cool season · Amaryllidaceae

Growing Red Onion in Utah

Red Wing and Cabernet are reliable long-day reds for Utah. Sweet enough for raw use, store 4-6 months.

Schedule (May 15 last frost)

When to do what

Start indoors

2/20

Transplant out

4/17

First harvest

8/7

Dates are calibrated for SUS Farms (Sevier County, Zone 6a, last frost May 15). For your own date, use the interactive calendar.

Planting

How deep, how far apart

Method

Start indoors

Seed depth

0.25″

Plant spacing

4″

Row spacing

12″

Germination temp

50–85°F

Days to maturity

110 days

How we grow it

Step-by-step

1.
Start indoors 12 weeks before last frost
Seed-starting mix in 2" or 4" cells. Bottom heat at 50–85°F until germination, then drop to room temperature. Light from a clip lamp 14–16 hours a day, 2" from the leaves.
2.
Harden off for 7 days before going outside
Day 1: 1 hour outside in shade. Add an hour and more sun each day. Skip the day if it’s windy or below 50°F. After day 7 the plants stay outside.
3.
Transplant 4″ apart in rows 12″ apart
Soil should be at least 50°F before transplanting — black plastic mulch laid down two weeks ahead helps in our cool springs. Water in deeply.
4.
Mulch and water consistently
2" of straw or wood chip mulch around the base. Drip line at the surface. Aim for 1" per week — including rain — measured at soil level, not by the calendar.
5.
Harvest around day 110
Days-to-maturity is a rough guide — taste, color, and size are the real signals. Red Wing and Cabernet are reliable long-day reds for Utah. Sweet enough for raw use, store 4-6 months.

From the farm

What we’ve learned growing red onion

On our farm, red onion is one of the rotations that grounds the season. We test new varieties every year alongside the staples — usually one experiment per row, surrounded by what we know works. Failures here are how we know what to recommend; the success stories make it into the catalog and onto our table.

Pests & problems

Amaryllidaceae family pressures in Utah

Red Onion shares its troubles with onions, leeks, garlic, chives, shallots. The pests and diseases below show up most years; the fixes are what we actually do, not what catalogs sell.

Scout weekly during the growing season — most outbreaks are 10x easier to manage when you catch them in week one.

Pest 1

Onion thrips — silvery streaks on leaves; reduces bulb size. Reflective mulch (silver plastic) confuses them; spinosad spray for serious outbreaks.

Pest 2

Onion maggot — larva tunnels into bulbs. Floating row cover early-season; rotate aggressively.

Disease 1

White rot — fungal sclerotia in soil persist 20+ years; cottony growth on bulbs. If confirmed, abandon the bed for alliums.

Disease 2

Pink root — pink-tinged roots; reduces yield. Solarize soil in summer; long rotations.

Companion planting

What to plant near (and away from) red onion

Most companion-planting charts you see online are folklore. The pairings below have either USU Extension research, Cornell vegetable MD pages, or our own multi-year farm logs behind them.

Plant near

Good companions

✓Carrots (mask each other's pest scents)
✓Brassicas (deters cabbage moth)
✓Tomatoes, peppers (general pest deterrence)
✓Strawberries

Plant away from

Bad companions

✗Beans, peas (alliums stunt legume growth)
✗Asparagus (some allelopathy reported)

Crop rotation

Alliums in their own light-feeder slot — can rotate with brassicas without issue.

Harvest & storage

Picking, keeping, preserving

When to pick

Days-to-maturity (110 days from transplant) is a baseline. The real signals are color, size, and feel. Red Wing and Cabernet are reliable long-day reds for Utah. Sweet enough for raw use, store 4-6 months.

How to harvest

Clean cuts with sharp pruners or scissors heal faster than ripped stems and reduce disease entry points. Harvest early morning when sugars are highest and the plant is fully turgid; afternoon-harvested produce wilts faster. Don't harvest when leaves are wet — fungal spores ride along.

