Onion in Zone 8B — Pacific Northwest

Onions are one of the most rewarding crops you can grow in our Pacific Northwest Zone 8B climate. Our mild summers and cool nights create ideal conditions for developing those complex, sweet flavors that make homegrown onions so much better than store-bought. With our 210-day growing season, you have plenty of time to grow impressive bulbs, and the long, cool fall extends your harvest well into November.

While our cool, wet springs can slow early growth and our dry summer months require consistent watering, proper timing makes onion growing straightforward in the PNW. Starting in late winter or early spring gives your onions the long, cool growing period they need to develop those large, flavorful bulbs we all want.

Starting onion seeds indoors makes sense if you want the widest variety selection or are aiming for the largest possible bulbs. Begin sowing from early to late January, about 10-12 weeks before your planned transplant date. Our slow spring start means indoor-grown seedlings get a valuable head start while outdoor soil is still too cool and wet.

Set up seed trays in a warm spot (65-70°F) with good light once seedlings emerge. Bottom watering works well for onion seedlings since it prevents damping-off issues that can occur in our humid indoor winter conditions. Keep soil consistently moist but not soggy.

Indoor-started onions typically produce the largest bulbs since they get such an early start. However, onion sets (small bulbs) are much easier for beginners and still produce excellent results in our climate.

Transplant your indoor-started onion seedlings from late March through mid-April, once the worst of our wet spring weather has passed. These young plants are quite hardy and can handle light frosts, but avoid transplanting during our heaviest spring rain periods when soil is waterlogged.

Harden off seedlings gradually over a week, starting with a few hours outdoors on overcast spring days and gradually increasing exposure. Space transplants 4-6 inches apart in well-draining soil - this spacing allows good air circulation, which helps prevent fungal issues during our wet springs.

Plant seedlings at the same depth they were growing in their containers. Don't bury the developing bulb. The cool, moist conditions of April are perfect for helping transplants establish before our dry summer months begin.

Direct sowing onion seeds works well from early March through early July in our climate, though early spring sowings generally produce larger bulbs. You can also plant onion sets (small bulbs) during this same window - sets are actually easier for beginners and less prone to bolting issues.

Wait until soil can be worked without creating muddy clumps, which usually happens by early to mid-March in most PNW locations. Our cool spring soil temperatures are fine for onion germination, though seeds will take longer to emerge than in warmer climates. Plant seeds ¼ inch deep and thin seedlings to 4-6 inches apart as they grow.

For succession planting, you can continue direct sowing through early summer. Later sowings won't produce bulbs as large as spring plantings, but they'll still give you fresh onions for fall harvest during our excellent growing weather.

Onions need consistent moisture throughout their growing season, which aligns well with our wet spring followed by dry summer pattern. During spring establishment, natural rainfall usually provides adequate water, but monitor soil moisture as we transition into our dry summer months starting in late June or July.

Once our dry season begins, provide about 1 inch of water weekly, applied slowly at the base of plants. Our mild summer temperatures mean you won't need the heavy watering required in hotter climates, but consistency is key. Use the finger test - soil should be moist 2 inches down but not waterlogged.

Stop watering when onion tops begin to fall over and yellow, typically from mid-July through August depending on variety. This signals bulb maturity, and continued watering at this stage can delay proper curing and reduce storage quality. Our naturally dry summer conditions are perfect for this final curing phase.

A light mulch of straw or compost helps retain moisture during our dry months while keeping weeds down, but avoid thick mulching that might harbor slugs during wet periods.

Your first onions will be ready from mid-June through early November, depending on planting time and variety. Spring-planted onions typically mature in mid to late summer, while succession plantings extend harvest into our mild fall weather. Look for tops that begin falling over and turning yellow - this indicates the bulb has stopped growing and is ready.

Pull bulbs on a dry day when possible, brushing off soil but leaving roots and tops attached initially. If tops haven't fallen naturally by late August, you can bend them over by hand to encourage bulb maturation. Green onions can be harvested anytime once plants are pencil-thick.

Cure harvested onions in a warm, dry, well-ventilated area for 2-3 weeks before storing. Our late summer and early fall weather is usually perfect for outdoor curing on screens or in mesh bags. Properly cured onions will keep for months in cool, dry storage.

With our first frost typically arriving in early November, aim to complete your onion harvest by mid to late October. Any remaining immature bulbs can be used fresh rather than stored.

Onion Thrips These tiny, slender insects cause silver streaks and stippling on onion leaves, eventually leading to yellowed, withered foliage. In severe infestations, bulb development suffers significantly. Thrips thrive during our warm, dry summer periods and can build up quickly once established. Remove damaged leaves promptly and encourage beneficial insects with diverse plantings nearby. Row covers during early growth can prevent initial infestations, and predatory mites naturally help control thrip populations in our mild climate.

Pink Root This fungal disease causes roots to turn pink, then purple, and finally black as they rot away. Plants become stunted and bulbs remain small. Pink root thrives in our cool, wet spring soils, especially in poorly draining areas or where onions have been grown repeatedly. Improve soil drainage, rotate planting areas yearly, and avoid overwatering. Choose resistant varieties when available, and plant in raised beds if your soil tends to stay soggy during spring.

Bolting Onions that send up flower stalks prematurely develop tough, bitter leaves and small bulbs as energy goes into seed production instead of bulb development. In our climate, bolting typically results from planting too early when plants experience cold temperatures, then warm weather. Plant sets and transplants after soil warms in late March or April, choose appropriate varieties for spring planting, and maintain consistent moisture to reduce stress that can trigger bolting.

Pacific Northwest Specific Challenges: Our combination of wet springs and dry summers creates unique challenges for onions. The extended cool, moist period can promote fungal diseases like pink root, while our mild summer temperatures mean slower bulb development compared to hotter regions. However, this also means less heat stress and reduced bolting risk once plants are established.

Onions make excellent companions for carrots, beets, and lettuce in our PNW gardens. Their strong scent helps deter carrot rust flies, while the different root depths mean they don't compete for nutrients. Lettuce benefits from the natural pest deterrent properties of onions, and both crops appreciate similar watering schedules during our dry summer months.

Avoid planting onions near beans and peas, as onions can inhibit nitrogen fixation by these legumes. This is particularly important in our region where cool spring soils already slow early growth - you don't want to further stress your nitrogen-fixing crops during their establishment period. Instead, use the space around beans and peas for herbs or flowers that won't interfere with their growth.