Orange trees can be a great addition to your garden or backyard. Precautions need to be taken however if your wish for an orange tree to grow and flourish. A dreaded frost could put the life and health of an orange tree in jeopardy.
The best way to protect your orange tree from frost is by considering the location and time of year that you plant your tree. Lighting, coverings, proper pruning and constant watering mechanisms should also be used in the fight against frost damage. Additionally, you should make sure that the tree you plant has a frost resistant rootstock.
Location of planting
The location in which you plant your orange tree can have amazing preventative effects on it receiving frost damage. You should consider planting your tree on some of the highest points in your backyard, as the air is slightly warmer at an elevated level compared to a lower surface. Having the tree exposed to the sun from a south westerly direction is also beneficial because this means it will potentially more sun. It goes without saying that you should bare in mind the sunniest positions in your backyard or property and consider planting you tree there. Additionally, planting your tree by a south facing wall that receives a lot of sun could also be beneficial for your tree. The wall in question may retain the heat from the sun and heat up the tree during the colder evening.
Lights can be attached to the tree in order to warm it up during the night so is it is less likely for it to receive frost damage. You can place a 100 watt outdoor light inside the canopy. Once attached, you can turn this on during the night. Be extremely responsible that the light doesn’t come into contact with water. If it does come in contact with water this could prove very dangerous, so caution needs to be taken. An alternative to a powerful outdoor light is to just use Christmas lights that are durable in outdoor conditions. These Christmas tree lights can be wrapped around the trunk and around the canopy branches itself. This can be a great idea in and around the run up to Christmas and the new year. Not only are these lights providing some needed heat for your orange tree during one of the coldest periods of the year, they also are a great decorative feature during the festive season that your neighbors could admire.
Small to medium-sized trees can can have wooden steaks or plastic pipes around the tree at equal distances, each a few inches from the tree’s corners. A sheet of plastic or frost cloth should snugly be able to wrap around them and the tree. Make sure that the number of pipes are about 12 to 24 inches higher than your orange tree. This would ensure that covering should easily be able to go over the top of the tree and cover all of it. This simple improvised cover protection, even with plastic sheeting, will do a world of wonders in the prevention of frost and freezing conditions from negatively affecting your tree.
If the tree is very small than you may just wrap plastic sheeting around the entirety of the tree, as it it may be the easiest option. At the other end of the scale, if the tree is too big to be protected by a frame covered in protective frost cloth or plastic, then you may wish to just cover the trunk and the most important branches with protective cloths and plastic sheeting.
Unfortunately freezing conditions can do a lot of harm to the leaves and branches of an orange tree. If a bad frost afflicts your tree than many parts of your tree could be killed. However, you should wait a few months before trying to distinguish what part of your tree are alive and what part is deadwood. By waiting until the end of spring or into the summer you will clearly be able to see what parts of the tree is dead. Scraping the bark on the part of the tree that you may think is dead can also provide an answer as to whether this tree part is is alive and not. If there is green beneath the bark then it is alive, if not then it is probably dead.
Time of Planting
You should hold off planting your orange tree until about February or March. The springtime could be a preferential time to grow your tree, because if it is grown earlier in the winter a young tree mightn’t be able to cope with the freezing cold weather. If you purchase your orange tree during the autumn time then you can always just place it in a container and leave it in a warm area near the house. If things start to get really cold then the orange tree can be easily moved inside the house. When the worst of the winter weather is over you should then try and move your orange tree to the outdoors. Although, this will do little to prevent the tree from being damaged or frozen in the future, but it will greatly reduce the chances of suffering from frost damage within the first year of you purchasing it which is often the time when such trees are most susceptible to frost damage.
If you water the ground below the canopy then this may help in the fight against frost damage. Moist soil is better at capturing and retaining heat then dry soil. Sprinkling the leaves with water throughout the night can also be at effective anti-frost measure. It needs to be highlighted that the water should continuously flows through out tonight and that there is no break in the flow of water until the morning. By doing this the leaves will be kept at an increased temperature and protected from the frost. This constant flow of water can turned off once air temperature has increased from sunlight. The temperature for stopping the flow should be when the air is at about 37 degrees Fahrenheit. 
Freezing conditions mightn’t only affect your tree’s branches, but it can also affect tree roots. Therefore, it is favorable if you have a rootstock that can withstand frost and doesn’t become damaged with cold weather. There are several different types of rootstocks that you may consider for your orange tree before you plant it. Trifoliate orange and sour orange are examples of just two rootstocks that you should consider for your orange tree. With that being said it is best to talk to your local nursery or garden center about what rootstock to use. Unfortunately, it is often times hard to get the rootstock that you wish for your tree. You may only be provided with a select few. Therefore, you should be willing to shop around and look for businesses that offer a wide variety of rootstocks. If one of them happens to offer trifoliate orange and sour orange rootstocks then you will be in luck.
 The University of Arizona, College of Agriculture and Life Sciences, Protecting a Citrus Tree from Cold (March 2001)