Arizona’s low deserts call for fall planting in October, while higher elevations wait until late winter for best results. Knowing the best time to plant onions in arizona can make the difference between a bumper crop and a disappointing harvest. Onions are cool-season crops that need specific day lengths and temperatures to form bulbs properly. In Arizona’s varied climate, timing is everything.
This guide will walk you through exactly when to plant onions in every part of the state. You will learn about soil prep, variety selection, and common mistakes to avoid. Let’s get started.
Best Time To Plant Onions In Arizona
The best time to plant onions in arizona depends entirely on your elevation and local microclimate. Arizona has three main growing zones: low desert (below 2,500 feet), mid-elevation (2,500–4,000 feet), and high desert or mountain areas (above 4,000 feet). Each zone has a different planting window.
Low Desert Planting Window (Below 2,500 Feet)
In places like Phoenix, Tucson, and Yuma, fall is the prime season. Plant onion sets or transplants from October through November. The soil is still warm from summer, which helps roots establish before winter cold arrives.
- October: Best month for planting short-day onion varieties
- November: Still acceptable, but earlier is better
- December: Risk of poor root development due to cooler soil
Onions planted in fall will grow slowly through winter and start bulbing in late February or March. You can harvest them by April or May. This timing avoids the intense summer heat that can cause onions to bolt or rot.
Mid-Elevation Planting Window (2,500–4,000 Feet)
Areas like Prescott, Payson, and Flagstaff’s lower edges have cooler winters. Here, you have two options: a fall planting in September or a spring planting in February or March.
- Fall planting: September to early October for overwintering
- Spring planting: Late February to early April for summer harvest
Fall-planted onions need protection from hard freezes. Use row covers or mulch to keep them alive. Spring planting is more reliable for beginners. Onions will bulb in June or July.
High Elevation Planting Window (Above 4,000 Feet)
In places like Flagstaff, Show Low, and Alpine, winters are long and cold. Wait until late winter or early spring to plant. The ideal window is March through April, after the soil has thawed and is workable.
- March: Start seeds indoors 8–10 weeks before last frost
- April: Transplant sets or seedlings outdoors
- May: Still possible but bulbs may be smaller
High-elevation growers should choose intermediate-day or long-day onion varieties. These handle longer summer daylight hours better. Harvest typically happens in August or September.
Understanding Onion Day Length Requirements
Onions are photoperiodic, meaning they bulb based on day length. Arizona’s latitude means you need the right type for your area. Using the wrong type leads to tiny bulbs or no bulbs at all.
Short-Day Onions For Low Desert
Short-day onions start bulbing when days reach 10–12 hours. This happens in late winter. Varieties like ‘Texas SuperSweet’, ‘Red Creole’, and ‘Yellow Granex’ work well in Phoenix and Tucson.
- Plant in October for March harvest
- Mild flavor, good for fresh eating
- Store for 2–3 months only
Intermediate-Day Onions For Mid-Elevation
These onions bulb when days are 12–14 hours long. They are versatile for Prescott and Payson. Try ‘Candy’, ‘Red Candy Apple’, or ‘Superstar’.
- Plant in September or March
- Good balance of sweetness and storage life
- Store for 4–6 months
Long-Day Onions For High Elevation
Long-day onions need 14–16 hours of daylight to bulb. They are perfect for Flagstaff and Show Low. Varieties include ‘Yellow Sweet Spanish’, ‘Walla Walla’, and ‘Red Baron’.
- Plant in March or April
- Large bulbs, excellent storage
- Store for 6–8 months
Preparing Your Soil For Onions
Onions need loose, well-draining soil with a pH between 6.0 and 7.0. Arizona’s soil is often alkaline and compacted. Proper preparation prevents disease and promotes strong root growth.
Testing And Amending Soil
Start with a simple soil test from your local extension office. Add sulfur to lower pH if needed. Mix in 2–3 inches of compost or well-rotted manure before planting.
- Test pH 2–3 weeks before planting
- Add 1 pound of sulfur per 100 square feet to lower pH by one point
- Work organic matter 8–10 inches deep
Fertilizing For Bulb Development
Onions are heavy feeders. Use a balanced fertilizer like 10-10-10 at planting time. Switch to a low-nitrogen formula (like 5-10-10) once bulbs start forming. Too much nitrogen causes lush tops but small bulbs.
- Apply 1 cup of 10-10-10 per 10 feet of row at planting
- Side-dress with 5-10-10 when leaves are 6 inches tall
- Stop fertilizing when bulbs begin to swell
Planting Onions: Seeds, Sets, Or Transplants
You have three options for starting onions. Each has pros and cons depending on your timeline and experience level.
Onion Seeds
Seeds give you the widest variety but take longer. Start seeds indoors 8–10 weeks before your planting date. In low desert, start in August for October transplanting. In high elevation, start in January for March planting.
