Citrus in the low desert, month by month

A lemon tree in a Phoenix yard is not waiting out a winter. It is waiting out a summer.

That is the first thing to understand about growing citrus here, and it is the opposite of how the rest of the country gardens. The low desert’s mild winter is the reason citrus belongs here at all: no long frozen ground, no dormant season to speak of, fruit that ripens on the tree in December and January while the rest of the country is looking at bare branches. What citrus has to survive instead is 111 days whose normal afternoon is at or above 100 °F, with the normal low in July sitting at 84.5 °F, so the tree never gets a cool night to recover.

Then, for about twelve weeks, the problem reverses and becomes a cold one. Both of those facts come out of the same two datasets: the NOAA NCEI climate normals for Phoenix Sky Harbor, 1991 to 2020, and the station record at the same site, which runs back to June 1933.

The Citrus Care Calendar turns the year below into a dated task list. This page explains the year itself.

The twelve weeks that decide the crop

Read the chart as two separate facts, because they are easy to blur together.

The first is the normals. The 1991–2020 monthly normals give Phoenix 0.4 nights below freezing in January, 0.1 in February, 0.1 in December, and nothing at all in the other nine months. Add them and a normal year contains 0.6 freezing nights. That is a real figure and it is why growers here get away with lemons and limes at all.

The second is the record. Since June 1933 the station has logged 511 nights at or below 32 °F, and 124 of those went to 28 °F or below. They fall in five months: 212 in January, 140 in December, 109 in February, 35 in November and 15 in March. April through October have never produced one: the coldest April and October nights in ninety-three years both stopped at 34 °F.

A normal year and a long record are not in conflict; they answer different questions. The normals describe what a typical January looks like. The record describes what January has done, and the answer is that a handful of them have been hard. If you plant a lemon on the strength of the 0.6 figure alone, you have read half the sentence.

The Frost & Freeze Date Explorer works from the record rather than the normals for this reason, and the companion article on frost dates and what to protect covers what each threshold does to a plant.

For citrus specifically, the threshold that matters most is not 32 °F. The extension’s December guidance for home gardeners puts fruit damage on exposed lemons and limes at 28 °F, and notes that it is the fruit on the upper and outer portions of the tree that goes first, which is the fruit you can see and reach. The record’s 124 nights at or below 28 °F are the nights that number is about.

Winter: the cover, and when to put it on

The University of Arizona Cooperative Extension’s handout on protecting a citrus tree from cold is blunt about the largest single factor: it is not the cover, it is where the tree was planted. A site that holds daytime heat (against a south- or west-facing block wall, on a patio, near rock) gives back some of it overnight, and that is often the difference between losing fruit and losing a tree.

The separate extension handout on frost protection adds the numbers and the method. Maricopa County’s first frost ranges from late November in Buckeye to mid-December in central Phoenix, and the last frost from early February to early April, with elevation and urban density accounting for the spread. Frost cloth of the right kind protects to roughly 30 °F, and some types to 20 °F. Cloth, not plastic. It should be draped to the ground (ideally over a frame) with the aim of trapping the heat radiating out of the soil, and it should not be gathered around the trunk. Take it off in the morning once the temperature underneath has reached about 50 °F, because leaving it on encourages growth you do not want in January.

Two more things from the same handout, both of which go against instinct. A trickle of water running at the base overnight helps, but not for more than three nights in a row. And pruning waits until spring: dead wood still holds warmth, and plants recover from freeze damage more often than they look like they will in the week after.

Spring: feed, and the last of the cold

The extension keeps a separate publication for each of the winter tasks (a fertilisation chart for Arizona, a guide to pruning citrus, a guide to irrigating it), which is a fair signal of where the work actually sits. Spring is when the pruning happens and when the season’s feeding begins.

The caution on the spring side of the calendar is that the last frost is not one date. In Maricopa County the extension puts it anywhere between early February and early April depending on where you are, which means that in a cold-pocket part of the Valley, March is still a month in which a citrus tree can be hurt. Waiting two weeks longer than you want to is cheaper than replacing a tree.

Summer: the long season

From 31 May to 18 September, the normal afternoon high is at or above 100 °F: 111 days. Inside that, from 18 June to 22 August, it stays at or above 105 °F, and the peaks are 106.5 °F in July and 105.1 °F in August.

Rainfall over the same stretch is close to nothing. The normal for May is 0.13 inches, June 0.02, April 0.22. The entire annual normal is 7.22 inches. Whatever water a citrus tree gets between April and September, you are the one bringing it.

Two things follow. The first is that irrigating citrus in the low desert is the year-round task, not a summer one; the frequency changes with the season, which is why the extension publishes the guide at all. The second is that a citrus tree’s bark can burn in this sun, and the damage tends to appear on the west side of the trunk, which is the side the afternoon sun lands on. The sun path tool will tell you which of your walls that is on a given date.

Autumn: harvest, and the second half of the year

Autumn is the good part. The normal high falls through the nineties in October and to 76.5 °F in November, the nights drop below 70 °F, and the tree gets the cool nights it has not had since May. Fruit ripens on the tree through this stretch and into the winter.

Autumn is also when a freeze stops being an abstract idea. The record shows 35 freezing nights in November and 140 in December, against a normal of 0.1 in December. The cover that has been in the garage since spring should be somewhere you can reach in the dark by the middle of November, because that is when the county’s first frost window opens on the western side of the Valley.

The year, as a table

There is one more number worth carrying into the year, because it is the one that catches people out in both directions. The normal number of nights below 40 °F is 6.8 in December and 4.7 in January, against 0.1 in December and 0.4 in January for nights below freezing. In other words, there are ten times as many nights that a citrus tree will notice as nights that will actually freeze it. The tree spends far more of the winter being merely cold than being in danger, which is why protecting it well on the few nights that matter is worth more than protecting it anxiously for three months.