All plants in their natural habitats tolerate humidity at some time; even the driest desert air can form dew after the sun sets and the temperature drops. Some orchids are dripping with moisture for much of the year from heavy rainfall and high temperatures, some orchids have seasonal humidity from coastal fog, and yet others are bathed in low cloud.
The first consideration for your greenhouse when thinking about humidity should be the genera you are attempting to grow in it. Even if you are strong-minded enough to stay with one genus (and very few of us are), there can be great variations in the needs of the different species. From what part of the world do your plants originate? In what type of habitat and at what elevation? Is the climate seasonal? You will probably not be able to satisfy the optimum requirements for every one of your plants but by shuffling them around you will eventually find the best place for each.
Humidity in the air can be measured by a hygrometer, although experienced growers sense when it is right for their plants just by feel. It is interesting to place several hygrometers around the greenhouse, noting the difference in humidity in different places. There are many ways of supplying humidity to your greenhouse; there is certain to be a method to suit you, or you may choose a combination of methods.
The way most obvious to the beginner is a hand sprayer. They are cheap to buy and easy to use. This is very effective but the results may only last a very short time so must be repeated several times during the day. Care must be taken to avoid spraying open flowers or brown spotting may spoil the blooms. Damping down by sprinkling water underneath the staging and on the path of the greenhouse works better, and the effects last longer than hand spraying, but both these methods are useful only for people who are available when it needs doing. What happens if you are out all day, or if you go away for extended periods?
Damping down can be automated, the water being run off your mains supply. This is achieved by attaching sprinkler jets underneath the benches via an automated control device that incorporates a non-return valve – not only is this a legal requirement it is essential to prevent contamination of the water supply.
My favourite humidifying trick, until recently, was capillary matting. A water storage tank is placed as high as possible in the greenhouse – it will need a strong stand, obviously. A ‘wick’ of capillary matting leads from the bottom of this top tank up over the rim of the tank to a ‘curtain’ of capillary matting suspended from the tank stand. The bottom of the curtain hangs over another tank on the greenhouse floor. A wick of one-and-a-half inches width will shift eight pints of water in twenty-four hours; this is taken up by the curtain. How much evaporates depends on the size of the curtain, the temperature and air flow. The bottom tank collects any water that has not evaporated. Adjustments to this system are simple. A larger or smaller wick will move comparatively more or less water; a larger or smaller curtain will provide a larger or smaller surface area for evaporation. If you only have room for a small curtain, a fan blowing over it will increase the rate of evaporation. For this system tap water can be used, and an electric pump will easily refill the top tank if you can’t do it manually.
Fogger jets are very good. Water is pumped from a storage tank inside your greenhouse and the jets break it up into a fine mist. Care is needed when siting the jets – will under the benches or high overhead suit your plants best? The system can be set up on a timer, several times a day if required. Use as large a tank as you can cope with to cover for periods when you are away. Tap water can be used for fogger jets, but if your supply is very hard it is best to use a mixture of tap water and some of your precious rainwater to prevent the system being blocked by calcium deposits. Murphy’s Law states this will only occur when you are not there to check.
A fountain or waterfall operated by an electric pump can be a very decorative feature as well as functional. If it splashes a very big reservoir will be needed to prevent it running dry and burning out the motor, but the splashes will add to the humidity. Yes, this type of thing takes up a lot of space and the water needs to be watched or green algae may build up, but you can always try a few Disas. They love their roots being bathed in a constant supply of fresh, cool water.
There is a type of humidifier that blows air vertically through a wet filter, producing lots of lovely moist air, but I have yet to see one with a reservoir larger than twelve pints. I found this lasted for ninety-two hours – just less than four days. Manufacturers please note – a larger reservoir is a must if you want to leave it home alone for more than a long weekend.
ust a few years ago there came on to the market a new type of fogger. It is a small electrical device containing a ceramic disc about one inch in diameter that oscillates at a very high frequency to break water molecules into a fine fog. Its drawback is that it has a very limited range of depth of water in which it will operate, but, unlike an electric motor, it does not burn out if the water evaporates until it is too shallow, it simply ceases to function.
And now for my new toy and current favourite. This year at Chelsea Show I saw somebody had an even better idea than the single disc. They have produced a larger version with five ceramic discs and a float. This produces lots and lots of lovely moist air. I have mine placed in a dustbin full of water – the float ensures that it is always at the optimal operating depth and it should be some time before I need to refill the dustbin. I have moved one of my fans to blow over it. The instructions state that these devices must not be used for more than eight hours at a time, but they can be turned on and off manually or, like all these electrically operated humidifiers, can be placed on timers to ensure that your plants have the humidity delivered to them at the times that suit them best.