Shallot

Microscopic soil-dwelling roundworms that burrow into plant roots and cause swollen knots (galls). Above ground, the plant looks stunted, yellowed, and wilted even with plenty of water. They attack over 2,000 plant species, so almost nothing is safe. They're most active in warm soil (70-85°F) and do more damage in sandy soils, where they move easily. Once a bed has them, populations stick around for years.

Triggers: Soil temperatures of 70-85°F are ideal for them; below 60°F they go dormant. Sandy soils make it easy for them to move and reproduce, while heavy clay slows them down considerably. In warm soil, a full generation completes in about 27 days.

Risk fades when: Activity drops sharply once soil cools below 60°F. Damage stops accumulating for the season, but the population stays in the soil and returns when warmth does.

Slugs and snails are nocturnal mollusks that chew irregular holes in leaves and clip off succulent seedlings. They leave characteristic silvery slime trails. Hermaphroditic and prolific, brown garden snails lay around 80 eggs per month for up to six clutches per year.

Triggers: Active at night and early morning in damp conditions. Coastal CA and southeast — active year-round. Spring rains and dense ground cover (mulch, debris, weeds) create harborage.

Risk fades when: Hot, dry summer weather drives slugs and snails into inactivity, so risk drops sharply once conditions turn warm and dry; populations are highest in cool, wet spring and fall, and in mild-winter coastal regions they can stay active year-round (UMN Extension, Penn State, UC IPM).

Golf-ball onions are almost always a day-length problem, not a fertility problem. Onions stop making leaves and start making a bulb when daylight hits a variety-specific threshold: short-day types begin bulbing at 10 to 12 hours, intermediate-day at 12 to 14, long-day at 14 or more. And here is the part that decides everything - Iowa State Extension puts it plainly: bulb size is largely determined by the number and size of the leaves at bulb initiation. Every leaf becomes a ring. Once bulbing starts, no new leaves form, and the onion can only fatten the rings it already has. So a short-day onion planted in the north hits its 10-hour trigger in early spring while the plant is still small, bulbs immediately, and Nebraska Extension calls the result exactly what it is - disappointingly small bulbs. Run it the other way and a long-day onion in the far south may never reach 14 hours and never bulb at all. Crowding does the same thing by another route: plants that never build big tops cannot build big bulbs. Match the variety to your latitude, plant as early as the soil can be worked, and give the tops every week of growth you can before the days get long.

Triggers: Iowa State Extension: short-day cultivars begin forming bulbs at 10 to 12 hours of daylight, intermediate-day at 12 to 14 hours, and long-day cultivars at 14 or more; bulb size is largely determined by the number and size of the leaves at bulb initiation. Nebraska Extension: short-day onions grown in northern regions start bulb development too soon, producing disappointingly small bulbs. UMN Extension: sweet or mild onions are short-day types and will generally develop small bulbs in Minnesota; overcast skies and cool temperatures during the season delay bulb formation. Crowding limits top growth and therefore bulb size. Note that bunching onions (Allium fistulosum) do not form true bulbs at all, and this disorder does not apply to them.

Risk fades when: Nothing can be done once bulbing has started - no new leaves form, so no new rings. The bulb will only fatten the rings it already has. Fix it at the next sowing: choose long-day varieties north of roughly the 36th parallel and short-day varieties south of it, intermediate-day in between. Plant as early as the soil can be worked, use transplants rather than direct seed in short-season areas to buy leaf-growth time, and thin to 3 to 4 inches. Small bulbs are perfectly good eating - they just will not size up this year.

Spider mites are tiny arachnids (1/50 inch) that feed on the undersides of leaves, producing characteristic silver-yellow stippling. Heavy populations produce visible webbing that interferes with pesticide coverage. They thrive in hot dry weather and drought-stressed plants. The two-spotted spider mite feeds on more than 180 cultivated plant species.

Triggers: Hot dry conditions; >90°F lifecycle <2 weeks. Drought stress amplifies. Broad-spectrum sprays (carbaryl, pyrethroids) trigger outbreaks by killing predators. Wisconsin Ext: 'as little as a month without significant rain during the growing season can favor a mite outbreak.'

Risk fades when: Populations build in hot, dry weather and then crash in late summer and fall as predatory mites catch up, host foliage declines, and cooler, wetter weather and rain set in, so risk fades once the heat and drought break (UC IPM, Colorado State Extension).

A thick-necked onion is a storage failure, not a growing failure - the bulb is usually large and looks fine. The neck is the stem where the leaves join the bulb, and at maturity it is supposed to dry down, shrivel, and seal. When it stays green, fleshy and moist, that open wet neck is a highway for rot. UMN Extension is direct about the cause: too much nitrogen results in late maturity, large necks that are difficult to cure, soft bulbs, green flesh, and poor storage quality. Watering late into the season does the same thing by keeping the plant growing when it should be shutting down. USU adds the discipline - stop nitrogen entirely once bulbs have initiated. The other route to a thick neck is pulling too early, before the tops have fallen and the neck has begun to dry on its own. Whatever the cause, extension guidance is unanimous and worth following: sort thick-necked bulbs out, keep them separate, and eat them first. Put one in a storage crate and it will rot and take its neighbours with it.

