Celeriac

About

Celeriac is celery selected for its base instead of its stalks, and it is the easier of the two to grow because nothing has to be blanched. What it asks for instead is time. Six months from a February sowing to an October lift, and the plant uses every week of it.

The commonest disappointment is a crown the size of a golf ball in October, and it has one cause almost every time. Sown too late, planted out too small, or allowed to dry out in July, the plant never builds the leaf area it needs to fill a crown.

The reward is the best-keeping root vegetable a temperate garden produces. Crowns stand in the ground through frost, improve for it, and lift through to March.

Appearance and character

A rough, knobbly, roughly spherical crown sitting at soil level, 8 to 12 cm across in a good year, with a mass of fibrous roots underneath and leaf scars across the top.

Skin is brown-grey and irregular, and peeling it wastes a third of the crown, which is normal rather than a fault.

Flesh is creamy white, dense and firm, discolouring quickly when cut. It tastes of celery with a nuttiness celery does not have, and it is better cooked than raw, though remoulade is the classic raw use.

Leaves are the same divided, toothed, glossy foliage as celery, on hollow grooved stalks, and are used as a herb.

A bolting plant sends up a branched flower stem to a metre with flat umbels of greenish-white flowers. The crown stays small and woody.

Where it is from

Celeriac is the same species as celery, Apium graveolens, and shares the wild ancestor that grows on damp brackish coastal ground across Europe, Britain included.

Selection for a swollen hypocotyl rather than thick stalks happened in the Mediterranean, and the root form is recorded in northern Europe from the seventeenth century.

Germany, the Netherlands and eastern Europe took to it far more thoroughly than Britain, and it is still described in British seed catalogues in terms that assume the reader has not met it.

The crown is not a root. It is a swollen stem base, a hypocotyl, which is why it sits on the surface rather than under it and why the true roots hang beneath it in a beard.

How it grows

Sow indoors in February at 15 to 20 deg C. The seed is tiny and needs light, so sow on the surface of moist compost and do not cover it. Germination takes two to three weeks.

Prick out into modules at the first true leaf and grow on above 10 deg C. A cold check here produces a plant that bolts in July.

Harden off over a fortnight and plant out in late May, once the nights have warmed. Plant with the crown at soil level rather than buried.

Space at 30 cm each way.

Through the summer, strip the lower leaves and any side shoots off the crown as it swells, which exposes it to light and air and gives a rounder, cleaner crown.

Lift from October. Crowns stand frost in the ground under a straw mulch and are lifted as needed until March.

Through the seasons

Late winter is for sowing, in February indoors on the surface of moist compost. Sowing later than early March costs the crop its size.

Spring grows the plants on above 10 deg C. Pot on rather than let them become potbound. Harden off through late May.

Early summer is for planting out at 30 cm each way, into ground dug with manure. Water in generously and mulch thickly.

Summer is watering and feeding, without a break. Start stripping lower leaves and side shoots in July as the crown begins to swell.

Autumn and winter are the harvest. Lift from October, and mulch the rest with straw to lift through the winter. Clear the bed by March.

Care difficulty

Difficult, and for the same two reasons as celery.

The season is the first. Six months means a February sowing under cover and a late May planting, and there is no way to shorten it. A plant sown in April makes a small crown whatever else is done for it.

Water is the second. The crop needs consistently moist soil for its whole season, and a dry fortnight in July costs size that is never recovered.

Bolting follows a cold check on a young plant, exactly as in celery.

Leaf spot is seed-borne, so buy treated seed.

Stripping the lower leaves is the one technique unique to this crop, and skipping it gives a crown buried in roots and leaf bases that is unpleasant to prepare.

Watering

Water weekly at absolute minimum, and more in heat. Around 20 litres per square metre a week, applied as a soak.

Mulch thickly at planting with 8 cm of compost or well-rotted manure, kept just clear of the crown. Celeriac has a shallow root system and the mulch does more than the watering can.

Water at the base rather than over the plant.

Never let the plants wilt. A check in growth shows up as a small crown in October and cannot be corrected later.

The critical months are July to September, which is when the crown fills.

Warning signs, diseases and pests

Celery leaf spot brings small brown spots spreading across the leaves in a wet summer, weakening the plant and reducing the crown. The fungus is carried on the seed, so use treated seed, space for airflow, and remove affected leaves.

Celery leaf miner tunnels white blotches through the foliage. Crush the larvae by squeezing the tunnel, and cover with fine mesh from May.

Slugs graze the crown and get under the leaf bases. Keep the ground clear and patrol on damp evenings.

Carrot fly larvae tunnel into the crown leaving brown channels. Fine mesh over the bed from planting, or a 60 cm vertical barrier.

Bolting sends up a flower stem and stops the crown developing.

Brown discolouration inside a sound-looking crown is boron deficiency, common on light, dry or over-limed soil. Apply borax at 3 g per square metre dissolved in water.

A small crown in October is a season or a watering fault rather than a disease.

Growing environment

Full sun to light shade.

Rich, deep, moisture-retentive soil is the requirement. Dig in a bucket of well-rotted manure per square metre the autumn before planting.

Ground that stays damp naturally suits celeriac better than free-draining soil.

Lime to pH 6.5 or 7 if the soil is acid, and test rather than guess, since over-liming causes the boron lock-up behind brown discolouration.

Space at 30 cm each way. Crowding gives small crowns.

Shelter from wind, which dries the plants and tears the leaves.

Rotate with the other umbellifers, counting carrot, parsnip, celery, parsley and fennel as one crop.

Feeding

Celeriac is a heavy feeder over a long season.

Dig in well-rotted manure or compost generously before planting.

Feed fortnightly with a balanced liquid feed from planting out until September. Liquid seaweed suits it well.

Use a nitrogen-rich feed in early summer to build the leaf area that fills the crown, then switch to a balanced feed from August.

Stop feeding at the end of September. Late soft growth is damaged by frost.

Add boron only where brown discolouration has appeared before, at 3 g of borax per square metre. Overdosing damages the crop.

Troubleshooting

A crown the size of a golf ball in October. Sown too late, planted out too small, or allowed to dry out in summer. Sow in February, plant out in late May, and water weekly without fail.

The plants flowered in their first summer. Bolting, from a cold check as young plants. Keep seedlings above 10 deg C and plant out in late May rather than early May.

Brown spots spreading across the leaves. Celery leaf spot. Use treated seed, space for airflow, and remove affected leaves.

White tunnels in the leaves. Celery leaf miner. Crush the larvae in the tunnel and cover with fine mesh from May.

Brown, bitter flesh inside a sound crown. Boron deficiency. Apply borax at 3 g per square metre and check the soil is not over-limed.

A mass of roots and leaf bases all over the crown. The lower leaves and side shoots were not stripped off through the summer. Do it from July.

The cut surface goes brown immediately. Normal oxidation. Drop peeled pieces into water with lemon juice.

Can I leave them in the ground over winter? Yes, under a straw mulch, and they improve for the frost. Lift as needed until March, then clear the bed.