Not many citruses can survive the frost, but trifoliate orange is an exception

By Lars Lund
l@rslund.dk

Soon the first night frost is expected, and the citrus plants will need more protection from the cold. Kumquats, which is one of the popular citrus varieties, can withstand frost down to 12 degrees, however, if it is already bearing fruit, the fruit will get damaged. I usually harvest kumquats twice a year. One time in July and once again in February. 

In mid-September I move my citruses inside the greenhouse. They have been outside since mid-May and benefited from the summer sun. The kumquat should be kept in the greenhouse until about the 1^st of October depending on the weather forecast. In October and onwards, I then keep the plant in my bedroom. It is placed next to a large glass door facing west so the plant can get as much light as possible. I keep the temperature at about 17-18 degrees Celsius. This is the best temperature for the plant and the small fruits can continue growing. In February, these fruits will be ready for harvest. A delicious marmalade, a good liquor and candied kumquats are all rather tasty.

The other sorts of citruses I have, are placed in my shed with windows. I call it my ‘plant hotel’. Here they get by fine. The temperature rarely gets below 5 degrees, and in general, it is between 10-12 degrees. Remember to air out especially if you have multiple plants in the shed. The humidity can sometimes be so high resulting in the plants rotting. In fact, it is more likely that a plant will die from rot than from the cold. Alternatively, you can keep the citrus in the greenhouse as long as you have insulated with bubble wrap and also heat the greenhouse in periods of frost. Especially the pots must be insulated and placed on top of some polystyrene.

The Bitter Orange

If you prefer to avoid all this moving around or maybe you do not have a lot of space for winter storage, then here is the solution. You simply need to get a trifoliate orange. The scientific name for this citrus is ‘Poncirus trifoliata’. It is also called the Bitter Orange. The trifoliate orange can be kept outdoor all year around and can become a decorative element in your garden. The fruit has a bitter taste – some will even say inedible – but I do not share the same opinion. You can for example use the fruit in marmalade or liquor.

In theory, the trifoliate orange can withstand frost down to 20 degrees, and if the temperature is 10 degrees below zero, you can cover the plant with a crop cover cloth. It is important to protect the root of the plant from prolonged frost. This you can do with straw or some leaves, but it is not an absolute necessity. 

The branches have long and thick thorns so if you want to keep the neighbour’s dog or cat out of your garden, you can have a hedge made of trifoliate orange. The hedge will not be that tall (approx. a couple of meters) and do not expect the hedge to become more than 25 years. In May it will grow some beautiful white flowers which in contrary to many other citruses will not have a citrus scent. The fruit will develop small mandarins with a lot of seeds.

You can buy the trifoliate orange online or at your local plant nursery. When you transplant it, it will need soil with a slightly acid pH and the soil must be airy. This means that the soil for example contains peat or a substitution of peat. In the growing season it will need fertilizing.

The sun can be harmful and damage many of your plants if it is frost. This includes the trifoliate orange. Plant it somewhere that is sheltered and preferably a place with a bit of shade. Also protect the plant from the strong winter sun in periods of frost.