Gadenin日本語Get the appDecide which fennel you are actually sowing
Two quite different plants share the name fennel. Herb fennel is a tall perennial grown for feathery leaf, flower umbels and seed. It lives for years, reaches five to six feet (1.5 to 1.8 m), and once it is settled it is very hard to remove. Florence fennel is an annual selection grown for the swollen white base, and it dies after it flowers whether you wanted it to or not.
They are treated in opposite ways. Herb fennel is sown once at the back of a border and then managed as a long-term resident. Florence fennel is sown repeatedly, on a tight schedule, into rich moist ground, and pulled entirely within about three months. Sowing the herb type expecting bulbs is the most common disappointment: it will never swell at the base no matter how well you feed it.
You can tell them apart at the seedling stage only by knowing what you sowed, so label the row. By the time the difference is visible, the herb type has already put down a taproot deep enough that removing it is a job with a fork rather than a hand weeder.
If a plant is growing back from the same crown for a second and third year, it is herb fennel. Florence fennel never overwinters as a productive plant; it either bolts or dies.
Direct sow: the taproot makes the decision for you
Both fennels are umbellifers and both make a taproot that goes straight down before there is much top growth. Breaking or bending that root tip is a check, and a check on Florence fennel means a flower stem instead of a bulb. This is why module-raised Florence fennel so often fails while direct-sown fennel a few feet away swells properly.
Sow direct into the final position, a half inch (1 cm) deep, and keep the surface from crusting until emergence. In soil at 60F (15C) expect seedlings in about ten days; in cooler soil it can stretch to three weeks. If you must start under cover because of slugs or a short season, use deep root trainers or three inch (7.5 cm) pots, sow two seeds per cell, and plant out within three weeks of emergence without touching the rootball.
| Item | Florence fennel (bulb) | Herb fennel (leaf and seed) |
|---|---|---|
| Sowing depth | Half inch (1 cm) | Half inch (1 cm) |
| Minimum soil temperature | 60F (15C), warmer is safer | 50F (10C) |
| Best sowing window | After midsummer, into shortening days | Spring, once frost risk has passed |
| Final spacing in the row | 8 to 10 inches (20 to 25 cm) | 18 to 24 inches (45 to 60 cm) |
| Row spacing | 18 inches (45 cm) | 3 feet (90 cm) or a single specimen |
| Sowing to harvest | 12 to 14 weeks to a usable bulb | Leaf from week 8, seed in year one or two |
| How often to sow | Every 3 weeks through the second half of summer | Once, ever |
Time Florence fennel into shortening days
Florence fennel bolts in response to long days and to cold nights early in life, and the two triggers catch people at opposite ends of the season. Sowings made in spring sit in lengthening daylight and read that as a signal to flower, so they throw a stem at eight inches (20 cm) tall with no bulb at all. Sowings made too late run out of warmth before the base has swollen.
The reliable window is the second half of summer, so that the plant does its bulking in shortening days and steady warmth. In practice that means sowing from around the longest day onward and finishing about twelve weeks before your first hard frost. A light autumn frost does not hurt a mature bulb, so the tail of the season is more forgiving than the start.
Spring sowings are still worth making if you accept what you will get. They give excellent fronds and, if you let them run, an early crop of seed. Treat them as a leaf crop and you will not be annoyed by the absence of bulbs.
- Sow into warm, already moist ground Water the drill before sowing rather than watering the surface afterward. Fennel seedlings that dry out once in their first fortnight often never bulb properly, and the check is invisible until eight weeks later.
- Thin in two stages, early Thin to 4 inches (10 cm) as soon as the first true feathery leaf shows, then to the final 8 to 10 inches (20 to 25 cm) when plants are about 5 inches (13 cm) tall. Late thinning is itself a check because the roots of neighbors are already tangled.
- Cut thinnings out, do not pull them Snip surplus seedlings at soil level with scissors. Pulling disturbs the taproot of the plant you are keeping, which is exactly the shock that sends it to flower.
- Sow a short row every three weeks Bulbs hold in good condition for only about two weeks in the ground before they go coarse and split. Three short sowings give a month of eating; one long row gives one glut and a lot of woody bulbs.
Herb fennel: sow once, and choose the spot carefully
Herb fennel is sown once and then lives with you. A single plant produces several thousand viable seeds in a season, they germinate readily in gravel, path edges, cracks and the middle of other perennials, and each seedling makes a taproot within weeks. Two years of ignoring it turns one plant into a colony spread across ten feet (3 m) of border.
Getting an established plant out is genuinely hard work. The taproot goes down eighteen inches (45 cm) or more and regrows from any crown fragment left behind, so a spade cut through the top simply produces two plants. Removal means loosening a wide circle with a fork and lifting the whole root, ideally when the ground is damp and the plant is still young.
Choose the position on the assumption that it is permanent: full sun, free-draining ground, at the back where a six foot (1.8 m) plant does not shade a bed, and away from gravel paths where seedlings are impossible to hoe. Growing it in a large tub sunk into the border keeps the root contained, though it does nothing about the seed.
The single most effective control is to cut the flower umbels off as the petals fade and before any seed browns. One season of letting a plant shed seed creates several years of weeding.