The two growing seasons of the low desert

The low desert does not have a growing season. It has two, and the whole of the gardening calendar here is arranged around the blocks of the year that sit between them. It is the single thing that separates a Phoenix garden from a garden almost anywhere else in the country, and it is the reason the local planting advice is written as two lists rather than one.

The chart below is the temperature year at Phoenix Sky Harbor, drawn from the 1991–2020 normals, with the two sowing windows marked. Everything that follows is read off it.

Why there are two seasons rather than one

A garden here has to get through two things that a garden in a temperate climate does not.

The first is a summer that runs for five months, from the end of May to the end of September, during which the normal high is at or above 100 °F for 111 days and the normal night stays at or above 80 °F for 84 of them. Almost nothing edible grows through that. The cool-season crops (lettuce, spinach, peas, broccoli, carrots) bolt or refuse to germinate in the heat, and the warm-season crops that were sown in spring have finished fruiting long before the coolest part of the night stops being in the eighties.

The second is a winter that is frost-free in the average year but not in the record. The normal number of nights at or below freezing in January is 0.4 and in December 0.1, and the normal year never gets there at all, which is why citrus is grown commercially in the Valley. But the station record back to 1933 is not the normals, and it does contain freezing nights, enough of them that a tender plant here is a plant with a plan.

Between those two blocks sit the two working windows. They are the spring sowing, from February into April, and the autumn sowing, from August into October. They are the same season in different clothes: the temperatures are similar, and what separates them is whether the days are getting longer or shorter.

The spring season, February to April

Spring starts earlier here than newcomers expect and ends faster than anybody expects.

February is the coolest of the three sowing months, with a normal high of 70.8 °F, and it is the month the cool-season crops go in (the leafy things, the roots, the peas) because they need to finish before the heat arrives. March follows at a normal high of 78.1 °F and is the month people describe as the best gardening month of the year. April jumps to 85.5 °F, and the warm-season crops go in behind the cool ones: beans, squash, cucumbers, the peppers and tomatoes that will fruit before the worst of June.

The speed of that is the thing to hold onto. March to May is a rise of sixteen degrees in ninety days, and the normal high has already crossed 100 °F by 31 May. A crop that needs fourteen weeks from sowing to harvest, put in the ground in April, is being harvested into the beginning of the hundred-degree stretch. The Planting Calendar exists to keep that arithmetic in view month by month.

The autumn season, August to October

Autumn is the second window and, for most vegetable gardeners here, the better one.

It opens in August, in the hottest month but one, which sounds wrong and is not. The seed goes into ground that is still hot enough to germinate it fast, and the plants do their growing in September and October as the temperature falls, a normal high of 100.4 °F in September and 89.2 °F in October. The harvest runs through the winter, which is the part of the year with no heat to speak of: December and January have normal highs of 66.2 °F and 67.6 °F.

That is why the autumn list is longer than the spring one. The tomatoes, peppers and squash are sown in August and picked in November and December, and the cool-season crops go in from September to November and are cut all winter.

The frost question, and how small it is

Here is where the two seasons meet the one genuine risk in a Valley winter, and where it is worth being precise rather than reassuring.

The two columns are deliberately different kinds of number. The right-hand column is what normally happens, and it says a Valley winter does not freeze. The two left-hand columns are what has happened since 1933, and they say that across 93 winters, January has produced 212 nights at or below freezing and December 140, with March and November contributing far fewer and the six months from April to October contributing none at all.

Both are true, and the useful reading is the one the Frost & Freeze Date Explorer is built on: frost here is a December and January event, it is not a season, and it is a thing you plan a cover for rather than a thing you plan your year around. A tender plant put in the ground in October is growing through the two months that carry the risk; the same plant put in the ground in March is not.

The two dead spots

Between the two seasons are two stretches that are not gardening weather in either direction.

The first is the deep summer, from roughly the start of June to the middle of August. The normal high runs 104.2 °F in June, 106.5 °F in July and 105.1 °F in August. Some things grow (the warm-season crops that are already established, and the heat-tolerant herbs) but this is the stretch the calendar is written around rather than through, and the sensible reading of it is to sow again in late summer rather than to fight the middle of the year.

The second is the turn of the year, and it is much smaller than people expect. December and January are the two coldest months and the two that carry the frost risk, and they are also the two months when the winter garden is at its best. What does not go in the ground then is anything tender.

Two windows, two gaps, and a frost risk small enough to plan for in a single afternoon. That is the whole shape of the Phoenix gardening year, and it is why the seed packets sold here carry two sets of dates rather than one.