When to plant tomatoes in zone 9b, Arizona

Arizona zone 9b averages its last frost on 1 February and its first on 12 December — 315 days between them. Every window below is worked out from those two dates and from the 168 days a year this zone spends at or above 90°F.

Spring planting

Sow indoors	7–21 December
Plant out	8–22 February
Harvest	4 April – 27 June

Fall planting

Sow outdoors	15 July – 15 August
Harvest	8 September – 28 October

Last frost 1 February
First frost 12 December
Growing season 315 days
Days at or above 90°F 168

You plant the fall crop into 110°F, on purpose. This is Phoenix and the low desert, a hundred and sixty-eight days a year at or above 90°F, and above about 90°F tomato pollen stops working — the plant flowers and sets nothing. So the University of Arizona has you put transplants in from mid-July through mid-August, in the worst of it, so the plant is large and flowering by September when fruit set returns. Shade cloth and daily water are not optional for those first weeks. The spring crop is the shorter one here: it goes in during February and is finished by the time June arrives.

Zone 9b is not zone 9b everywhere

8 states contain zone 9b. They share a coldest winter night and almost nothing else: the last frost spans 24 January to 15 April, and the number of days at or above 90°F runs from 0 to 168. A planting calendar that stops at “zone 9b” is giving all 8 the same advice.

Zone 9b, state by state. Arizona in bold.
State	Last frost	First frost	Season	Days ≥90°F
NV	24 January	11 December	323 days	144
AZ	1 February	12 December	315 days	168
FL	2 February	4 January	332 days	96
LA	5 February	18 December	312 days	78
TX	5 February	11 December	309 days	113
CA	11 February	6 December	301 days	78
OR	19 February	10 December	292 days	0
HI	15 April	1 December	230 days	0

Other zones in Arizona: see all Arizona zones for tomatoes.

Where these dates come from

The frost dates for Arizona are NOAA’s 1991–2020 US Climate Normals — the 50% probability of a 32°F freeze, taken from weather stations standing inside zone 9b itself, not averaged across the state. The fall window follows University of Arizona AZ1005 / AZ2078, low desert calendar. Everything else on this page is arithmetic on those two dates: the same inputs give the same answer every time.

Made only for gardeners in the United States

Your zone is one number about one winter night.

Every date above is worked out from Arizona’s own frost records and its summer heat, read together — which is why two gardens in the same zone can be a month apart. ZoneSprout does that for 2,700 plants, in every zone of every state, and tells you what to do this week.

Get ZoneSprout How the dates are worked out