Plants need darkness

How much light do indoor plants need?

Status: 04.12.2020 2:41 p.m.

Plants need light to live. Deficiencies are easy to spot in winter when they shed their leaves. But light is not only the energy supplier, it also controls processes in the plant.

by Helge Masch

If there is enough light in a room for people to orientate themselves, that does not mean that there is enough light for the indoor plants for photosynthesis. The illuminance - measured in lux - indicates how much light is affecting a surface.

Plants often lack light in winter

The summer sun brings it to 100,000 lux, an overcast summer day to around 20,000 lux, an overcast winter day, on the other hand, only 3,500 lux. In winter, at best, 2,000 lux are available for our indoor plants - when the sky is clear of clouds. If a plant is in the middle of the room, you can subtract around 1,000 lux for every meter of distance from the window. It is generally said that indoor plants need about 1,000 lux to grow.

Light requirements are very different

What do the pictograms on the plant labels mean?

When buying house plants, there is usually a care and location notice in the form of pictograms. What do they mean?

shadow: low light requirement, 300 to 800 lux
Penumbra: medium light requirement, 800 to 1,000 lux
Sunny: high light requirement, 1,000 to 1,500 lux

The light requirement of a houseplant also depends on whether the plant grows in its natural location in full sun or in the shade. If a plant has so-called variegated leaves, i.e. yellow or white components, there is correspondingly less chlorophyll available to absorb light. Such varieties have a higher need for light. In general, it can be said: In the low-light autumn and winter time, all plants should ideally be placed directly at the south-facing window.

In order to determine the correct location for a plant, it is not necessary to purchase a measuring device, a so-called lux meter. It is enough to observe the reaction of the plant to its location and to make it lighter or darker in time.

This is how plants react to a lack of light

  • The distances between the leaf nodes become significantly longer. The plant stretches towards the light.
  • Since there is not enough energy available to supply all organs, the plant sheds leaves in order not to exhaust itself in times of lack of light.
  • New leaves stay smaller.
  • Green plants with variegated leaves. This means that chlorophyll forms in, for example, white areas.
  • Typical leaf structures, such as the holes and bulges in the window leaf, do not appear.

Examples of indoor plants with low light requirements

There are plants that are very frugal and only need little light (300 to 500 lux), for example ivy, tree friend, window leaf, cobbler palm, bow hemp and dragon tree. African violets, mono-leaf and flamingo flowers are flowering plants that also do well in the shade.

Light controls plant development

The poinsettia needs darkness to form red bracts.

Plants differ in "short-day plants" and "long-day plants". The critical day length is twelve hours in each case. A short-day plant like the poinsettia can only bloom if it receives less than twelve hours of light a day. A long-day plant such as the shrub marguerite, on the other hand, requires more than twelve hours of light for flower formation.

Pay attention to the light requirement when sowing

The need for light also plays an important role when sowing plants. A distinction is made between light germs and dark germs. The seeds of the dark germs only begin to germinate when they are so deep in the earth that no more light can reach them. The seeds of the light germs (lobelia, begonia), on the other hand, must not be covered with substrate. If there is insufficient exposure to light, the ability to germinate is greatly reduced.

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NDR 90.3 | The garden | 04/01/2017 | 8:00 p.m.