Short-term storage

Refrigerate at 32-40°F with high humidity (90%+). Most cool-season crops keep 1-3 weeks if cleaned and stored properly. Don't wash before storing — water on leaves accelerates spoilage.

Long-term preservation

Freezing, canning, drying, and fermenting all preserve red onion. Pick the method that matches your kitchen and how you actually use the harvest — frozen tomatoes are great for sauce but bad for sandwiches; dried herbs work everywhere; fermented vegetables shine in salads.

Mistakes we’ve made

Common ways to fail at red onion

Each of these has cost us a season at some point. Easier to learn from someone else’s lost crop than your own.

Planting too deep

The fix:Red Onion seeds want exactly 0.25" of cover — about a quarter of an inch — about the diameter of a pencil. Deeper than that and the seedling exhausts itself before reaching light. Carrots and lettuce especially: shallow is right; sprinkle, then cover with a dusting of soil and tamp gently.

Skipping the harden-off step

The fix:Plants raised under indoor lights have soft cuticles and weak stems. Move them straight outside and they sunburn, snap in wind, or wilt and never recover. The 7-day gradual sun exposure is mandatory, not optional.

Watering on a calendar instead of by need

The fix:Stick a finger or screwdriver 4" into the bed. Damp at depth = wait. Dry at depth = water deeply. Calendar watering ignores rain, heat waves, and seasonal evapotranspiration — leading to either drought stress or root rot.

Ignoring soil pH

The fix:Most Utah backyard soil tests at pH 7.4-8.4 (alkaline). Iron and zinc become unavailable to roots above pH 7.5 — leaves yellow, growth stalls. A $20 mailer test from USU Extension tells you exactly what your soil needs. Sulfur amendment in fall, foliar iron mid-season as needed.

Letting heat-bolt happen mid-season

The fix:Cool-season crops bolt (go to seed, become bitter) when night temps stay above 70°F. Plant for an early-spring AND late-summer harvest, with a heat gap in between. Fall plantings of lettuce, spinach, and brassicas are often better than spring ones in Utah.

Common questions

Frequently asked about red onion

+When should I start red onion indoors in Utah?

In Sevier County (last frost May 15), start 12 weeks before — that's roughly February. If you're at higher elevation (Park City, Logan), add 2 weeks. Lower elevation (Salt Lake, St. George), subtract 2-4 weeks. Use the interactive seed-starting calendar at /seeds/calendar to dial it in for your specific frost date.

+Why are my red onion leaves turning yellow?

Three usual suspects. (1) Iron chlorosis — yellow leaves with green veins is the Utah classic; the cause is alkaline soil locking up iron. Foliar iron rescues the season; sulfur amendment in fall fixes it long-term. (2) Nitrogen deficiency — entire leaf yellow including veins, starts with old leaves. Side-dress with compost or a slow-release organic fertilizer. (3) Overwatering — yellowing accompanied by soft, mushy stems means the roots are drowning. Check drainage; reduce water frequency.

+Will red onion survive a late frost in Utah?

Red Onion is a cool-season crop and tolerates light frost (down to ~28°F) once established. Tender seedlings just out of the greenhouse are more vulnerable — cover with floating row cover when overnight forecasts show below 35°F. After hardening off properly, mature plants of this family typically shrug off late-spring frosts that would kill warm-season crops.

+How long does red onion take from seed to harvest?

110 days from transplant. Add 84 days for the indoor seed-starting phase, so total time from sowing seed to first harvest is roughly 194 days. Days-to-maturity is a baseline — cool springs add a week or two; hot summers can speed up by similar amounts. Use it for planning, not as a strict calendar.

+What's the spacing between red onion plants?

4" between plants in the row, 12" between rows. That gives mature plants room to fill in without competing. Closer spacing reduces yield per plant; wider spacing wastes garden space. The numbers come from average mature plant size at full vegetative growth — adjust slightly for compact varieties (closer) or large heirlooms (wider).

Same family (Amaryllidaceae)

Related crops

Onion

110 days · cool

Leek