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Light and humidity are two needs that are variable to the survival of a plant. Ideally the degree of light and humidity that a plant gets should be similar to what that plant would have in its natural habitat.

Light and growth

One of the biggest challenges in growing houseplants is getting the lighting just right. Read on to see why it is so difficult.

The pigments in the leaves capture the light to give the leaf its colour. But light can vary in both intensity and quality. Plant growth is dictated by plant hormones, called auxins. They circulate through the plant and are responsible for the length and width of the cells in the stems, leaves and flowers. Outdoor plants can capture light rays from all directions, which enables the auxins to circulate in a uniform manner through the plant. Growth is then even all over the plant. But the story is different for houseplants. They often get light from only one side. This affects the circulation of the auxins and the result is a lop-sided plant.

The trick is to keep turning your houseplants a quarter of a turn every time you water them. That way light will reach all sides of the plant.

Light intensity

Light intensity is the amount of light that actually gets to the plant and the ideal will vary from plant to plant. If a given plant gets too much or too little light then the process of photosynthesis will be disrupted and growth will be stunted. Too much light destroys the chlorophyll and the leaves will tend to yellow and develop patches that turn brown and dry out. Too little light makes a plant go leggy. The distance between the buds gets too long, the stems weaken, the leaves turn yellow and new ones are smaller. Two-tone leaves turn green and flowering plants stop blooming.

Light quality

Light is generated by the movement of energetic particles called photons. Each colour in the light spectrum has its own wavelength and with the human eye we can basically see seven: red, orange, yellow, green, blue, indigo and violet. These are the rays situated between 400 and 750 nm. Plants use mostly the rays between 445 and 750 nm: violet/blue and orange/red. Blue rays are important for compact and leafy plants, while red rays encourage flowering.

Sunlight is perfectly adapted to the needs of plants, unlike the classic incandescent light bulb. The light emitted form a light bulb is too intense (650-700 nm) to be effective for use by plants. Traditional fluorescent tubes do better but do not offer as complete a spectrum as grow-lights. Grow-lights are more expensive but they are recommended for growing plants indoors. To make a long story short, your plants need either a good source of sunlight or light from specialized grow-lights. Your average incandescent light bulb just doesn't make the grade.

A Light for Every Season

A south-facing window is where you will get the most light and heat into your house. From November to March the sun's rays are less strong and so a southerly window is required for many plants to compensate for the drop in light in the winter. But be careful, during the warmer months, if a plant that doesn't require great amounts of light is put in a southerly window, its leaves are apt to burn.

East-facing windows provide sun in the morning and then the rest of the day they are cooler. There is enough bright sun but it isn't too intense. This is ideal for your ferns, gloxinias etc.

West-facing windows are the place to put your plants that require only a few hours of sun a day. Your north-facing windows are probably too dark in the winter and may be good only for certain plants during the summer months.

Humidity in the Air

Successfully growing houseplants often hinges on being able to recreate as closely as possible the natural habitat of your plants. Have a look at the table below to see the amount of humidity required by different categories of plants.

Category Ideal Relative Humidity
Cacti and Succulent Plants Very low humidity
Ferns 30 - 50 %
Tropical plants 50 - 75 %

Most plants can thrive with the humidity between 40%-60%.

One look at a plant will tell you how high or low the humidity is in the room. For example, if the leaf tips and edges start to dry out or curl up, then you will know that the humidity is too low. You may need to install a humidifier. Sometimes you can simply place your plants that need more humidity in a more humid place, such as in the kitchen or bathroom. Apartment plants often suffer from too little humidity, particularly in the winter months when baseboard heaters tend to be on for hours. To increase the humidity in the air sometimes it is enough to simply place the plant in a saucer of water with a bit of gravel in the bottom. Don't let the water touch the base of the pot, otherwise you might rot the roots. As the water evaporates it will humidify the plant. You can also spray your plants with water two or three times a week, preferable in the morning. If you have time, do it every day.

Watering, misting, a quarter turn, repotting… Don’t we often say that indoor plants need TLC or even a few kind words.