Indoor gardening: Getting the light right

Plants need light when they are actively growing, but not necessarily while dormant or resting. Exactly when and how much light a plant requires depends on its origin and can affect where it is placed in a room.

Woodland and forest plants such as dracaenas, cordylines and aspleniums, naturally grow in dappled shade, so can cope with the semi-shade or filtered light of a sunless window, or should be placed about 1.5m (5ft) away from a sunny window.

Desert cacti, succulents, pelargoniums and other sun-loving flowering plants revel in the bright light of a sunny windowsill.

Most houseplants need something between these two extremes—an east or west-facing window, or bright but not direct sunlight. Very few plants thrive in deep shade—that is, 2m (6ft) or more from a source of natural light.

You can help your houseplants to grow better by taking into account the following points:

Top tip: Plants grow towards light, so give their pots a quarter turn clockwise once a week to encourage a well-balanced shape.

Any indoor lighting can enhance forms and colours and add drama to a display of houseplants, but to improve growing conditions requires a special type of light bulb—ordinary tungsten light bulbs are ineffective. Professional growers use mercury or sodium plant lamps. Fluorescent tubes are an effective supplement to natural daylight, but only if they are suspended 60–90cm (2–3ft) above the foliage.

To make a real difference, you need 1000 lux, that is about 120 watts, per square metre (yard), which would require four 32-watt fluorescent tubes or a single 125-watt high-pressure mercury bulb.

Even though light levels are at their highest in the house during summer, many houseplants appreciate a period outdoors, where gentle rain can refresh their foliage, and light and humidity levels are often higher.

The time to move them out is around the longest day, once there is no risk of frosts, and strong gales are unlikely. Houseplants can stay outdoors until the nights draw in and begin to cool down in late summer.

Plants that benefit from an outdoor holiday include evergreens with shiny leaves, such as citrus and dwarf pomegranates, and forest cacti such as Christmas cactus and epiphyllums.

Shade-loving species such as aspidistras that enjoy cool or moderate temperatures can also enjoy the summer outside. Never move out plants with furry leaves, such as African violet (Saintpaulia) and some begonia species. Tender plants, such as anthuriums and prayer plants (Maranta), that need high summer temperatures must be kept indoors or transferred to a warm, shaded greenhouse where they are safe from wind and bright sunlight.

If you live in a flat or have no suitable garden, your plants can still benefit from extra summer light and ventilation through an open window or door. Pelargoniums, busy lizzies (Impatiens) and most succulents will enjoy this so long as they are not in a draught.

Most houseplants that enjoy summer outside need a sheltered place where they are shielded from wind and scorching sunlight. The shady side of a house or wall, or near a hedge (not under trees, though, because drips can damage their foliage) is ideal, or you can stand them in a shaded coldframe with the lid removed.

Check them daily because their compost can dry out very quickly. To save on watering, you can plunge pots into the ground or into larger containers filled with sand or shingle.