Indoor gardening: Getting the temperature right

Every plant has an optimum temperature range within which it grows well. Aim for steady warmth—close to the top of the preferred range—during the plant's growing season, and nearer the bottom level while it is resting, or dormant.

Houseplants and temperature 

Houseplants come from a wide range of environmental conditions, from cool shady forests to hot sunny deserts. Most benefit from seasonal changes in their ambient temperature, depending on whether they are in full growth or temporarily resting.

Although plants dislike great fluctuations in temperature, which can cause their leaves to turn yellow quickly and drop, most plants are fairly adaptable and will frequently adjust to slightly cooler or warmer conditions. In general, the difference between day and night-time temperatures should be no more than 5°C (10°F).

The majority of houseplants come from tropical and sub-tropical areas, but few thrive indoors in very warm conditions because the atmosphere is too dry—high temperatures in the natural habitat would usually be matched by high levels of light and humidity. If the temperature is not to their liking, plants can show signs of distress:

  • Too warm: thin spindly growth, especially in poor light; leaves turn brown at the edges; flowers fade prematurely.
  • Too cold: leaves curl and turn brown; growth is slow; compost stays wet, causing roots to rot.

 

Rest periods

A plant rests as autumn approaches

As temperatures fall in autumn and the hours of daylight continue to decrease, the rate of photosynthesis needed for growth slows down and many plants enter their annual resting period, or dormancy.

Some bulbs and tuberous plants, such as gloxinias and gloriosas, lose their foliage and become fully dormant. They can be moved and kept almost completely dry in temperatures about 5ºC (10ºF) cooler than in summer.

The growth of other houseplants, especially evergreens, merely slows down in winter. Give these plants less water than in summer; sometimes, it's beneficial to provide cooler conditions elsewhere in the house, but don‧t let the compost dry out completely.

Check each plant about once a week for dead or discoloured leaves. These may indicate the presence of pests such as mealy bugs and red spider mites, which can still be active. Also clear up any fallen leaves and trim dead or damaged shoots as soon as you notice them.

The rest period is over at the first signs of new growth. Water the plant and, if appropriate, move it back into its preferred growing position in the house. This may also be a good time to repot.

 

Winter flowering plants

Some winter-flowering houseplants and bulbs that have been forced for early flowers start their rest period as the days lengthen, rather than shorten. These include tulips, cyclamen, hippeastrums and Christmas cactus (Schlumbergera). Others—including Jasminum polyanthum, kalanchoe and Primula obconica—develop new growth in spring, and unusually their brief rest period coincides with their flowering one.

In this case, plants should be kept cool and watered carefully. Always check a plant's winter preferences to avoid over-watering or keeping it at the wrong temperature.

Top tip: Tender flowering plants need to be kept in a frost-free environment, so move them away from windows at night-time during winter.