Planting vegetables through a desert year
Vegetable gardening in the low desert runs on two workable windows either side of a summer that stops the harvest. What the normals say about each one.
Short answer
There are two planting seasons here, and neither of them is spring. The cool season runs from about October to March and the warm season from February to early June, and the four months between them are a stretch for keeping plants alive rather than starting them.
A vegetable garden in the low desert is not a spring project with a summer break at the end of it. It is two short seasons with four difficult months wedged between them, and the temperature normals at Phoenix Sky Harbor draw both of them fairly precisely.
Everything below comes from the 1991–2020 climate normals for that station, which is the official observation site for the city. A normal is a thirty-year average, so it describes a typical year rather than the year you are about to have, a distinction that matters more in a garden than almost anywhere else.
The year, in four windows
The calendar most seed packets are written for assumes a cold winter and a mild summer, which is the wrong way round here. What the normals actually give a gardener is a cool season with real warmth in the afternoons, a warm season that ends early, and a four-month stretch in which the only question is whether a plant can be kept going.
The figure below shows the windows that define that year. Each is the run of days on which the normal high or the normal low crosses a threshold.
The four temperature windows that shape a desert planting year
Each band runs from the first day the normal crosses the threshold to the last day it crosses it, 1991–2020 normals. A band is a climatological window, not a rule: a given year can differ by a week or more at either end.
Derived from the NOAA NCEI 1991–2020 daily normals for Phoenix Sky Harbor (USW00023183) by taking the first and last calendar day on which the normal high or low crosses each threshold. 29 February is excluded so the windows line up year to year. The day counts are the length of each window, not days above the threshold in a given year.
Two things follow from that figure, and both of them contradict the habit of planting in spring.
The warm season ends in June, not in September. The normal high reaches 105 °F on 18 June and does not fall back below it until 22 August, and in between the nights are the part that matters: from 20 June to 11 September, the normal low never drops below 80 °F. Sweet corn, beans, squash and tomatoes will not carry a harvest through a stretch like that.
The cool season is the long one. From 18 May to 5 October the normal low stays at 70 °F or above, which is 141 nights, but the cool-season crops that feed a winter kitchen are not growing through most of that. They grow from October to March, when the afternoons are between 66 °F and 89 °F and the nights fall into the forties and fifties.
The cool season, October to March
This is the season that surprises people who move here from a colder place, because it is not really winter. It is a long, bright, mostly frost-free spring that happens to begin in autumn.
| Month | Normal high | Normal low | Normal rainfall | What the month does |
|---|---|---|---|---|
| October | 89.2 °F | 65.6 °F | 0.56 in | The heat breaks. First reliable month for direct sowing of cool-season crops |
| November | 76.5 °F | 53.7 °F | 0.57 in | The best growing month of the year for leafy crops |
| December | 66.2 °F | 45.3 °F | 0.74 in | Slow growth, real frost risk, longest nights |
| January | 67.6 °F | 46.0 °F | 0.87 in | The coldest month by normal low, and the widest gap between afternoon and night |
| February | 70.8 °F | 49.0 °F | 0.87 in | The last cool month; warm-season seed goes in at the very end of it |
| March | 78.1 °F | 54.5 °F | 0.83 in | The crossover month, when both seasons are briefly possible |
October is the month to notice: a normal high of 89.2 °F sounds like summer, and the first half of the month is. The normal high is still 100.4 °F in September and the last day it reaches 100 °F is 18 September, which is why the local planting guidance puts the start of the winter garden in the second half of September at the earliest and mid-October for anything sown directly.
The three winter months are also where this article has to be honest about the limits of a normal. The normal number of nights at or below freezing at Sky Harbor is 0.4 in January, 0.1 in December and 0.1 in February, numbers small enough to read as never. The station record from 1933 tells a different story: 212 January nights at or below freezing out of 2,727 observed, 140 December nights out of 2,724, and a January low of 17.1 °F. The average is not the risk. A single December night can take a winter garden that has been growing well since October, so a gardener here plants the tender things with somewhere to go, or accepts the occasional loss.
The warm season, February to June
The warm season is the one people imagine when they think of gardening in Arizona, and it is shorter than it looks.
