Growing green manure in pots or containers: possible or not?

Why grow green manure in pots or containers?

A plant, whether ornamental or a vegetable, needs water and nutrients to grow. Nutrients that it draws from the soil. Plants cultivated in pots, containers or raised vegetable beds therefore benefit from rather limited nutrient reserves, as the substrate becomes depleted over the seasons. This is why it is essential to nourish this soil. Among the various ways to enrich the soil, green manures are certainly the most comprehensive, as they are not limited to their fertilising effect alone.

The primary function of green manures is to release mineral elements such as nitrogen through their decomposition, elements that will later be used by the plants or vegetables planted in the ground. This holds true in open ground, in vegetable gardens or flower beds, but also in all containers used for cultivating plants, vegetables or small fruits in soilless conditions. Similarly, when green manures decompose, their organic matter is returned to the substrate, eventually producing humus.

In pots, containers or raised vegetable beds, the substrate becomes depleted. Hence the usefulness of green manures to enrich the soil

But the role of green manures doesn’t end there. Sown between two crops, they prevent the substrate from remaining bare:

• They occupy pots and containers between two crops, thus limiting weed proliferation.
• They reduce erosion, especially in winter.
• They improve the substrate’s structure thanks to their root system that breaks up particles.
• They stimulate the biological activity of the substrate.
• They promote biodiversity through their flowering, often melliferous and nectariferous, attracting and nourishing pollinating insects and beneficial insects.

Which species of green manure to choose for container growing?

There are several species of green manures that play different roles.

Fabaceae

Fabaceae, commonly known as legumes, enrich the soil with nitrogen. Indeed, thanks to a symbiosis with a bacterium of the genus Rhizobium, the root nodules have the ability to fix atmospheric nitrogen and return it to the soil. The most commonly cultivated Fabaceae are broad beans, peas and beans, which can serve as green manures. But you can also sow:

• The sainfoin, ideal for dry substrates, is perfectly frost-resistant. Its flowers are very melliferous.
• The white clover, purple or crimson is very interesting at the foot of a small fruit tree grown in a pot. It is often combined with a grass.
• The spring vetch or winter vetch benefits from very rapid growth, with dense and covering vegetation.
  

  Fabaceae are nitrogen-rich green manures

Grasses

Grasses provide abundant foliage that constitutes a choice standing crop. Similarly, their deep root system aerates the soil. Finally, grasses are not from the same family as some of the vegetables grown in pots, which limits the spread of diseases and parasites. As a result, crop rotation is perfect. You can sow:

• The rye, which can be sown alone or mixed with a legume.
• Oats, which have a dense root system rich in nutrients. They adapt to all climates.
  

  Rye and oats are green manures to grow in pots.

Brassicaceae

Among the Brassicaceae, white mustard is the main green manure. It has the advantage of growing very quickly and working the soil deeply thanks to its taproot. It should not be sown after vegetables from the same family such as cabbages, turnips, radishes or rocket.

Other green manures

Among other plants commonly used as green manures, we can also mention phacelia from the Boraginaceae family, which has several advantages: it has rapid growth, is frost-resistant, produces a lot of standing crop and inhibits weed germination. It also creates a break in crop rotation, as its botanical family is different from other vegetable families. Finally, its melliferous flowering spreads over several weeks.

Phacelia is a very melliferous green manure

You can also sow buckwheat, from the Polygonaceae family, with qualities similar to those of phacelia. It belongs to a different botanical family from that of common vegetables, it grows quickly, it is frost-sensitive in winter so disappears on its own, and its small flowers attract many insects.

How and when to grow these green manures in pots?

Growing green manure in containers isn’t complicated, provided you follow a few basic rules. Even though pots offer less depth than open ground soil, many species adapt very well.

The substrate should be lightly worked on the surface to loosen it. Then, the sowing is done “broadcast style”. The seeds are just covered, firmly pressed down, and watered with a fine spray. Watering continues if needed. That’s all.

The sowing period depends on the green manure species and the desired effect. Green manure can be sown between late winter and early spring, just before planting certain hungry vegetables, such as tomatoes, squashes, courgettes, peppers… These green manures will have time to develop within two months, allowing for vegetable planting by late April or early May. In spring, you mainly sow mustard, buckwheat, spring vetch, phacelia…

By sowing green manure in late summer, they will occupy the substrate in pots and containers to prevent the growth of adventives. Late August to early September, you can sow mustard, sainfoin, crimson clover, winter vetch, rye, and buckwheat.

How to use green manures in containers or pots?

Once the green manures have grown well and reached the beginning of their flowering, it’s time to cut them and incorporate them into the substrate.

Cut them before they go to seed, as once they’ve seeded, some plants can become invasive, especially in pots. Moreover, it’s just before flowering that plants have accumulated the most nutrients. You can cut them by hand, with secateurs or even scissors, depending on the size of the container.

Then, you have two options. The first is to leave them on the surface, as a mulch. This protects the soil, limits water evaporation and gently feeds microorganisms.

The second option is to incorporate the residues into the soil. Chop them finely, then lightly mix them into the top third of the soil with a small hand cultivator. After this, it’s best to leave the container undisturbed for two to three weeks before replanting. This allows enough time for the organic matter to begin decomposing without interfering with future plantings.

To go further:

• Green manures: all you need to know!
• Green manures: why? How?