Container vegetable garden: what depth and what soil mix

In a container, 20 to 30 cm of soil is enough for most vegetables. Brussels Environment (Bruxelles Environnement) recommends it. English and American guides say the same. Yet in the ground, a lettuce's roots go down more than a metre. Depth therefore matters less than you think. The volume of soil per plant and the water it holds matter more.

How far roots go down

In 1927, two botanists from the University of Nebraska, John Weaver and William Bruner, published a rare piece of work. They exposed by hand the roots of dozens of vegetables, in the deep soils of Lincoln. In their drawings, the roots go much deeper than you would imagine.

The same book says something else, more useful for a container. In lettuce, all the thick lateral roots start in the top 25 centimetres. Its network of fine rootlets is densest in the top 30. The round radish absorbs mostly between 5 and 20 cm. The tomato grows most of its lateral roots between 5 and 25 cm in the first month. In a one-month-old bean, the bulk of the absorbing roots runs between 5 and 20 cm. The authors describe it as distinctly shallow.

In deep open ground, a root therefore goes down far. But the plant feeds mostly in the top 30 centimetres. A well-watered container of 20 to 30 cm covers this zone.

Depth and volume, vegetable by vegetable

The table gathers the advice of the RHS, the British horticultural society, and of the Virginia Tech extension service. The volume is the minimum size of the container, not a volume per plant.

Sources: Weaver and Bruner (1927), Virginia Tech (2026), RHS.

Three crops need more room. Tomato, courgette and aubergine want at least 19 litres each, according to Virginia Tech. Brussels Environment allows for a pot 30 to 40 cm across for a courgette. For carrot, choose a short variety. The detail is in Carrots in pots.

What height for the container

Brussels Environment asks for 20 to 30 cm of depth, whatever the container. Virginia Tech writes that most plants need at least 15 to 20 cm. The RHS aims higher, with at least 45 cm deep and wide. Otherwise, it says, you will have to water and feed often.

These figures do not contradict each other. A 20 cm container grows salads if you water often. A 45 cm container needs less water and less fertiliser. For a first container, 30 cm is a good compromise.

To know how much soil to buy, multiply the area by the height. A 1 m² container filled to 30 cm takes 300 litres. Brussels Environment counts 250 to 300 litres for a 1 m² container. A container 1.20 m by 80 cm, filled to 30 cm, takes 288.

The soil mix

You have three options.

A shop-bought, peat-free potting mix. Brussels Environment advises an organic mix or one based on coconut fibre. Avoid peat, taken from natural habitats. The RHS also recommends a peat-free mix. It cites John Innes No. 3, a soil-based mix that is easier to manage. A soil-based mix lasts three to four years before being replaced.

A homemade mix. The RHS suggests two parts soil to one part well-rotted organic matter, with a fertiliser added. It is economical for a big container. This mix is not sterile. It can bring diseases or pests from the garden.

With manure. For organic growing, the RHS mixes well-rotted manure into the lower half of the container, at 20 % of the volume.

If your town soil worries you, read the guide on polluted soil in town.

Reusing the soil the following year

Plants use up the nutrients in potting mix. Brussels Environment advises enriching a mix that has already been used with a third of compost by volume. For a 300-litre container, that makes 100 litres of compost well mixed in.

Also feed during the season. According to Virginia Tech, a mix enriched with fertiliser feeds plants for eight to ten weeks. A soluble fertiliser every two to three weeks then takes over. The RHS gives a liquid fertiliser every two weeks. Do not exceed the dose. In a container, there is no large volume of soil to cushion the excess.

The container itself

Almost anything will do, from a wooden box to an old tub, if it has holes in the bottom. Brussels Environment advises against containers that can release toxic residues, such as tyres, treated wood or painted or stained pots. Virginia Tech also rules out wood treated with creosote.

Virginia Tech advises checking containers at least once a day, twice in hot, dry or windy weather. Small pots dry out faster than big ones. Group your containers. They shade each other and the potting mix stays cooler.

The full balcony guide, month by month, is in A balcony vegetable garden in Belgium. The varieties that hold up in pots are in the Balcony & pots collection. The app calculates when to sow each vegetable in your container, according to the climate of your town. Start here.

Photo: Markus Spiske, Unsplash