A garden greenhouse in the vegetable patch? Everything you can do there

The first morning frosts have appeared, and it's a bit of the beginning of the end in the vegetable garden, unless, like me, you have a garden greenhouse.

The term greenhouse is broad and encompasses various types of structures that offer varying levels of protection against the cold. For vegetable cultivation, the most commonly used model by market gardeners and gardeners is the tunnel greenhouse.

Although unheated, this greenhouse, known as a "cold greenhouse", is nonetheless very practical. The main advantage is that temperatures rise quickly inside. In the heart of winter, even the slightest ray of sunshine can provide a gentle warmth, conducive to vegetable development. Additionally, by blocking the wind, the greenhouse also limits the effects of the cold.

Let’s explore everything you can do in it, as well as the precautions to take when cultivating under cover.

A greenhouse in the vegetable garden: possibilities and advantages

Significantly extend the seasons

The greenhouse, by creating a favourable microclimate and protecting vegetables from light frosts, allows for earlier and later cultivation and harvesting. The soil warms up quickly in spring, enabling seeds to germinate rapidly. Thus, in a mild climate, it is possible to sow carrots, early lettuces, radishes as early as January, and to harvest courgettes in June, as well as tomatoes, aubergines, and cucumbers until the end of October.

Increase productivity

The warmth of the greenhouse also allows for successful cultivation of a wider range of vegetables, particularly those "of the sun" such as tomatoes, peppers, and other more exotic varieties like sweet potatoes or kiwano, which are often less productive in regions with a "hostile" climate. Planted in a warm environment that suits them perfectly, they grow better and faster, thus offering higher yields.

Growing under a tunnel allows for grape harvests, even in a cold climate. Photo credit: Local Food Initiative

Protect against certain diseases

As mentioned in our advice sheet: "Tomato: downy mildew, other diseases and pests", the best protection is to cultivate them under cover. Indeed, the greenhouse, by sheltering from rain, significantly reduces the risk of cryptogamic diseases.

Sow and propagate without clutter

Anyone who grows their vegetable plants knows: from March to May, the house is usually very cluttered with tender vegetables that cannot be planted in the garden before the Ice Saints. Although the cold greenhouse is not suitable for very early sowing, sometimes done as early as February, it still allows for storage, or even planting, as early as April, with appropriate additional protection (thick fleece, frames, or specific installations like a heated box) in anticipation of cold nights.

The greenhouse is also the ideal place to grow cuttings, such as those from blackcurrant and gooseberry bushes prepared in autumn, which will be planted in the following spring.

The cold greenhouse is well-suited for sowing and sheltering cuttings. To protect your plants, consider adding extra protection to prevent potential damage from a cold night.

The limitations of the cold greenhouse

Temperature regulation

It may not be immediately obvious, but in an unheated tunnel greenhouse, it can freeze. It can also get very hot very quickly. The thermal inertia is very low, and without your intervention: temperatures can, within 24 hours, swing dramatically: dropping to -5 °C at night and rising to +40 °C during the day. This temperature range is uncomfortable for plants!

To avoid this, there is only one solution: ventilation… which is usually done by opening the doors or side openings, if the tunnel has them. Regular ventilation also helps to expel excess humidity that encourages the appearance of diseases like powdery mildew. As you can see, managing the climate of a greenhouse can sometimes be a bit tricky and requires, above all, almost constant presence!

Under cover, regulating ambient humidity is important to avoid diseases.

Watering management

In the greenhouse, don’t rely on the sky for watering (unless you opt for the "Cabriolet" model from Serres Tonneau) but on your own strong arms or, if not, your clever mind… This may seem obvious, but to cultivate in this enclosed environment, planning for water supply, storage, and possibly an autonomous watering system is crucial. That said, the issue of watering is certainly the easiest to resolve. In our case, the greenhouse is over 50 meters from the house: the collected rainwater is transferred via a small pump to a dedicated reservoir for the greenhouse. The automatic watering system is gravity-fed and does not require electricity.

Questionable aesthetics

While glass greenhouses generally blend well into gardens, let’s be honest: the same cannot be said for tunnel greenhouses. At night, under the full moon, it may look almost pretty, but the same cannot be said during the day! Nevertheless, there are solutions to integrate it: cultivate around tall plants (Jerusalem artichokes, giant sunflowers, cardoons…) or place it at the back of the garden.

However, know that the appearance of the greenhouse is quickly forgotten when you experience the joy of cultivating and gardening under it on rainy days!