Onion

Sow onions early, and match the day-length type to where you live

Start onion seed indoors about eight weeks before you plan to set plants out, then plant transplants, sets, or direct-sown seed in early spring as soon as you can cultivate the ground. Pick the type by latitude, not by the picture on the packet: short-day in the South, long-day in the North, intermediate in the band between. In the Deep South and the Gulf states, short-day onions are a fall- and winter-planted crop rather than a spring one - set them from late fall through February and ignore the spring timing above. Onions bulb on a photoperiod trigger, not on age. Short-day types start bulbing at about 10 to 12 hours of daylight, intermediate at 12 to 14, and long-day at 14 to 16. A short-day onion in the North begins bulbing while the plant is still tiny and gives you marbles, and a long-day onion in the South never receives the signal at all. The bulb is built out of the leaves the plant made before bulbing started, so every week of early growth is bulb size.

Set plants shallow and wide, and do not hill soil over them

Set transplants and sets about an inch deep, pointed end up, and space them by the size you want: 2 inches apart for green onions, 4 inches for moderate bulbs, 8 inches for large ones. Weed by hand or cultivate shallowly, and never pull soil up around the plants. Do not rush plants into cold ground. Onion size is set by spacing because the plants are weak competitors and each bulb needs its own light and water. Soil mounded against the stem holds moisture on the neck and invites stem rot. Young plants that sit through a cold spell read it as winter and flower the following spring instead of bulbing, and a bolted onion never makes a keeper.

Water hardest while the bulb is sizing, then taper off

Keep the soil near field capacity from the moment bulbs begin to form through bulb enlargement, watering little and often rather than deeply and rarely. As the tops start to go over late in the season, gradually cut the water back. Onion roots only occupy the top 6 to 8 inches of soil, so the plant has no reserve to draw on and hits moisture stress within days, and bulb enlargement is the stage where that stress costs you size directly. Late water keeps the neck green and wet when it needs to be drying for curing.

Feed nitrogen while the leaves are growing, then stop when bulbs swell

Work about a third of the season's nitrogen in before planting, then side-dress the rest in splits during early leaf growth, placed a few inches out from the base rather than against it. Once the bulbs start to swell, cut nitrogen off entirely. Onions build the bulb out of leaves they already made, so nitrogen only pays while leaves are still forming. Nitrogen applied after bulbing begins keeps the neck thick and green, and a thick neck will not dry down, will not cure, and rots in storage.

Count thrips down in the neck, not on the leaves

Pull the neck open on a few plants scattered across the planting and count adults and larvae quickly before they scatter. Treat at about 15 thrips per plant early in the season on a susceptible variety, or about 30 per plant later on a tolerant one. Silvery streaking on the leaves means they are already established. Thrips shelter at the base of the youngest leaves in the center of the neck, so a leaf-surface look badly undercounts them. Hot dry weather runs a full generation in three to four weeks, which is how a few plants become the whole bed; feeding injury kills leaf area and the bulb shrinks with it.

⚠ Organophosphates and pyrethroids have widespread resistance in western onion thrips - rotate insecticide classes rather than repeating one, and check the pre-harvest interval before you pull.

Downy mildew: watch the cool wet spells, and keep the leaves dry

Through cool damp weather, check leaves for pale patches carrying a fuzzy violet-grey growth. Water at the base rather than over the canopy, keep rows open to prevailing wind, and rotate three years away from onions, garlic, and other alliums. Clear allium debris and volunteers. Infection needs only 2 to 3 hours of leaf wetness between 43 and 61 degrees, and spores need 95 percent humidity in the canopy - which is exactly what a sprinkler and a crowded row deliver. Once infected, a plant takes 8 to 16 days to sporulate, and those spores infect the neighbours immediately. The disease shuts down above about 82 degrees, so it is the cool end of the season that decides it.

Thrips: aim into the neck, and mind the wait

A hard water spray knocks thrips back. If you spray, spinosad is the home-garden material - add a spreader and aim down into the leaf axils, or you will miss them. Thrips shelter deep in the neck where the leaves meet, so a spray that only wets the outside of the leaf does almost nothing.

⚠ WAIT 1 DAY after spinosad before pulling - the same wait whether you are pulling scallions or lifting dry bulbs. Neem and pyrethrins are 0 days; malathion is 3 and is highly toxic to bees, so spray at dusk. NOTE the number is crop-specific: spinosad is 1 day on onion but 7 on potato.

Row cover at planting for onion maggot

Lay floating row cover over the bed immediately after sowing or transplanting and seal the edges down with soil. Do not cover ground that grew onions last year - pupae left in that soil emerge underneath the cover, on the wrong side of it. Rather than tilling culls and volunteers under, gather them into deep piles, then bury the piles or haul them away. The maggot overwinters as a pupa in soil where onions grew, and the spring flies lay at the base of young plants; the newly hatched larvae crawl in behind the leaf sheath and eat out the stem and developing bulb, opening the way for soft rot. Small plants die outright, and cover only works if the flies are outside it rather than emerging underneath it. Culls tilled under feed the next generation; gathered into a deep pile, fly reproduction is confined to the pile's surface layer, which knocks populations back further than tilling does.

Harvest storage onionsJul – Aug

Lift onions when most tops fall over and begin to dry, then cure bulbs with tops attached in a dry airy place. Waiting for tops to fall signals maturity; curing dries necks for better storage.

⚠ Keep curing onions out of rain.

Cure in a single layer, then store cold and dry

After lifting, spread the bulbs in a single layer in a warm, dry, well-ventilated place for two to three weeks, until the tops and necks are thoroughly dry and the outer scales are papery. Then store at 32 to 40 degrees with 65 to 70 percent relative humidity. Storage rot walks in through the neck, and a neck is only closed once it has dried shut - piling bulbs deep or curing them in humid air leaves the door open. Cold and dry then holds the bulb dormant; warm or damp storage restarts sprouting and rot.

⚠ Keep curing onions out of the rain, and out of direct sun in hot climates, which scalds the bulbs.