Humidity for Houseplants

Humidity is the amount of water vapour contained in the air. Air humidity is measured on a relative humidity scale, where “0%” means the air is completely dry, contains no moisture content and does not occur naturally, to “100%” where the air is completely water-saturated. Warm air dries out quickly so needs more water adding than cold air does, in order to maintain required humidity levels.

Many types of houseplants require indoor humidity to be relatively high, especially tropical rainforest plants such as the syngonium, plus bromeliads and some orchids. Fleshy-leaved houseplants thrive on moisture, while air plants or tillandsia carry scales on their foliage which collect moisture from the air and nutrients from dust particles within it.

Some of the houseplant species with thin leaves require high levels of humidity. Even cacti and succulent plants require around 30% humidity as a general rule. This is a reason why the rooms where humidity is often highest, such as bathrooms and kitchens, are often particularly suitable for some kinds of houseplants.

Room humidity in temperate climates during summer usually ranges from around 40% to 50%, though this is a generalisation. 40% is satisfactory for most houseplants. In winter, when heating is used within rooms, the air humidity will drop to around 20% or lower. Whilst warm air generally benefits plants, the reduced humidity damages them. Hence humidity should be maintained, at least in the immediate vicinity of the plant itself.

Rescuing Houseplants Damaged by Inappropriate Humidity

Relative humidity levels that cause damage to houseplants manifest themselves in slightly more extreme versions of the signs of the inappropriate levels of watering. With too little humidity, leaf tips shrivel and/or become slightly yellow quite suddenly at their edges, while those houseplants that flower may drop their flowers or buds very quickly. Some ferns can be particularly prone to too little humidity.

Humidity indicators of high humidity levels in houseplants are often signs of rot or mould. Increasing the humidity for a houseplant will reduce the need for watering. Grey mould, or botrytis, occurs particularly in still air and is most distinctive from other signs of distress when it occurs in the centre of leaves, or invades the centres quickly from the edges.

Such leaves are best removed quickly. Compost may need changing and a systemic fungicide spray will be required, though improving the air circulation is the fundamental additional remedy. Rot needs to be caught quickly if the houseplant is to be saved, and if so, removing the diseased areas, improving ventilation and reduced watering are called for.

Measuring Humidity

Humidity monitoring using specialized humidity meters, or installing portable humidifiers for rooms may be appropriate for enthusiasts, as might some form of scientific metered measurement or test measurement on an occasional basis. Most houseplant gardeners will research plants before they attempt to grow them and usually know from experience which houseplants will grow where, but from time to time a simple and inexpensive test might be useful.

Such a way of humidity testing for the general level of humidity in a room can be done using an empty glass. If the glass is left in a fridge overnight, it can then be placed in the position of the houseplant in the morning and left for five minutes. The houseplant position should already be away from draughts.

If the frost on the surface of the glass clears almost completely in such a short time the air has readily absorbed the moisture so must be too dry. If it has stayed fairly well-frosted, then the air humidity is likely to be at a reasonable level akin to that of a normal room in summer. If there is still some frosting but defrosted water is running down the glass because it has not been absorbed by the air then the air is quite humid and only those houseplants requiring very high humidity will need more attention. While this is a general guide it is suitable as a “quick fix” for determining humidity level for most houseplant purposes.

Ways to Maintain Humidity for Houseplants

Regular maintenance of houseplant humidity may cover a range of activities and research of specific plants’ needs is worthwhile, though none of this is unduly time-consuming. Useful practices include:

• Misting houseplant foliage regularly with a light spray of tepid, soft water is the usual method. Spraying flowers directly in such a way is best avoided on some houseplant species.
• Standing houseplant pots in trays filled with pebbles that are kept moist is often suitable, so that the moisture from the pebbles will moisturise the air around the plant as it evaporates.
• Growing appropriate houseplants in close proximity to each other, as well as being a display opportunity, is also suitable and helps maintain similar humidity across the group.
• If a water feature is employed, growing appropriate plants around it is another method.
• Placing houseplant pots in outer pots, sometime termed outer- or double-potting, so that an appropriate moisture-retaining mixture can be maintained between the two pots without over-watering the houseplant is suitable in some instances.
• Lining hanging baskets with moss can be suitable for retaining moisture but may require closer day-to-day management than other methods in some instances.
• Moss poles, known also as moss sticks or totem poles, are usually used as supports for display and growth with tropical plants in particular. They also help maintain humidity.
• Some houseplants also benefit from an occasional gentle hosing or even from a gentle shower as a quick and occasional practice.

Employing many of the above methods can be an integral part of the houseplant display. Another useful precursor to keeping foliage houseplants healthy in terms of their humidity is to occasionally wipe their leaves with a damp cloth. Some care is needed as this will not be suitable for all houseplants and as a general rule, young leaves should be left undisturbed. Indoor alternatives for specialized requirements are terrariums and bottle gardens, which help create a microclimate around the plants in which it is easier to maintain the humidity levels.