Grow in the Dark: Crops That Don't Need Light

26 February 2010, written by Jeremy Dore

The winter months are often characterized by low temperatures, darker days and a sense of waiting for spring to start. If you planned ahead last year then your garden may still be providing supplies of root crops, Brussels sprouts, kale and other winter-hardy vegetables but even so there is a distinct lack of new growth and fresh tastes during this period. Rather than wait for the lighter days of spring to start there are some plants that can be persuaded to produce a new crop in the dark, a process known as ‘forcing’. Although it sounds technical it is not as tricky as you might expect and can yield impressive results when fresh produce is scarce.

Do you remember being taught at school that plants need light to grow? It is actually not true! Plants use light to store energy by means of photosynthesis which uses sunlight to convert carbon dioxide from the air into stored forms of energy like sugars. However, remove the light and the plant can keep on growing using this store of energy. This is how seeds manage to sprout and push up through the soil before they reach the surface.

This simple fact is the basis of forcing plants. By covering a plant or bringing it indoors the warmer temperature means that new growth can be stimulated during the colder months. Forcing often has the added advantage of making the crop taste better because excluding light prevents chlorophyll forming which can be responsible for bitter taste.

The Victorians were experts at this process and experimented with forcing plants by covering them up with huge terracotta forcing pots or by bringing them into dark cellars. By the end of the 19th century forcing was big business in the UK where the cheap coal was used to heat huge forcing sheds for crops like rhubarb. There was even a special express train run from the area known as the rhubarb triangle to London between Christmas and Easter!

Rhubarb

When produced commercially rhubarb is grown outside for two years before digging up the root stock of the plant (the ‘crown’) bringing it inside and forcing it. At the end of this process the root is exhausted and is usually composted.

Few gardeners will want to lose their rhubarb plants, so a more practical way is to leave the plant in the ground and cover it with a large container or bin. The container can be weighed down to stop it being blown over and packed around the outside with straw or mulch to help it retain heat for an earlier harvest. Keep checking it and you can produce beautiful pink forced rhubarb which is sweet and tender in 4-6 weeks, often a month earlier than un-forced rhubarb.

Because forcing deprives the plant of photosynthesis it does weaken it and should only be done in alternate years. For this reason it is worth having more than one plant so that you can alternate the forcing.

Beansprouts

In my opinion the easiest crop to grow in the dark is Chinese beansprouts. Popular in oriental cooking, these are quick to grow and make a great addition to stir fries, with their crunchy texture. They are grown from mung beans which are first soaked overnight before placing in a sprouter or on a layer of something absorbent like cotton wool or paper towels. If using a sprouter, they need rinsing twice a day as they need to be kept moist. Light can be excluded by covering with a cardboard box or placing them in a cupboard. The beansprouts are usually ready in a week and need the small leaves and remains of the bean removing as these are slightly bitter.

Chicory

Popular in Italy, forced chicory is a great way to produce fresh, crisp salad leaves when there is little else available. They have less of the distinctive bitterness of normal chicories and there are special varieties available suitable for forcing such as Witloof and Zoom F1.

The process starts in summer when the chicory is sown outside. The plants are thinned to 30cm (6") apart and kept weeded and watered until the weather starts to turn cold when they are uprooted and stored in a box covered in sand. This can be kept in a cold garage or frost-free outdoor storage area until you are ready to force them on. At this point they can be transferred into pots or packed upright in moist sand and brought into a dark indoor area where the temperature is above 10°C (50°F). In about four weeks the white-green hearts called ‘chicons’ will have grown and be ready for harvesting. At least, that is the theory. I didn’t manage to get my chicory started in time last year, so the plants weren’t big enough for transplanting and I am now growing them on as a spring crop. However, it is definitely high on my list of gardening challenges for this year!

Others

There are other plants that can be forced using similar methods to those described above. Dandelion leaves are forced in France for salads and historically seakale was often forced – so much so that a law was passed in Britain to prevent it being harvested from the wild and preserve stocks.

So, if you fancy a challenge then why not give it a go? Produce a tender crop of crisp leaves or succulent pink rhubarb and you can be enjoying the fruits of your labour when most gardeners are waiting for the warmer weather!