Humidity and Tropical Plants: What Gardeners Need to Know

Humidity simply refers to the amount of water vapour present in the air.

For gardeners, the term we usually hear is relative humidity, often written as RH and expressed as a percentage.

Warm air can hold more water vapour than cool air, so relative humidity can change throughout the day even when the actual amount of water in the air does not change very much.

This is why humidity is often higher during the night and early morning, then becomes lower during the hotter part of the afternoon. In tropical climates, however, humidity may remain fairly high for much of the day.

Plants are constantly exchanging water with the air around them.

Water is absorbed by the roots, transported through the plant, and eventually released through tiny openings on the leaves called stomata. This loss of water is known as transpiration.

Humidity affects how quickly this happens.

• When the surrounding air is dry, water leaves the plant more quickly.
• When the air is very humid, water loss becomes slower.

This can influence:

• how quickly a plant uses water;
• how fast the growing media dries;
• how easily the plant cools itself;
• how nutrients move through the plant; and
• how favourable the environment is for pests and diseases.

There is no single perfect humidity level for all tropical plants.

Many common tropical houseplants and shade plants grow comfortably at around 50–80% relative humidity, although some species prefer even more humid conditions.

Plants originating from rainforests, shaded forest floors or moist mountain habitats often appreciate higher humidity. Examples include:

• many Anthurium species;
• Philodendron;
• Alocasia;
• Calathea and related plants;
• ferns;
• mosses; and
• many tropical orchids.

However, many palms, fruit trees, herbs, flowering shrubs and hardy tropical ornamentals do not require extremely high humidity as long as they receive enough water from the roots.

One common misunderstanding is that tropical plants should always be kept as humid as possible.

In reality, excessively humid and stagnant conditions can create problems. High humidity reduces evaporation from leaves and growing media. If the air is also still, leaves may remain wet for long periods after rain or watering.

You may eventually notice problems such as:

• leaf spots;
• fungal growth;
• stem or crown rot;
• algae growing on potting media;
• moss forming excessively on pots;
• slow soil drying; and
• mould developing on dead leaves or organic matter.

A shaded corner surrounded by walls and dense plants, for example, may remain wet for many hours after rain even though another part of the same garden dries quickly.

Good airflow is one of the easiest ways to manage plants in humid tropical conditions.

Air movement helps wet leaves dry faster and reduces pockets of stagnant air around dense foliage. You do not need strong wind. Even gentle natural air movement can make a big difference.

When arranging plants, avoid packing everything too tightly together. Dense planting may look lush, but it can also reduce airflow and make disease problems more difficult to control.

Regular pruning can also help. Remove:

• dead leaves;
• damaged leaves;
• overcrowded stems; and
• fallen organic matter accumulating around plants.

For indoor collections, a small fan may be helpful if the room is very enclosed, especially when growing many humidity-loving plants together.

When humidity is high, water generally evaporates more slowly from both the leaves and the potting media.

This means a plant may need watering less often than expected.

For example, a plant kept in a shaded area with high humidity, a large plastic pot and moisture-retentive potting media may remain wet for a long time.

Another plant of the same species grown under brighter light with stronger airflow may dry considerably faster.

Misting plants is often recommended as a way to increase humidity.

In practice, the effect is usually temporary. A light mist may increase humidity immediately around the leaves for a short period, but the water soon evaporates.

For outdoor tropical gardens, regular misting is generally unnecessary. Our surrounding humidity is already high enough for most plants.

Indoors, misting may be enjoyable and can help remove dust from some leaves, but it should not be treated as the main solution for low humidity.

Repeatedly wetting leaves may cause problems if:

• the room has poor airflow;
• leaves stay wet for long periods; or
• the plant is susceptible to fungal or bacterial disease.

For most outdoor tropical gardens, no. A humidifier is normally unnecessary.

It may be useful indoors if you grow humidity-sensitive plants in a strongly air-conditioned room where relative humidity becomes quite low.

Even then, you do not necessarily need to create jungle-level humidity. Moderately increasing the humidity may already be enough.

Good airflow is still important. Very high humidity inside an enclosed room can lead to condensation, mould and other unwanted problems.

Plants naturally release moisture into the surrounding air through transpiration.

Because of this, grouping several plants together can create a slightly more humid microclimate around them. This can be useful for humidity-loving plants.

However, avoid packing them too closely. A good arrangement gives the plants some shelter while still allowing air to move between the leaves.

Humidity can vary greatly within the same garden.

A shaded area below trees may feel much more humid than an exposed balcony. Similarly, a narrow space between buildings may stay humid because there is very little air movement.

This means moving a plant just a few metres can sometimes improve its growing conditions considerably.

Humidity plays an important role in many plant diseases. Many fungal spores germinate more easily when leaves remain wet or when surrounding humidity is high.

However, it is useful to distinguish between humid air and wet leaves.

A plant can grow perfectly well in humid air if its leaves dry reasonably quickly. Problems become more likely when moisture remains on the plant for long periods.

You can reduce this risk by:

• improving airflow;
• spacing plants properly;
• pruning dense growth;
• removing infected or dead leaves;
• watering the growing media instead of repeatedly wetting the leaves; and
• avoiding unnecessary watering late in the evening where disease is already a problem.

For most outdoor gardeners, probably not.

You can usually judge whether a location is humid, dry, windy or stagnant by observing your plants and the surrounding environment.

A small digital temperature and humidity meter can nevertheless be useful for indoor plant collections, terrariums, greenhouses or enclosed growing spaces.

Still, do not become too obsessed with the number. A reading of 68% versus 72% humidity is unlikely to make much practical difference for most common plants.

For most tropical gardeners, humidity management can be kept quite simple.

For beginners, there is no need to chase a particular humidity percentage. Instead, ask three simple questions.

This approach is usually much more useful than trying to maintain an exact humidity number.

Humidity can sound quite technical, but for beginners I think it is better to start with simple, practical observations rather than chasing a perfect humidity reading. These are a few things I have found especially useful.

Humidity is an important part of tropical gardening, but it should not be considered on its own.

Tropical plants respond to a combination of humidity, temperature, sunlight, airflow, soil moisture and drainage.

In Malaysia and similar tropical climates, outdoor humidity is already suitable for a very wide range of plants. In fact, gardeners are often more likely to face problems from too much moisture and poor airflow than from humidity being too low.

So instead of trying to make your garden more humid, pay attention to the overall environment.

Observe how quickly the soil dries, whether leaves remain wet after rain, how much airflow the plants receive and how each species responds.

Over time, you will begin to recognise the different microclimates in your own garden and know which plants grow best in each location. That understanding is much more useful than chasing a perfect humidity percentage.