HOW TO OVER-WINTER CITRUS PLANTS OUTSIDE
With the effects of global warming becoming ever more prominent in the news, our seemingly milder winters are making possible for those living in a northern European climate to grow some of the hardier varieties of citrus plants. But not just outside over the summer period, with a little forethought and preparation you can successfully over-winter them too.
The talk of citrus plants thriving in our northern climate does sound a little challenging. This is especially so when you consider that the citrus genus originated from the areas of India, Indonesia and southern China where the climates are more tropical rainforest. However, given the right conditions, varieties such as the Meyers lemon, the Satsuma, Mandarin, Clementine and Calamondin are capable of surviving temperatures as low as -9°Celsius, although to be fair they may not look like much come the spring.
Perhaps the biggest factor on whether your citrus plant will survive the winter is soil type and positioning. Citrus like a rich, slightly acidic, free-draining soil that will get as much sun throughout the day as possible. If you intend to grow your citrus as a wall shrub you can increase the available light by fixing a mirrored surface to the wall behind your trellis or wire work. If the use of glass is unacceptable due to risk of damage then consider the following alternatives as mirrored acrylic, aluminium sheets or even humble tinfoil will also do the job. Alternatively, if you have a well positioned pond you can plant your citrus near the southern end of it to make the most of the additional reflective light.
WHERE TO POSITION YOUR CITRUS PLANTS
Cold air wafts downhill, so planting citrus on the higher ground will be - to some extent - warmer than a position at the bottom of a slope which can also carry the risk of being a frost pocket. Planting on the south side of your garden is important as this will allow the plant and surrounding soil to make the most of the ambient heat and sunlight. The positioning and distance that your citrus plant is from the house can also be a crucial factor and is often overlooked. Plant your citrus as near as you can to the south side of your house but without leaving it in the shade. Most cold weather will come down from the north and northwest so you will be using your house as a windbreak, forcing the cold air up and over your citrus trees and leaving the area near the south side somewhat warmer. The house itself will also radiate a certain amount of heat, some of which will be absorbed by your adjacent citrus plants. Overhanging trees can also maintain a slightly higher air temperature within their canopy and will also reduce the incidence of ground frost. Although this does give a clear over-wintering temperature advantage, growing citrus plants beneath other trees is not a fantastic option as their summer shade will result in poorer growth and lower fruit yields.
PREPARE THE GROUND FOR OVER-WINTERING
The ground below and around a citrus tree should be completely free of weeds, grass, mulch or anything else for that matter during the winter period as these will all act as an insulator barrier. This will prevent solar heat from entering the soil during the day which will mean that there is less heat stored for release from the soil at night.
Moist soils will absorb more heat than dry soils, so trees should be carefully watered 2-3 days before a bad frost or freeze is predicted. Water too much though and you will risk water-logging the root environment which can cause root death so make sure that before you plant your citrus you have provided a free-draining soil!
Good tree health and nutrition is also important when it comes to helping citrus trees withstand freezing temperatures so make sure that you follow a good maintenance regime during the growing season. Fertilization and pruning should come to an end in August or September as this will allow the trees to harden off completely before severe frosts are encountered.
Young citrus trees should be ‘banked’ for the first 3-5 winters until they are mature enough to withstand the colder temperatures. Banking is a technique where the surrounding soil is pulled up into a mound around the tree to cover the bud union and lower trunk. Even if the exposed parts of the tree are completely killed off through cold damage, the bud union under the bank should still survive to grow back in the spring.
Citrus trees may be covered temporarily with blankets, quilts, paper or other material as a protective barrier against hard frosts or snow. Remember to remove all covering protection in the morning to allow citrus trees to take full advantage of the warmth and light – however weak – from the available sun.