Celery

Start seed indoors 10 to 12 weeks before your last frost

Sow indoors about 10 to 12 weeks before your last spring frost, pressing the tiny seed onto the surface rather than burying it. Expect 14 to 21 days to germinate, with 50 to 70F the optimum range, and plan on 8 to 10 weeks of growing before the seedlings have the 3 or 4 mature leaves and established roots they need to go out. Buy disease-indexed seed, or use seed that is three years old. Celery germinates slowly and needs a long head start to size up in a short season. The seed also carries the late blight fungus Septoria apiicola, and MSU reports those spores live up to two years on seed, so three-year-old seed is clean seed.

Transplant only once nights hold above 55F, or the crop bolts

Set plants out 6 to 7 inches apart in rows 24 to 34 inches apart, but wait until temperatures stay above 55F. Harden off by easing back on water, not by chilling the seedlings, since cool temperatures induce bolting. If a cold snap is forecast in the first weeks after planting, throw a floating row cover over the bed. Celery is a biennial, and MSU reports that plants exposed to temperatures below 55F for 7 days or longer often form a seedstalk in their first year. It builds a seedstalk instead of the thick petioles you meant to eat.

One to two inches a week, from below, never let it check

Give celery 1 to 2 inches of water per week and never let the bed dry out. Use drip or a soaker hose rather than a sprinkler, and mulch once plants reach 6 inches tall. Water needs climb sharply above 85F. Celery is shallow rooted, so it feels drought fast. Any check in moisture makes stalks stringy and strong flavored, and because calcium moves through the plant only in the water stream, a dry spell in heat starves the growing heart of calcium and turns it black.

Sidedress nitrogen at four weeks after transplanting, then again at eight

Wait about four weeks after transplanting, then sidedress a quarter cup of 21-0-0 per 10 feet of row, and repeat at eight weeks. Do not push nitrogen harder than this in a home bed. Celery demand for nitrogen is low during its first 25 days in the field, then climbs rapidly as it builds petioles, so fertilizer applied at planting is largely leached or wasted before the crop can use it.

Aster leafhoppers: check once or twice a week while plants are young

Look for small wedge shaped leafhoppers once or twice a week, most closely while plants are young. A floating row cover over young plants keeps them off entirely. Pull and destroy any plant showing twisted, yellowed, stunted growth, and keep grassy weed hosts down around the bed. The leafhopper itself does little feeding damage. It matters because it injects the aster yellows phytoplasma, which has no cure once a plant has it. The phytoplasma must incubate about three weeks inside the leafhopper before that insect can transmit it, so infective leafhoppers arriving in your bed are ones that fed on an infected host well upwind.

⚠ Row cover and roguing are the home garden controls. UC IPM states there are no pesticide controls for the phytoplasma and that insecticides alone will not prevent leafhopper transmission. If you do treat, follow the pre-harvest interval printed on your product's label, and never spray an insecticide on plants in bloom.

Late blight: spray at the first spots, not after

Watch for small yellow spots peppered with small, dark, round bodies (pycnidia); older lesions turn tan and papery. At the first appearance of symptoms, spray chlorothalonil, or copper hydroxide if you garden organically, and reapply at the interval your product's label specifies while wet weather lasts. Stop overhead watering, and stay out of the bed while the foliage is wet. Septoria apiicola spreads when splashing water moves its spores from leaf to leaf, and rain, heavy dew, fog, or sprinkler irrigation above 70F favor infection. Development ceases entirely below 55F. Every pass you make through a wet planting can carry spores from diseased to healthy plants.

⚠ Chlorothalonil carries a 7 day pre-harvest interval on celery, copper hydroxide 0 days. Confirm celery is named on your product's label, since home garden chlorothalonil labels vary by brand, and follow the label's own reapplication interval and rate limits.

Cut whole heads inside the six to eight day window

Take outer petioles any time once they pass a foot in length, which lets one plant feed you for weeks. For whole heads, cut at the base once the plant is 3 inches or more across. The inner stalks stay the most tender. MSU reports that once celery is growing rapidly it stays at harvest quality for only about six to eight days. Past that the petioles go pithy and hollow, so the window is short and worth watching for.