California Bluebell

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).

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).

Tiny sap-sucking insects that erupt off the leaf in a white cloud when you brush the plant. Despite the name they are not flies at all - they are Hemiptera, relatives of aphids, scales and mealybugs, and the name comes from the mealy white wax coating the adult's wings and body. They pierce the phloem and drain sap, so heavy populations yellow the leaves, dry them, and drop them. The bigger nuisance is honeydew: a sticky sugary excretion that coats the foliage, grows black sooty mold, and draws ants, which then chase off the natural enemies that would otherwise keep whitefly in check. UC IPM notes outbreaks often begin exactly that way - when biological control gets disrupted - and that once populations are high the pest is genuinely hard to manage. Bemisia nymphs go further, causing distortion, discoloration, and the leaf silvering that gives silverleaf whitefly its name. Both species vector plant viruses. Indoors and under cover they are far worse than in the open garden, because it never gets cold enough to knock them back and their generation time collapses to about 18 days at 80-90°F. The single highest-value habit is inspecting every new plant, undersides included, before it enters the greenhouse or the garden.

Triggers: UC IPM: whiteflies become abundant in vegetable and ornamental plantings especially during warm weather, and outbreaks often occur when natural biological control is disrupted. Greenhouse whitefly develops egg to adult in as little as 18 days, with fastest development between 80 and 90°F, so populations compound quickly under cover or on overwintered indoor plants. They favor succulent, actively growing tissue. Host range is broad - UC IPM lists alfalfa, beans, cucumbers, eggplants, grapes, lettuce, melons, peas, peppers, potatoes, strawberries, tomatoes and many ornamentals. Ants tending the honeydew interfere with the natural enemies that would otherwise suppress them.

Risk fades when: UC IPM: abundant especially during warm weather. Development is fastest at 80-90°F, so protected culture and indoor overwintering sustain populations outside the normal outdoor season - which is exactly when houseplants and tender herbs brought inside get hit.

Wind-flop is what happens when storms knock down plants that were standing fine. Different from regular flopping (which is gradual under the plant's own weight) — wind-flop is sudden and often dramatic. Corn after a thunderstorm, Brussels sprouts after a windy week, sunflower heads bent over from a single gust. Plants with shallow root systems (recent transplants, plants in containers) and tall plants with weak base stems are most vulnerable. Plants partially lodged (leaning but not broken) often recover if the soil is firmed back around their base. Plants snapped at the stem are usually a loss. Prevention: stake tall plants from the start, hill up around corn and tall brassicas as they grow, plant windbreak rows of taller plants to break gusts, choose dwarf or short-stem varieties for windy locations.

Triggers: Sudden wind events overwhelm structural support. Saturated soil after rain compounds risk because root anchoring is weaker. Tall corn, Brussels sprouts, sunflowers most commonly affected.

Risk fades when: Re-stake leaning plants by gently lifting and firming soil. Plants with snapped stems below the lowest leaf node are a loss.

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).

Self-seeding (or volunteer reseeding) happens when a plant is allowed to flower, set seed, and drop those seeds in the garden. Many cool-season crops like cilantro, dill, fennel, arugula, lettuce, and many flowers (calendula, cosmos, sunflower, poppy, nigella) reseed reliably year after year — often more reliably than deliberate sowing. Hybrid varieties typically don't reseed true-to-type — the seedlings may show traits of the parent lines rather than the labeled hybrid. Open-pollinated and heirloom varieties reseed faithfully. If you don't want volunteers, remove flower heads before seed sets. If you want them, leave seed heads on through fall and they'll sprout when conditions are right next season.

Triggers: Self-seeding follows any time a plant flowers, sets seed, and is left in place long enough for seed to mature and drop. Open-pollinated varieties reseed true; hybrid varieties may not.

Risk fades when: Self-seeding isn't a problem per se — it's the plant doing what plants do. Manage by deciding whether you want volunteers next season.

This looks identical to heat decline in July and needs the opposite advice in September, so the distinction is worth getting right. These are cool-season annuals, and an annual is a plant with one job. UConn Extension puts it bluntly: an annual's main purpose in life is to set seed. Penn State describes what follows - once seed formation begins, the flowering process is suspended and the plant directs all its energy into seed. Growth slows, new buds stop forming, and the plant dies. Heat simply accelerates the whole sequence. Kansas State observes that annuals which thrive in the cool of spring, such as pansies, fade as June weather arrives, while heat-adapted annuals like zinnia and lantana bloom on until frost. So a bed of nigella, flax and baby blue eyes that collapses in midsummer is not sick and is not going to recover. It has finished. The one lever a gardener has is deadheading. MSU Extension explains the trick: removing the spent flower before it sets seed convinces the plant it must keep blooming to reproduce. Do that faithfully and the display stretches. Miss a week in a heat wave and it is over. The other lever is the calendar - sow early, and sow again in batches.

Triggers: UConn Extension: an annual's main purpose in life is to set seed; removing spent flowers, termed deadheading, will prolong the blooming period, and should be planned on a weekly basis. Warm-season annuals such as zinnias, tithonia and portulaca tolerate summer heat and bloom until frost - the plants here do not. Penn State Extension: when a plant sets seed the flowering process is suspended and the plant begins to use all its energy to form seeds; deadheading as soon as blooms begin to fade will promote a second bloom. MSU Extension: deadheading is simply removing the spent flower before it sets seed, which tricks the plant into believing it must keep blooming to produce seed for the next generation. Kansas State Research and Extension: annuals that thrive in the cool of spring, like pansies, fade as June weather arrives; if an annual develops seed pods, the plant puts its energy into seed production instead of flower production. Penn State also notes that forget-me-nots are among the plants best left undeadheaded if reseeding is wanted. NOTE ON EVIDENCE: deadheading is reported to be much less effective on some very short-lived annuals, including nigella and poppies, for which succession sowing every two to three weeks is the recommended approach; that specific claim was not found in an extension source, though it is consistent with the curator guidance for nigella. Blue flax (Linum lewisii) is a perennial rather than an annual - heat shortens its flowering flush, but the plant persists and reflowers, so it does not senesce like the others here.

On Phacelia California Bluebell: Heat arrives early or plants dry severely after budding

Prevention: Sow in the proper season and keep moisture moderate while blooming

Risk fades when: There is no recovery to wait for. Unlike the cool-season perennials that stall in heat and rebloom when nights cool, an annual that has set seed is done, and no amount of shearing, watering or feeding restores it. Everything useful happens earlier. Sow as early as the season allows so the plants flower in cool weather. Deadhead weekly, and during a heat wave more often than that, removing faded flowers before the seed pod swells - once pods form, the switch has already flipped. Sow successive batches two to three weeks apart, which is the only reliable method for the shortest-lived species such as nigella. Keep moisture even, since drought accelerates seed set independently of heat. And if you want the planting to return next year, do the opposite: stop deadheading late in the season and let it seed itself. These plants reseed readily, and a self-sown stand germinates at the right moment without any help.