How to Grow Carrots: Stony Soil Is the Whole Problem

Carrots are the crop that most exposes your soil. Everything else about growing them is easy, and the difference between long clean roots and a handful of forked stumps is almost entirely what the seed had to push through. That, and the fly.

The short answer

Sow into deep, stone free soil that has not been manured recently, because fresh organic matter makes roots fork. Sow thinly, a seed every 2 to 3 cm, so you barely thin, and keep the drill damp for the two to three weeks germination takes. Cover with fine mesh or build a 75 cm barrier, since carrot fly flies low. On stony or heavy ground, grow a short variety or use a deep container instead.

Soil decides the crop#

A carrot is a taproot trying to go straight down. Anything in the way diverts it.

Stones

The obvious one. Each stone forks a root. Sieve the top 25 cm if you are serious, or accept short varieties.

Fresh manure or compost

Causes forking and lush tops with thin roots. Carrots follow a manured crop by a year rather than being manured themselves.

A compacted pan

Common in beds that were dug once and then trodden. The root reaches it and stops, giving a stumpy blunt end.

Heavy clay

Resists the root and holds water. Short stumpy varieties or a raised bed are the realistic answers.

Lumps of undecomposed matter

Half rotted material in a new bed acts like stones. Sheet mulched beds are often best given a year before carrots.

If your ground fails on any of those, do not fight it. A 30 cm deep container of compost mixed with a third sand grows better carrots than a stony bed ever will.

Sowing, thinly and patiently#

Two things go wrong at sowing: too thick, and drying out.

Sow 1 cm deep in a shallow drill, aiming for a seed every 2 to 3 cm. The seed is tiny, so mix it with dry sand to spread it, or buy pelleted seed. Firm the drill back gently and water with a fine rose.

Then keep it damp. Germination takes two to three weeks, which is longer than most crops, and a single dry spell in that window loses the sowing. Covering the drill with fleece or a plank until the seedlings show holds the moisture and stops the surface crusting.

Buy fresh seed each year. Carrot seed loses viability faster than most, and a failed sowing that was actually old seed wastes a month.

Carrot fly, and the 75 cm rule#

Carrot fly is the reason people give up on carrots. The larvae tunnel into the roots and ruin them, and by the time you see the damage the crop is done.

The insect flies low, generally below about 60 cm. That single fact gives you two reliable defences.

Key points

• Cover the bed with fine insect mesh from sowing, sealed at the edges. This is the most reliable method and it also excludes other pests.
• Or build a barrier 75 cm high around the bed from mesh or fleece. The fly will not climb over it.
• Or grow in containers raised above that height, which works for the same reason.
• Sow thinly so you barely thin, because crushed foliage releases the smell that attracts the fly.
• Thin in the evening, water afterwards, and take every thinning away rather than leaving it on the soil.

Companion planting with onions is widely recommended and the evidence for it is weak. Mesh works; the smell of alliums is not a substitute.

Thinning, watering and feeding#

If you sowed thinly you may not need to thin at all, which is the goal. If seedlings are crowded, thin to about 5 cm apart for maincrop and 3 cm for earlies, in the evening, and remove the thinnings.

Water consistently rather than heavily. Irregular watering causes splitting, the same mechanism as in tomatoes: the root swells faster than its skin after a drought.

Do not feed. Carrots in reasonable soil need nothing, and nitrogen gives you magnificent foliage and disappointing roots.

Harvest and storage#

Start pulling at any usable size. There is no need to wait for a target, and pulling every other root gives the rest room.

Water the row before pulling if the soil is dry, which stops the root snapping.

Maincrop carrots can stay in the ground into winter in mild areas, covered with straw or fleece if hard frost is forecast. Otherwise lift them, twist off the tops rather than cutting, and store in boxes of dry sand somewhere cool and frost free. They keep for months that way.

What it costs#

A packet of seed

$2 to $5, and buy it fresh each year rather than using the remains of last year's.

Fine insect mesh, 2 m

$15 to $40. The single purchase that decides whether the crop survives, and it lasts years.

Hoops for the mesh

$10 to $25, or free from old plastic pipe.

A 30 cm deep container

$20 to $50 if your ground is stony. Better carrots than a poor bed.

Sand for the compost mix and for storage

$8 to $18 a bag, and it does both jobs.

Where this needs adjusting#

On stony or heavy ground, switch to short stumpy varieties like the Chantenay and Paris Market types, which are bred for exactly this, or grow in containers and stop trying.

In a hot climate, sow earlier and later and avoid the hottest months, because carrots germinate poorly in hot soil and roots become woody.

In a very short season, stick to early varieties and sow under fleece to get the soil warm enough for germination.

Where the guidance here comes from#

The no-manure rule and the 75 cm carrot fly barrier are conventional and both are things I ignored once and paid for. For variety choice by soil type, sowing depth and storage the RHS advice on growing your own is the reference I check, and Garden Organic is the better source on carrot fly barriers and on why the companion planting claims do not hold up well.

The short version#

Deep, stone free, unmanured soil, or a deep container instead. Sow thinly every three weeks, 1 cm deep, and keep it damp for the three weeks germination takes. Mesh over the bed or a 75 cm barrier around it. Do not feed. Pull at any size, and store maincrop in sand.

Related reading#

• Garden planner: rotation, and why carrots follow a manured crop rather than being manured.
• Container gardening: depth and compost mix for growing them in pots.
• Gardening tools for raised beds: hoops and mesh, which do several jobs at once.
• Garden soil vs potting soil: what to fill a deep container with.