Triggers: UMN Extension: onions need a good supply of nitrogen, but too much causes late maturity, large necks that are difficult to cure, soft bulbs, green flesh and poor storage quality. USU Extension: refrain from applying any nitrogen fertilizer once bulbs have been initiated, and undercut onions at maturity. Nebraska Extension: harvest when about two-thirds of the tops have fallen over and dried naturally and the necks have started to dry; onions with a thick neck should not be stored. MSU Extension: thick-necked onions still contain green, moist tissue, will not keep, and if stored with the rest they will rot and may ruin the others. Iowa State: use thick-necked bulbs as soon as possible.

Risk fades when: A thick neck does not cure out. Sort those bulbs into a separate 'use first' pile - they are sound to eat but they will rot in storage and spoil neighbouring bulbs. Cure the rest at 75 to 90°F in a warm, well-ventilated place for two to four weeks until outer scales are dry and the neck is tight; poor curing causes decay regardless. For next season: stop nitrogen once bulbs begin to swell, taper irrigation as tops start to go over, and wait to harvest until about two-thirds of the tops have fallen and the necks have begun drying naturally.

Thrips are tiny (1/16 inch) slender insects with fringed wings that puncture and rasp leaf surfaces, leaving silver stippling with black frass dots. The biggest concern is virus vectoring: western flower thrips is the principal vector of tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV), which affect more than 600 plant species. Greenhouse and high tunnel infestations can be devastating.

Triggers: Hot dry weather; greenhouse/high tunnel environments. Female lays eggs inside leaf tissue. 2 larval stages feed; 2 non-feeding pupal stages in soil/litter. Lifecycle 10-21 days. Many overlapping generations. Bridge crops (spring wheat, peach, strawberry per NC State) build populations before vegetable hosts available.

Risk fades when: Infestations are worst in hot, dry weather; cool conditions slow development and heavy rain or overhead irrigation knocks populations down sharply, so risk fades as the weather turns cool and wet and rebuilds the following spring (UK Entomology, Cornell IPM).

Onions lose fights with weeds, and they lose badly. The reason is anatomical: an onion has a shallow, sparse root system and upright, narrow leaves that never close a canopy or shade the ground. Nebraska Extension states it plainly - due to their shallow root systems, onions compete poorly with weeds. A weedy onion bed does not just look untidy. Every week of shaded, crowded growth costs leaves, and because bulb size is set by the number and size of leaves at bulb initiation, lost leaves mean a permanently smaller bulb. The window that matters is from emergence through bulb initiation; after the bulb starts sizing, weeds cost less. The catch is that the same shallow roots that lose to weeds are easily severed by a hoe. Cultivate shallowly, close to the surface, and do not hill soil up around the plants. Mulch lightly instead: it suppresses weeds and conserves moisture without root damage.

Triggers: Nebraska Extension: due to their shallow root systems, onions compete poorly with weeds; shallow cultivation is necessary and the soil should not be hilled up around the plants. Light mulch decreases weeds and conserves soil moisture. The critical period runs from emergence through bulb initiation, because per Iowa State Extension bulb size is largely determined by the number and size of the leaves at bulb initiation - leaf growth lost to weed competition cannot be recovered once bulbing starts.

Risk fades when: Leaf growth lost to weeds is not recoverable - once bulbing begins the onion makes no new leaves, so the bulb is capped at whatever top growth survived. Weed early and often rather than waiting for a big clean-up. Cultivate shallowly and close to the surface; onion roots sit just below it and a deep hoe cuts them. Never hill soil up around onions. A light organic mulch suppresses weeds and steadies soil moisture at the same time, which also helps prevent split and double bulbs.

Leafminers are tiny fly larvae that feed between the upper and lower surfaces of leaves, creating winding pale tunnels or blotchy patches. They rarely kill plants but can ruin the marketability of leafy greens grown for foliage. Allium leafminer is an emerging pest in the eastern US (first detected in Pennsylvania in 2017) that damages onions, garlic, leeks, and chives.

Triggers: Liriomyza trifolii: 1 generation in ~1 month at typical greenhouse temps, 14 days at 95°F, 64 days at 59°F (UC IPM). Adults active mid-day. Allium leafminer emerges late March-early April, second flight September-October. Broad-spectrum insecticides trigger outbreaks by killing parasitoids.

Risk fades when: Parasitic wasps usually bring populations down as the season progresses unless broad-spectrum sprays disrupt them; damage peaks in warm spring and summer flushes and eases in cooler weather, so risk fades with natural-enemy buildup and falling temperatures (USU Extension, PNW Pest Handbook, UC IPM).