- Cheapest option
- More variety choices
- Requires patience and indoor setup
Onion Sets
Sets are small, dormant bulbs sold in bags. They are easy to plant and mature faster. However, they may bolt (flower early) if exposed to cold. Choose sets that are dime-sized or smaller.
- Fastest to harvest
- Less work than seeds
- Limited variety
Onion Transplants
Transplants are young plants started by a nursery. They are ready to go in the ground immediately. This is the most reliable method for beginners.
- Best success rate
- Available at garden centers in season
- More expensive than seeds or sets
Step-By-Step Planting Guide
Follow these steps for a successful onion crop in Arizona.
Step 1: Choose The Right Location
Onions need full sun, at least 6–8 hours daily. Avoid low spots where water pools. Raised beds work well in heavy clay soil.
Step 2: Prepare The Bed
Remove weeds and rocks. Loosen soil to 12 inches deep. Mix in compost and fertilizer. Rake the bed smooth.
Step 3: Plant At The Correct Depth
For sets and transplants, plant so the tip is just above the soil surface. Burying too deep causes rot. Space plants 4–6 inches apart in rows 12–18 inches apart.
Step 4: Water Thoroughly
Water immediately after planting to settle soil. Keep soil consistently moist but not waterlogged. Onions need about 1 inch of water per week.
Step 5: Mulch To Conserve Moisture
Apply 2–3 inches of straw or shredded leaves around plants. This keeps soil cool, reduces weeds, and retains moisture. In low desert, mulch is critical for fall planting.
Common Mistakes And How To Avoid Them
Even experienced gardeners make errors with onions. Here are the most common problems in Arizona.
Planting Too Late In Low Desert
If you plant onions in January or February in Phoenix, they will not have enough cool weather to develop roots. They may bolt or produce small bulbs. Stick to October planting.
Using Wrong Day Length Variety
Planting long-day onions in the low desert results in tiny bulbs. The days are too short for them to bulb properly. Always match variety to your zone.
Overwatering In Heavy Soil
Clay soil holds water. Overwatering leads to root rot and fungal diseases. Water only when the top inch of soil is dry. Use drip irrigation for best control.
Not Hardening Off Transplants
Transplants from a nursery need gradual exposure to outdoor conditions. Set them outside for a few hours each day for a week before planting. This reduces transplant shock.
Harvesting And Storing Onions
Knowing when to harvest is as important as planting at the right time. Onions are ready when the tops fall over and turn brown.
Signs Of Maturity
- About 50% of the tops have flopped over
- Bulbs are firm and have papery skins
- Neck feels tight when squeezed
How To Harvest
Gently loosen soil with a garden fork. Pull onions by the tops, not the bulbs. Brush off excess dirt but do not wash. Lay them in a single layer in a shaded, dry area.
Curing For Storage
Cure onions for 2–3 weeks until the necks are completely dry. Cut off tops 1 inch above the bulb. Store in a cool, dark, well-ventilated place. Ideal temperature is 40–50°F.
- Short-day onions: Store 2–3 months
- Intermediate-day: Store 4–6 months
- Long-day: Store 6–8 months
Frequently Asked Questions
Can I Plant Onions In Arizona During Summer?
Summer is too hot for onions in most of Arizona. They prefer cool weather. Fall or early spring planting is best. Summer heat causes bolting and poor bulb formation.
What Is The Best Onion Variety For Phoenix?
Short-day varieties like ‘Texas SuperSweet’ and ‘Yellow Granex’ work well. They bulb in late winter and are ready by April. Avoid long-day types.
How Deep Should I Plant Onion Sets In Arizona?
Plant sets so the tip is just above the soil surface, about 1 inch deep. Deeper planting can cause rot, especially in clay soils common in Arizona.
Do Onions Need Full Sun In Arizona?
Yes, onions need at least 6–8 hours of direct sunlight daily. Partial shade reduces bulb size. Choose a sunny spot in your garden.
Can I Grow Onions From Kitchen Scraps In Arizona?
You can regrow green onion tops from root ends, but bulb onions need a full season. It is not practical for large harvests. Use sets or seeds instead.
Final Tips For Arizona Onion Growers
Success with onions in Arizona comes down to timing and variety selection. Start with a soil test, choose the right day length type, and plant at the correct window for your elevation.
Water consistently but avoid overwatering. Mulch to protect roots from temperature swings. Harvest when tops fall over and cure properly for long storage.
With these guidelines, you can enjoy homegrown onions from your Arizona garden. Whether you live in the low desert or the high country, there is a planting time that works for you.
Remember that local microclimates vary. Observe your garden’s conditions and adjust by a week or two as needed. Gardening is a learning process, and each season teaches you somthing new.
Happy planting, and enjoy the fruits of your labor. Onions are a rewarding crop that stores well and adds flavor to countless dishes. With the right timing, you will have a bountiful harvest.