Most plants grow best with relative humidity of over 50%. Although many plants will tolerate lower levels, only those native to arid regions will tolerate humidity levels below 25%.
Problems usually appear in winter, when dry home heating robs the air of moisture. The best way to raise humidity levels is to use a humidifier. If you do not have a humidifier, you can place a plant's pot in a large saucer filled with gravel and water. Be careful not to let the bottom of the pot sit in the water, or the roots may rot. As the water evaporates, it will fill the surrounding air with moisture. You can also mist the leaves with water.
While it would be ideal to give each plant the conditions that suit it best, most indoor plants will adapt to normal room temperatures. This is especially true in summer, when growing conditions are optimal.
Most indoor plants grow more slowly in winter because of the shortage of light. So it is best to maintain slightly cooler temperatures, keep the plants away from heat sources and reduce the frequency of waterings at this time of year. Plants that require a dormancy period in winter can be kept at even lower temperatures (13 to 15°C).
In their natural habitat, most plants experience a drop in nighttime temperatures. It is a good idea to lower the thermostat by 3°C in the evening, because too much heat at night will weaken some plants and interfere with blooming.
Most plants need to be watered when the top centimetre of soil in the pot is dry to the touch. Plants adapted to arid regions, including cacti and succulents, can be allowed to dry out even more. They should be watered more frequently in summer, however, when growing conditions are optimal.
Avoid watering too little, too often. Thoroughly soak the soil surface until you see water coming through the drainage holes, and remove any excess water sitting in the saucer a few minutes later.
Use room-temperature water. Cold water can slow the growth and produce spots on the leaves of various tropical plants. In addition, because some plants are sensitive to the chlorine in tap water, letting the water sit for a day before using it will allow the chlorine to evaporate.