New Mexico · zones 6a–8a
When to plant onions in New Mexico.
Sow or transplant between Feb 10 to Mar 26 in a typical year, keyed to New Mexico's median last frost around Apr 20. A second, fall sowing fits Aug 30 to Oct 11. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical New Mexico season.
1 · Start indoors
Dec 30 to Feb 12
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 10 to Mar 26
Keyed to the ~Apr 20 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 30 to Oct 11
Counted back from the ~Oct 25 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
New Mexico spans more than one climate: Zone 7 is the most common zone, but the state runs 6a to 8a. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
New Mexico growing notes
Onions are the one crop in this bed that answers to daylight instead of elevation, and the wrong type bulbs small or not at all.
The trigger every other crop here can ignore
Every other vegetable in this bed lives or dies mostly by elevation: which zone the ground sits in, how hot the soil runs by midsummer. An onion shrugs at that. What tells a bulb onion to stop making leaves and start making a bulb is day length, set by latitude, not elevation or soil heat. Extension guidance for onions here splits the state along one line, the southern counties against the central and northern reach of it, since that line tracks the daylight swing an onion answers to.
That line decides which onion type belongs in the ground, not the variety names printed on a seed rack. A short-day onion, bred for a milder winter farther south, is what a southern garden here plants in fall, sitting through a mild low-desert winter to size into a mature bulb by early summer. An intermediate-day onion is the spring-planted type, the one choice for a central or northern garden where the ground runs too cold to carry a fall-set transplant, and it doubles as a second crop behind that fall-planted short-day one further south. A long-day onion is the type to leave off the order: bred for the far edge of northern summer daylight, it may not bulb in this state.
One planting, and every leaf banked before the switch counts
Onions here go in as transplants, set a shallow half inch deep and spaced 4 inches apart in the row. A bulb needs about 95 days to size up fully, and it gets one planting per window not a running succession. Both a spring transplanting and a fall one work in this state, and the choice between them is about banked growth: every leaf an onion puts up before day length forces the switch to bulking becomes a ring in the finished bulb, so more growing time ahead of that trigger means a bigger bulb at pull time.
In the low desert, a transplant set in fall can often sit through a mild winter and start bulking with a head start no spring planting matches. Higher up, where a winter cold snap risks a small transplant still finding its roots, spring planting once the ground has warmed is the more dependable route. Either way, a plant given its full 4 inches of room fills out a bulb worth the 0.5 pounds this bed can expect per plant; crowd the row tighter and every bulb comes up smaller for the space saved.
Dry air favors one pest and slows two more
Onion thrips are the pest this state's aridity favors, building up fastest in the hot, dry conditions a New Mexico summer offers for months. They are pinhead-sized and hide down in the crease where a leaf meets the neck, so the scouting test is running a finger through that crease and checking what comes off against something pale. Onion maggot works earlier, its larvae tunneling into the base of a young transplant until it wilts and yellows for no obvious reason; a lightweight cover cinched over the bed at transplanting keeps the egg-laying fly off, since a maggot already inside a bulb has nothing left to treat.
Downy mildew shows up as a grayish-purple fuzz on the leaves, and needs the kind of humid stretch this state mostly does not offer except during summer monsoon, when muggy, overcast days hand it moisture it cannot get the rest of the year. White rot is the one worth taking seriously beyond this year's crop: it lives in the soil around the roots and can sit there long after this year's onions are pulled, the argument against planting onions or garlic twice running in the same ground. A plant that yellows and topples with white, fluffy growth at the base, small black flecks scattered through it, is not coming back, and neither is that patch of soil anytime soon.
In New Mexico: watch for
The pests that find onions here.
A middle-length season, about 188 frost-free days, means onion, bulb overlaps onion thrips for a stretch each summer. Check plants every week or two and treat early.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: New Mexico gives onions about 188 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in New Mexico: questions
When should I plant onions in New Mexico?+
In a typical New Mexico year, sow onions or set out transplants between Feb 10 to Mar 26, keyed to the state's median last frost around Apr 20. There's also a fall window: Aug 30 to Oct 11. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in New Mexico?+
Start seeds inside about 6 weeks before set-out: roughly Dec 30 to Feb 12 for a typical New Mexico season.
What USDA zone is New Mexico?+
New Mexico spans zones 6a–8a, mostly zone 7. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.