles of air

The bubble wrap that insulates should be specially made for greenhouses. It consists of a UV resistant plastic film of 8 mm thick bubbles with air. Combined with the greenhouse's own glass, it gives a U-value of 2.4. This means that if you need to heat your greenhouse so that it is frost-free, you will save 45% of your heating costs if you have bubble wrap on the windows.


The U-value is a designation for how large a quantity of heat measured in Wh, passes through a structure of one square meter in one hour when the difference temperature between the inner and outer side is just a single degree of heat. The lower the U-value, the better the pane or material insulates. A pane with one layer of glass has a U-value of 6. The old, double-glazed windows with air between the glasses have a U-value of 3. Two-layered energy windows with the gas argon11 or a three-layered energy pane with ordinary argon between each glass have a U-value of 0.6.


Standard greenhouses have a single layer of glass. In other words, by setting up bubble wrap, you minimize the risk of frost. You can buy bubble wrap online at various greenhouse companies or in a DIY store. There may be a difference in quality even for bubble wrap with the same bubble dimension. If you shop in the hardware store, you should pull out 3 feet of the roll. The piece you pull out must not collapse. If it does, it has probably been squashed in a large container, so that the bubbles have leaked. If you buy online, you do not have the opportunity to check if it is ok before you have it, so if it has completely collapsed, you must immediately contact the supplier. My own experience, however, is that both the hardware store and online suppliers are careful not to squash bubble wrap together. Also, be aware that there can be reasonably large price differences.


Build a tent or buy a ready-made

How many bubbles you need to set up depends on how big your greenhouse is and how many plants you have. In the vast majority of cases, the exotic plants can stand on something resembling a pallet. It is a good idea to raise the plants slightly above the ground, put them on a pallet or a thick flamingo plate. Make a sample setup. Now you know how much material you need to cover your needs. The materials just need to be cheap laths or clamping strips. To attach the bubble wrap to the profiles of the greenhouse, you can find some special plastic fittings that hold the bubble wrap. Therefore, if you build your bubble house in a corner, you already have two walls. If you are really smart, you hang a wire in the roof of your greenhouse, on which you can lay bubble wrap. In that way, you lower the roof, making all or part of the space in your greenhouse smaller, but also stronger.


If you do not want to throw yourself into a construction project with laths and bubbles, you can buy small tents with poles and bubble wrap. However, you do not have to put together the tent. The tents that are stable and have good large bubbles also have many poles to assemble. It is easy, but it takes time, and if you do not have a lot of space after the winter, the tent must be disassembled again and packed down. Of course, so should the lath solution. I have tried both and personally, I prefer the lath solution.


Additional heat

It's been many years since we've had a winter for a really long time. Often it is only a few days in a row that the frostbites, and most often at night. When it is cold, the sun likes to show, and even if it is low, a greenhouse can quickly get a lot of free heat from the sun. On days when it is bitterly cold and even the catfish freeze, heat must be added to the greenhouse. There are both small and large petroleum heaters, fireplaces, gas heaters and electric heaters on the market. I have experimented with all types. Petroleum heaters do not always have clean combustion and use a lot of liquid. A fireplace burns cleaner. A greenhouse gas heater with a thermostat, turns the heat up and down by itself, has an output of 2.5 KW and uses 179 grams of gas per hour. Electric heaters are available with different effects, and the price of consumption depends on your electricity company. Both petroleum and gas provide water during combustion, and you must be aware of that. It happens that people forget to air out or place the plants too close so that there is no air around them, and then the plants rot. That is not exactly the intention. Despite the water, I personally prefer the small special gas heater with a thermostat. I use approx. 1 - 2, 11 kg bottles a year. In addition to placing plants in the greenhouse, I have also built an actual plant hotel. It is an insulated shed with energy windows. Both walls, floor and ceiling are insulated. In there I have an electric heater. Its thermostat is set to start if there is frost in the room. It has not happened since I built the shed about 8 years ago.


The plants

Figs, olives, eucalyptus, common flax are among the plants that one should worry about the least. As long as the temperature is not below ten degrees for a long time, they all tolerate frost. The fig tolerates more when it is in the ground outside or inside the greenhouse, but if it is in a pot, the pot should be wrapped in bubble wrap and preferably placed on a piece of thick flamingo. The same goes for olives and eucalyptus. I have permanently dug my two olives into the ground in the greenhouse and put leaves on the ground. If it is 20 minus degrees, I wrap them in bubble wrap for the period. Common flax planted in the ground tolerates being outside. When the sun is a little too harsh, they can be wrapped in a crop cover cloth. Especially in early spring, they can be damaged by the harsh sun.


My very large and evergreen African lilies do not like frost, but they take up a lot of space in the greenhouse and my small plant hotel. In recent years, I have therefore put them outside against the gable of the house and under the large eaves. As soon as I hear frost is coming, I cover them with bubble wrap. It has been going well for the last three years. Almost all non-evergreen African lilies tolerate frost.


I have no angel’s trumpets, but my mother-in-law always cut hers back, and place them in the basement, where the temperature is 15 degrees, to overwinter. They got some light from the window. There they will be with the geraniums. Some I also put inside my house. I always have my kumquat in the bedroom by a bright window, and there it thrives very. well and sets a bunch of fruit in January. In the bathroom are beautiful and red bell-shaped chillies. Should the smell in the bathroom become too strong, you can always blame the chillies.

Om Lars Lund

Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.  

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