Humidity in the home is one of the hardest things to get right, and even if you strike that perfect level it's very difficult to maintain constantly and consistently. Fortunately in most cases humidity for houseplants is only of little importance, the section that follows below looks at the problems with humidity and the effect on indoor plants.
Fortunately in most cases humidity for house plants is only of little importance
Moisture is extracted from any material that is exposed to dry air, which is why washing dries when you put it on the line outside.
On a bright sunny slightly windy day the washing will dry very quickly, because the air is dry and the humidity is low. You'll notice that if the day is particularly "damp" it takes longer for washing to dry and even then may not dry completely, on days like this the humidity is high.
Considering houseplants and humidity. If the average indoor plant is surrounded by low humidity then water will be more easily taken away from the plant's leaves. In most cases it will result in the plant's roots taking up more moisture to replace what has been lost, resulting in the soil drying out quicker and you needing to water it more. If the water loss is too great for the plant to replace what is lost then you will get side effects such as crispy brown leaf edges, leaf drop and in some cases the flower buds falling off.
If the humidity is very high for a long period of time you will avoid the above problems (yay!), but attract fungal disorders, white mold on the surface soil of house plants, rotting and certain pests (boo!) all of which relish in such damp conditions.
Low humidity is sometimes a problem during the Winter months because the majority of homes have central heating, or fires which dry out the air considerably. The upside to this is that in Winter months houseplant's don't grow very much or are instead in a rest/dormancy period so don't need as much water. Therefore dryness at the roots and low humidity for short periods isn't normally a huge problem at this time of year.
High humidity normally only occurs in the bathroom and kitchen, because both places have running hot water from bath, showers, cooking etc which creates steam and therefore moisture. Fortunately once again it's very rare that the bathroom or kitchen is constantly saturated with steam and therefore these places can make ideal locations for plants which demand high humidity.
Let's say you have a problem location with low humidity all year round i.e. in an air conditioned office, or dry city apartment, and you have noticed your indoor plants are suffering, what can you do?
Place washed gravel / sand / pebbles in a large shallow water filled tray. Then put the plant pot on top. This creates a small level of water below the pot which cant be absorbed by the plant's roots. Instead the water gradually evaporates and boosts humidity surrounding the plant. Be sure not to let the pot sit in the water though as this will encourage the roots to rot.
By far the easiest solution for temporary levels of low humidity is misting. It's just a case of buying a spray bottle and misting the plant every so often. If temperature are high you may need to mist several times day.
If you group several house plants close together they will create a natural increase in humidity.
If you don't have the time to constantly mist, fiddle around with pebble trays or you aren't able to group a number of plants together, think about investing in a humidifier. These machines add moisture to the air. Like plants, a lot of people don't like constantly dry air either and humidifiers can alleviate this problem.
What can you do when you've noticed a lot of problems associated with high humidity such as mold on the soil of your house plants, rotting and various pests?
It's unusual for areas of the home to be very dank and stale if good ventilation is provided. In Winter however this can be an issue as most ventilation comes from opening a window, which at this time of you year you might not want to do! If you can't increase ventilation try the following:
Take extra care when you water. It's that simple. Spend a few extra minutes making sure you haven't over watered the plant, and drain off any excess water collecting in it's drip tray etc.
Think about moving the plant to a drier place in the home. Even if it's just for a few months over the Winter season.
If the above isn't helping or cant be done, you could think about investing in a dehumidifier. These machines remove moisture from the air, keeping your plant happy, but also removing problems from your home associated with high humidity, such as mold and fungus growing on your walls and ceilings.