Citrus trees vary in their frost-hardiness, and even a few degrees makes a difference. Some of the citrus trees most commonly grown in the Desert Garden, listed from most hardy to least hardy:Kumquat
Almost exactly two years ago, Arizona's Maricopa County experienced two or three days of killing freezes that reminded us of exactly how devastating a freeze can be. It was also an interesting demonstration of how much the different citrus trees can vary in their hardiness, and how location can make a difference.
But first things first: "hardiness" and "hardy" always refer to a tree or plant's ability to withstand cold and freezing temperatures. You'll sometimes hear the word misused in describing a plant's ability to withstand difficult growing conditions such as heat, drought, wind, poor soil, or general neglect, but, to be accurate, "hardy" refers only to frost-resistance. For a plant that has good performance under bad conditions, use the word "tough" instead; the plant will appreciate the compliment.
In the Great Freeze of 2007, just the citrus here at The Ranch provided a good illustration of differing hardiness.
After two nights of prolonged freezing temperatures, the Seville Orange trees looked a little off color, but suffered no fruit or foliage damage. The Minneola Tangelo lost tips on a number of leaves, but suffered no fruit or wood damage.
The lemon tree lost all of its fruit on the outer areas of the tree canopy, lost all of its flower buds, a few small twigs, and most of the leaves showed some frost damage. The Mexican Lime lost all of its flower buds, most of its leaves, and most of the branches above a certain point were frozen through and subsequently died; it was a mess (that's the lime tree pictured above).
If you look carefully at the photo, you'll notice that the lime tree is planted right next to a covered patio; that patio cover probably saved its life. The patio stayed just a couple of degrees warmer than the surrounding area, and the foliage and wood nearest that warmer air suffered no damage at all; that was also the only part of the tree that produced fruit later that year.
When you plant citrus, pay attention to the micro-climates in your landscape – those areas that are just a bit warmer or colder than the rest of the landscape. Plant the least-hardy citrus, such as limes and lemons, in the warmer and more protected areas. When your citrus are smaller, you can use frost cloth to protect them from damage, but all except the dwarf varieties will eventually grow too large to cover easily; planting them in the right place to begin with will make all the difference.