Planting Guides

When to Plant Onions in Phoenix: Complete Guide + Best Varieties for Zone 9b

Phoenix, Arizona

USDA Zone 9b

Last Frost: Feb 15

Last updated: October 30, 2025

Learn when to plant onions in Phoenix with specific dates for Zone 9b. Compare 6 varieties, get a complete timeline, and discover which onions grow best in Arizona.

AAlejandro Vega

October 30, 2025

Fresh onions harvested in Phoenix showing successful desert growing with proper timing — — Image © PlantReference.org 2026

Quick Answer

Plant short-day onions in Phoenix October 15-December 15 for spring harvest. Long-day varieties won't form bulbs in desert latitudes.

TL;DR

Phoenix onion success requires fall planting of short-day varieties between October 15-December 15 for Zone 9b's desert climate. Plant Allium cepa 'Texas Super Sweet' and other short-day types only—long-day varieties won't bulb in Phoenix's latitude. Harvest occurs March through May before summer heat arrives.

Frequently Asked Questions

When is the best time to plant onions in Phoenix?

Plant onions in Phoenix between October 15 and November 30 for optimal results. This timing allows establishment before winter cold while providing sufficient growing time before summer heat. Only short-day varieties work at Phoenix's latitude—long-day types won't form bulbs regardless of planting date.

Can you plant onions in spring in Phoenix?

Spring planting is not recommended in Phoenix. The rapid transition from winter cold to summer heat confuses onion plants and often causes premature bolting. Fall planting is essential for Phoenix success, allowing onions to grow through the pleasant winter months and mature before May heat arrives.

What onion varieties grow best in Phoenix?

The best onion varieties for Phoenix are short-day types: Allium cepa 'Texas Super Sweet', Allium cepa 'Red Creole', and Allium cepa 'Yellow Granex'. These varieties are bred for southern latitudes and trigger bulbing with only 10-12 hours of daylight, perfect for Arizona conditions.

Do onions need shade in Phoenix?

Yes, onions benefit from 30% shade cloth in Phoenix during March through harvest. While onions need full sun in cooler climates, Phoenix's intense desert sun and low humidity cause leaf burn that reduces bulb development. Light shade actually increases yields in desert conditions.

How often should you water onions in Phoenix?

Water onions every 2-3 days in Phoenix winter, increasing to daily light watering as spring temperatures rise. Desert air pulls moisture from plants faster than anywhere else in the country. Consistent moisture is critical—never let onions completely dry out as this immediately stunts bulb development in low humidity conditions.

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Tagged with

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Written By

Alejandro Vega

Alejandro is a landscape designer in Phoenix who focuses on low-water, desert-adapted plantings. He grew up in Tucson surrounded by Sonoran Desert plants and got frustrated watching neighbors install water-hungry lawns in a city that gets eight inches of rain a year. After earning his landscape architecture degree, he started designing residential gardens using native and desert-adapted species. Alejandro's approach is practical—he works with the climate rather than against it, using microclimates, shade structures, and efficient irrigation to create gardens that look good without draining the aquifer. He's a regular speaker at local xeriscaping workshops.