By Nicolas Ancion (ULg), David Volk (EBF), Ismaël Baraud (CSTB), Laurent Reynier (CSTB), Bernard de Gouvello (CSTB), Marcel Deravet (IFSB), Nicolas Brulard (Gally)

What can be produced in an RTG?

All kinds of plants can be produced in an RTG according to your project: vegetables, herbs, as well as ornamental plants can be cultivated.
The choice of plants is first determined by the market, through a market study. What do you want to sell, and to whom? What kind of product? Which price are the customers ready to pay? Is there any competition?
Then the building characteristics and the RTG will help to define technically the possible and relevant economic modes of production.
Several kinds of plants with the same climatic needs or one specific crop can be cultivated.
As the urban context is unique, we recommend focussing on the specific need of the market, using freshness and proximity of production as an asset.
The intensive production of root vegetables is not really of interest in RTGs as compared to field production.

What plant production techniques do you recommend?

It is not possible to recommend a single production technique. It must be adapted to the chosen species, environmental constraints, and is very dependent on productivity expectations. For example, a mono-specific tomato production can be carried out in a vegetable garden, in pots or in a soil-less culture substrate, but the surface area, the equipment and the cultivation technique (watering, fertilisation, ...) will have to be adapted.

How to plan production?

Once again, this question needs to be considered according to the yields and expectations inherent to the crop and the selected varieties. Planning should first be consistent with market expectations and workforce availability. It is obvious that some crops have a seasonality of consumption, so it is easy to plan production from seedlings or young plants. For example, basil is mainly consumed in summer, and its cultivation is known to require 6-8 weeks on average to reach a marketable stage from seedlings. Staggered harvests are also possible to produce in the long term, but the principle remains the same: it is necessary to know how to grow the plant and to adapt the conditions (light, temperature, fertilisation, etc.) if necessary in order to have an early or late production compared to the normal period.

Is lighting indispensable?

The addition of artificial light is neither mandatory nor necessary. It can be beneficial to certain crops or for forcing others, but it represents costs and substantial electricity consumption. Urban integration also has to be considered. Does artificial light match with municipality rules, neighbour expectations or the global image of the project? Many horticulturists whose production comes from classical greenhouses do not use any lighting.

Is heating indispensable?

It is not necessary to have a heating system, but it substantially widens the scope of possibilities in terms of crops and also makes it possible to grow plants all year round. Depending on the regions and the winter temperatures and in spite of a good mulching, heating can mitigate cold-induced damage. A lot of traditional field greenhouses are not heated.
Anyway, bioclimatic design dramatically reduces the heat needs and maintains a minimum temperature in winter.
Finally, it is recommended to couple the heating of the greenhouse to that of the building when possible to optimise energy flows.

What crops can we grow in a cold greenhouse in the wintertime?

In an unheated cold greenhouse, it is possible to produce mainly winter-flowering horticultural species such as pansies or primroses, winter vegetables such as cabbages, beets, turnips and chard for example, but it is also possible to consider the multiplication of nursery plants, which could be very interesting for municipalities for example.
In urban areas, the heat island effect and local microclimates can create specific roof conditions favourable to crops (for example, ~30% of the building’s energy lost through the roof can maintain good temperatures in the greenhouse).

Is a ventilation system indispensable?

It is indispensable and essential! It allows the temperature in the greenhouse to be regulated by lowering it when it exceeds the set or desired temperature. It is also necessary for the good health of the greenhouse and of the plants to remove excess moisture and renew ambient air.

What types of substrates can be used?

There exist many horticultural substrates adapted to different types of crops (herbs, seedlings, vegetables, horticultural plants, etc.). They differ from one another in their composition, structure, lightness, fertilisation, etc.
They greatly facilitate the success of crops. In the case of soil-less culture, several possibilities are also available, including straw, sphagnum, peat, sand, etc. The choice will be made according to the crop type, the species, and care must be taken to adapt fertilisation.
Growing media formulation requires great expertise for a perennial substrate adapted to the context (substrate weight, CEC, water retention and drainage, acidity). Contact growing media suppliers.

How should I feed my plants?

Most commercial potting soils are enriched with fertiliser, but the amount is often insufficient for the entire crop. It is therefore necessary to add mineral or organic fertilisers in a solid form at the base of the plants or in a liquid form in the irrigation water, known as ferti-irrigation. The latter is essential in the case of soil-less culture and must be managed according to the developmental stage of the crop, its nature and what is expected from it.