Controlling Humidity For Optimum Houseplant Growth

When we talk of humidity in gardening terms we mean relative humidity. This is the proportion of water vapour in the air. Humidity has to be considered in conjunction with temperature. Cool air is usually more highly charged with moisture, while warm air is normally much less highly charged.

Humidity is measured on a scale that starts at zero per cent (when the air is ‘bone dry’) and climbs to 100 per cent (when the air is said to be saturated). Even in deserts, which we think of as having dry air, humidity levels are likely to be around 40 per cent for some of the time.

spray-misting

Some of the rooms in our homes have a level of humidity similar to deserts, and some of them have even less humidity.

A steamy atmosphere

A saturated level is one where we can actually see the vapour in the air, like a thick fog. It is most often seen in a bathroom when mirrors steam up. Then we see steam condensing on the ceiling and running down the walls and woodwork.

humidity-in-desert-region

Somewhere between these two extremes of very dry air and very moist air is a level that most house plants prefer and need.

Without our intervention they would have to try to cope with the almost desert-like level of 40 per cent humidity. Plants are losing water from their leaves all the time whenever their stomata (pores on the leaf surfaces) open for them to breathe. Air that is highly charged with moisture to start with does not draw as much moisture from a plant as does dry air.

As the temperature rises, so the need to increase the level of humidity increases. More moisture has to be added to warm air to bring it up to the same relative level. There are several ways you can do this.

high-humidity-swamp-region

Some methods will give a long-term boost to the humidity, while others just provide temporary relief for the plant.

To increase the humidity level to the desirable level of 60 per cent you will need to do a great deal more than simply misting your plants with a hand sprayer.

Raising the humidity level will stop the plants losing excessive moisture in an attempt to cool down when temperatures rise. It also discourages certain insects like red spider mites, and so ensures healthier plants.

Can I measure the level of relative humidity in the air in my home?

Yes, simple aids are available. Your garden centre is likely to stock a hydrometer, which will do this.

Can I tell at a glance which plants need high levels of humidity?

Yes, as a general rule plants that have thin papery leaves (some Tradescantias and Caladiums) or very finely divided leaves (Maidenhair Fern, Mother Ferns, and some Asparagus Ferns) like high levels of humidity.

What would be a suitable level of relative humidity for a collection of plants including Philodendron, Ferns, Fitonnia, and a Polka Dot Plant?

Around 60 per cent upwards would suit all of these plants.

  • For most plants to grow well and thrive in the dry unnatural air within the home you will have to provide the right level of humidity.
  • One way to create humid conditions is to stand the plant on a moist base. Cover the base of a large dish or tray with pebbles, gravel, peat, moss or sand, and pour water into the tray to moisten it. Stand the plant on the moist material.
  • As water evaporates you will need to replenish the level in the tray. It is surprising how much water can evaporate from a 30 x 30cm (1 x 1ft) tray. It can be as much as half a pint in a week.
  • Another way is to stand a pot on a wooden block inside a large container. Pour water into the container up to the base of the plant pot. It will be drawn up and evaporated through the wood.
  • Grouping several plants together is another way to raise the humidity level. When water is transpired by one plant it can be taken up by its neighbours.

Creating the right conditions

A good way of keeping a high level of humidity is to stand the pot on moist pebbles. When the pot is put on a bed of moist pebbles, make sure the water level does not reach the roots.

Misting your plants

Misting is a short-term way to raise humidity levels. It has to be done regularly during the day when the weather is hot for it to be effective.

If you do spray, do it early in the morning before temperatures rise and the plant begins to work at cooling down. Do not spray in full sunlight, as there is a danger of scorching the plant when the sunlight dries the droplets of water. It is best to use tepid, soft water. Avoid misting plants that have hairy leaves, as the water may cause them to rot.

Although misting helps discourage red spider mites, it cannot raise the level of humidity adequately in hot weather and is best used along with other methods.

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