Fennel

Direct sow herb fennel after the last frost

Once hard frost has passed, soak the seed a day or two and direct sow a quarter inch deep where the plants will stay, thinning seedlings to 4-6 inches apart when they are about 2 inches tall. Do not start it in pots and move it later. Fennel drives a deep taproot early and transplants badly once that root is disturbed, and root disturbance is itself a trigger for bolting.

⚠ Fennel sap irritates the skin of sensitive people, so wear gloves if you handle cut stems for long.

Florence fennel: sow midsummer for a fall bulb crop

Sow bulbing fennel in midsummer, a quarter inch deep in loamy well-drained soil at pH 6.5 to 8. Count back your variety's days to maturity from your first fall frost — most Florence types run 60 to 90 days — and add a couple of weeks of slack; fennel tolerates light frost, so the crop does not have to be finished before frost arrives. Thin to 8-12 inches apart once the bases start to swell so each bulb has room. Spring-sown Florence fennel runs to flower in the long days and heat of midsummer and the bulb never forms, while a midsummer sowing does its bulbing in the cooling, shortening days of late summer and fall.

Keep fennel evenly wet or it will bolt

Give fennel 1 to 2 inches of water a week and never let the soil dry out between waterings. Dry spells are what push a first-year plant into flowering, and once a Florence fennel bolts the bulb turns stringy and splits and no amount of water brings it back.

Hill soil around the swelling bulb to blanch it

As the bulb enlarges, mound loose soil up around its base, leaving the fronds exposed. Light turns the exposed leaf bases green and coarse, and burying them keeps the bulb white, tender and milder in flavor.

Side-dress nitrogen as the bulbs begin to swell

When the stem bases start to thicken, side-dress about 3 tablespoons of a nitrogen fertilizer such as 21-0-0 per 10 feet of row and water it in. Phosphorus and potassium are usually already sufficient, so add them only if a soil test calls for them. The bulb is built almost entirely during this swelling phase, and nitrogen is the nutrient that limits how thick those leaf bases get.

Check for caterpillars, aphids and crown rot, but leave the swallowtails

Look over the foliage every week or two for aphids and for striped green caterpillars. Knock aphids off with a hard spray of water or treat a heavy infestation with insecticidal soap. Leave the caterpillars alone, or move them to a spare plant. Also check the crown and stem base for soft, brown or collapsing tissue, especially after a wet spell — that is stem and root rot, and the fix is drainage, not a spray. Fennel is a larval host for black and anise swallowtails, especially the bronze types, and even a plant chewed bare recovers. Aphids and slugs are the only other regular insect pests, but stem and root rot in wet or poorly drained soil is the one problem that actually kills the plant.

⚠ Never spray any insecticide, including soap, on open fennel umbels. The flowers are heavily worked by bees and by the tiny parasitic wasps and hoverflies that control aphids in the rest of the garden.

Cut the bulb at tennis-ball size, before it bolts

Harvest once the bulb is firm and about the size of a tennis ball, under 4 inches across, by cutting with a knife just above the taproot. Snip fronds for the kitchen any time along the way. A bulb left past this point starts elongating toward flower, and the tight overlapping bases loosen and go fibrous. Firm and slightly small beats large and stringy.

Cut the umbels as they brown, for seed or to stop reseeding

Cut the seed heads as soon as they turn brown and before they shatter, on a dry afternoon. For seed, hang the cut heads in a paper bag in a cool airy place and collect what falls. If you do not want seed, cut and remove the spent umbels anyway. Ripe fennel seed drops on its own within days of browning, and fennel self-seeds freely and has escaped gardens and naturalized. In mild-winter regions it is genuinely invasive, so removing the heads is the whole control.