Carrot pot depth: match compost to the root you grow

A balcony trough looks generous until a ruler goes into the compost. Carrot pot depth is the inside measurement, from the settled surface to the base. A baby root and a blunt Nantes you mean to grow on do not share it. An RHS container note asks for more than 20 cm of compost for baby roots. Long roots need a deeper pot or an early pull. When watering is the limit, RHS guidance on vegetables in containers points to a 45 cm pot.

Those figures come from the RHS notes, from a Which? remark on 10 or 15 litre pots for baby carrots and against a rich feed, and from the amazon.co.uk listings in our carrot seed set. We did not grow a trial pot, and Which? scores are not copied. Prices and stars are the UK listing figures recorded for this comparison in October 2026. Onward links to amazon.co.uk are affiliate links. A qualifying purchase can earn a commission at no extra cost to you.

In short

Give baby carrots more than 20 cm of compost, measured inside the pot after watering has settled the level. A Nantes-type root needs a deeper pot, or you harvest it while it is still small. Use a 45 cm pot when watering is the constraint, skip a rich feed, and treat a packet that only says "yes, pots" as incomplete until it states a depth.

More than 20 cm for baby roots

The figure to keep is more than 20 cm of compost, and it applies to baby roots. The RHS April container note uses that depth, and in the same note the seed is covered by about 1.5 cm. Measure after you have filled the pot and watered it once. Compost sinks. A pot that looked deep enough in the shop can sit short of 20 cm the next morning.

Measure the compost, not the outside of the pot. The rim and any crocks steal space the root cannot use. Push the ruler to the inside base and read it at the surface. Which? mentions 10 or 15 litre pots for baby carrots, and litres are volume, not depth. A wide trough can hold 15 litres and still give the roots less than 20 cm to travel. A narrower pot can clear 20 cm and hold less water. Use the litre note and the ruler together.

More than 20 cm is a baby crop, not a storage carrot left until autumn. If the settled compost is short of that, keep the pot for a shallower crop and label the depth you measured.

Deeper compost for a Nantes-type root

RHS guidance for vegetables in containers names Nantes or Amsterdam types and tells you to thin to about 8 cm. Round-rooted carrots suit most pots. Long roots need a deep pot or an early pull as baby roots. A Nantes root is the blunt early kind, so "it can go in a pot" still leaves you to decide whether you will pull it young or give it more compost than the baby-root minimum.

The top pick for that blunt early root is the Meldon Early Nantes single at £3.10, with 4.3 from 267 reviews on amazon.co.uk. The seller says it can be grown in pots, troughs or beds, describes a blunt root in 65 to 75 days, and says to cover the seed by about 3 cm. The listing states no pot depth. Those day and cover figures are the seller's, not an RHS trial of this packet. The RHS April note covers seed by about 1.5 cm. Do not average 1.5 and 3 into a private rule. The sowing steps sit on how to sow carrots.

Use this packet when the pot is deeper than the baby minimum, or when you will pull the roots while they are small. A pot that has only just cleared 20 cm will not finish full-sized blunt roots if you leave them for the seller's 65 to 75 days. Reviewers mention crops they liked, and others mention no germination, a damp packet, or roots that stayed small. That is a signal about the packet, not a test of pot depth.

Compare the listing with the rest of the set on the carrot seed comparison. Which packets answer the pot question is on carrot seeds for containers. The title says 1,000 seeds and the pieces line says 2,000. That clash does not change the depth. It does mean the listing is not a complete specification.

The 45 cm pot when watering is the limit

A 45 cm pot is the RHS answer when you want less fuss with water, not a rule that every balcony carrot must be that size. Compost in a shallow trough on a sunny rail dries at the surface on a warm afternoon. Extra depth holds a reserve under that surface.

These notes give no daily watering timetable. Press a finger into the compost and water when the top has dried. A 45 cm pot in the same sun usually needs that top-up less often than a trough that has only just cleared 20 cm, and a hot spell can still empty either container. The compost needs to drain. Depth that cannot escape becomes a sump, and waterlogged compost is a different failure from a pot that dried out.

The 45 cm figure does not authorise a long maincrop left to full size. If the packet is silent on depth, do not assign it 45 cm. Pull long types early, or keep them for a bed that is actually deep.

Compost and feed in the pot

Fill the pot with a compost meant for containers. Which? pairs 10 or 15 litre pots for baby carrots with a clear limit: no rich feed. A strong fertiliser does not add the centimetres a short pot lacks.

Water the filled pot before you sow. After that first watering the surface often drops, so top it up if the settled depth has fallen short, then sow. Sowing first leaves the seed deeper than you meant.

No compost brand is named here, and no Which? score is copied. "No rich feed" is not a rule against water. Some bags already include a modest nutrient charge, which is different from dosing a pot like a hungry bed in July. If the pot is ordinary rather than deep, choose a round or stump type on Nantes or Chantenay carrots. Do not invent a centimetre depth for a stump packet that never states one.

What the packet still has to say

A yes without a number is still unfinished. The Early Nantes listing says pots, troughs or beds and stops there. You can apply the RHS split yourself: baby depth if you pull small, more compost if you want the root to size. You cannot read a pot depth off that page.

A question with no answer is weaker still. The container line on the Autumn King 2 listing does not confirm a pot and states no depth. Treating that silence as permission for any trough is how a long root ends up forked or stubby. If the container has only just cleared 20 cm, plan a baby harvest or a round type, and leave the storage carrot for a bed.

Window boxes, spacing and carrot fly

A window box is a carrot pot only when the settled compost is more than 20 cm deep. Measure after watering, with the box where it will stay, because a box on a bracket can hold less than the same box on the ground. If the depth is short, keep it for a shallower crop.

Where the box clears that depth, thin to about 8 cm. In a narrow box that is often a single row. Sow a little more thickly than the final stand, then remove the extras while they are seedlings. A thick strip then produces threads, because the roots share a shallow layer and the water goes faster than in a 45 cm pot.

Carrot fly still applies on a balcony. The RHS notes that the fly is a weak flier, which is why a pot on a table is sometimes suggested. That note is not a guarantee. Fleece or mesh still has to cover the crop, with no gap at the rim, for the weeks the flies are about. Lift the cover to water, then close it again.