The normal high first reaches 90 °F in the first days of May and 100 °F on 31 May. From there it climbs to 104.2 °F in June before the monsoon arrives to put the humidity in the air and knock the high down to 106.5 °F in July, still the hottest month of the year.
For a vegetable garden the practical consequence is a front-loaded season with a hard stop. Warm-season crops sown in February and March are harvested in May and early June. Anything sown in April is gambling on a fruit set that has to happen before the nights warm up: the normal low goes from 60.8 °F in April to 69.5 °F in May to 78.6 °F in June.
The rainfall normals make the same point from a different direction. April brings 0.22 inches on average, May 0.13, and June 0.02: the driest month of the Phoenix year, with the highest temperatures in the run-up to it. Any warm-season bed from April onwards is an irrigated bed, and June is the month an irrigation fault does the most damage.
The four months that are not a season
June, July, August and September carry 98.4 of the year’s 111.3 days at or above 100 °F, and the normals for the four months read 104.2, 106.5, 105.1 and 100.4 °F as normal highs.
Two things do happen in that stretch. The monsoon delivers 0.91 inches in July and 0.93 in August (more than any other pair of months) but it delivers them in short, violent falls that run off a hard, dry soil rather than soaking into it, and it arrives with wind and dust. And the warm-season crops that were sown in March are finishing, not starting.
The reason to keep the summer bed covered is the surface temperature of bare ground. The site’s own surface figures put bare soil in full sun at 135 °F on a 105 °F afternoon: an estimate placed inside the measured range from the Georgia Extension chart, and the source page says so. A cover crop, a shade-cloth structure or a mulch layer keeps the soil and its biology in a range October can work with. The planting calendar handles this month by month if you want the task rather than the reasoning.
Water is the other calendar
No planting decision here is really separate from a water decision, because rainfall supplies 7.22 inches a year across 33.4 days with measurable rain, and a vegetable bed in June, July and August needs more than that in a fortnight.
That is why the Hydrozone Yard Planner belongs beside the planting calendar. A vegetable bed is always a high-water zone, and it should sit on its own irrigation line rather than sharing one with the shrubs, because the two need different amounts at different times of year. Grouping by water use is not an aesthetic exercise; it is the difference between a timer that works and a timer that starves one part of the yard to satisfy another.
The two growing seasons is the longer version of the same subject, including what happens in an ornamental bed. For the irrigation side, how often to water desert plants covers frequency and depth rather than crops.
What a normal cannot tell you
Four limits are worth stating plainly.
The normals are thirty-year averages, and a garden experiences one year at a time. A February that runs five degrees above normal will push a bed a fortnight ahead of this calendar; a cold snap in the first week of December will move it back a month.
The Valley is not one place. Sky Harbor sits in the urban heat island, and a garden in Apache Junction or on the west side of the White Tanks will run cooler at night and warmer in the afternoon than these numbers in different proportions.
Soil changes everything about water. Caliche, sand and imported fill all behave differently, and the frequency in any general guide has to be adjusted to what happens when you push a trowel into your own ground.
And nothing here is a chemical recommendation. What to feed a bed, and how much, is a question for a soil test and for the Extension guidance your county publishes, not for a climate article.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, monthly summaries, Phoenix Sky Harbor (USW00023183)
Every monthly normal high, low and rainfall figure in this article, including June's 0.02 inches and August's 0.93 inches.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, daily summaries, Phoenix Sky Harbor (USW00023183)
The dates the normal high and normal low cross each threshold: the 100 °F window from 31 May to 18 September, and the 84 nights from 20 June to 11 September whose normal low stays at 80 °F or above.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, annual and seasonal summaries, Phoenix Sky Harbor (USW00023183)
The annual figures quoted here: 7.22 inches of rain, 33.4 days with rain, and 111.3 days a year at or above 100 °F.
- NOAA NCEI Global Historical Climatology Network: Daily (GHCN-Daily), station USW00023183, Phoenix Sky Harbor
The freeze counts from actual observations back to 1933: 212 January nights at or below freezing, 140 in December, and the 17.1 °F January low.
- University of Arizona Cooperative Extension
The crop timing and the two-season planting pattern this article follows. No figures are quoted from the Extension; the temperature and rainfall numbers all come from the NOAA files above.