Since the summer newsletter I’ve made progress on my new hothouse. The structure is now complete and most of my tropical ferns have taken up residence. Most importantly, the living room is now cleared of the stacked IKEA plastic tubs that were acting as a temporary fern nursery. I’ve been spending the first few weeks trying to stabilise and optimise the environmental conditions.
The final panels of glass had barely been positioned, a smell of silicone still lingered in the air, and I was already moving a few ferns into the new hothouse. I was going away for a few days, so rather than rush in with any delicate ferns, I decided to move my collection of small tree ferns in there for the few days I was away. The ferns in the cold greenhouse were beginning to flush with fronds and would appreciate a little elbow room. I also added a small electronic thermometer and hygrometer, so I could monitor the temperature and relative humidity whilst I was away. I figured before rushing out and buying heaters and humidifiers, I should at least see how the environment behaved with no extra support.
The hothouse is essentially an aluminium, double-glazed, lean-to conservatory. It fixes to a north-east facing wall, but the south facing elevation isn’t glazed apart from the roof section. So, during the summer the structure doesn’t get much direct light apart from the early rays of sun, which hit a small section of the structure but are filtered by several tall silver birches when they are in leaf. The conservatory company advised not to use an automatic roof ventilation system, as they usually go wrong and they wouldn’t be able to guarantee. Instead, I opted for a small side window, should additional ventilation be required, and the plan was to install a wall ventilation fan in the future.
Over the few days the temperature inside the structure maintained a steady few degrees above the outside temperature. There was a slight increase in temperature with the morning sun relative to the outside temperature but nothing like I’ve seen in the other greenhouse structures in the garden. So, the thermal glass was working, to the point where I was almost wishing there was slightly more thermal gain from the morning light. It meant the heating bill would be higher than anticipated.
The humidity was essentially the same as outside but raised by about 20 %RH after watering the plants and wetting the concrete floor. As one would expect, this gradually dropped to match the outside humidity over the course of a couple of days. So, how could I achieve a constant 18 °C and 80–90 %RH? I had an old oil convection heater. It had setting of either 1000 kW or 2000 kW. With a simple probe thermostat, on the lower heat level setting, I rigged the heater up. The temperature soon stabilised with a gentle oscillation of about 1 degree. Even though the floor print of the hothouse is small, only
3 × 2 m, it’s relatively tall, and what soon became apparent was the positioning of the probe of the thermostat made a huge difference to the average temperature reading. As I moved the thermostat around, it was clear the floor temperature was several degrees cooler than higher in the structure. The introduction of an oscillating fan sufficiently remedied this effect and lessened the difference across the structure but didn’t remove it completely. The fan is on continuously and in my opinion, reduces the amount of time the heater is on, as the heat is distributed more evenly.
After considering a commercial fogger, I opted for a domestic ultrasonic humidifier. Much cheaper and one had served me well in the hot cupboard. Should it not work, I would be able to use it elsewhere, so it wouldn’t be wasted. I filled up the humidifier, a 6 l model, and set it to maintain 80 %RH. Within the course of two days, I needed to refill it. This was a problem. I don’t use a reverse osmosis system, so my only option was tap water. Buying 18 l of deionised water from Tesco every week wasn’t practical and I had tried rainwater in humidifiers in the past but algae becomes a problem—not to mention a risk of disease from bird droppings and so on. You may remember in my attempts to raise the humidity in the house, I quickly coated every surface in a fine calcium dust. Vapourising 18 l of hard tapwater in the hothouse weekly wouldn’t be healthy for the plants as they would be quickly covered in the deposit. I didn’t outwardly panic at the stage, I buried my head in the sand and I began to move a few more ferns into their new home.
To begin with I added nothing I felt was too delicate. Mainly a few large Nephrolepis which were suffering inside—ironically from low humidity—and my Drynaria baskets. What I noticed was as more plants went in, the humidifier was on less and less. Miraculously, move forward a couple of months, a few more ferns, benches with capillary matting, and the humidity remains constant at between 85–95 %RH and the humidifier is rarely on. If the door to the hothouse has been open for a brief period, it will switch on for a few minutes but nothing more.
As we’ve moved into summer, the thermal gain has increased slightly, so mid-morning I’m finding the temperature goes up to around 22 °C. The hottest it’s reached outside (in the shade) this year is 25.4 °C on the 17 May 22. Inside the hothouse on the same day the most it got to was 24 °C. For comparison, the polycarbonate lean-to (cold greenhouse) went up to 31.4 °C on the same day and the hot cupboard (wood and perspex) 26.7 °C. Both these structures get more sunlight during the course of the
day. So, the positioning and type of glazing has certainly helped to keep the
temperatures more stable in the new hothouse.
At this point I’ve never felt the need to open the side window and I have no immediate plans to install the planned ventilation fan. I’ll monitor what happens as we hit the higher temperatures of summer. I’m aware that these conditions aren’t fundamentally far off what you would expect in a closed terrarium. Of course, I’m on the lookout for problems with mould, but I’m hoping that if I can keep the air circulation sufficiently high I can avoid problems associated with a lack of ventilation or stagnant air. It’s early days but my Microgramma seem happier in the ‘windy’, humid conditions than they were in the hot cupboard, so I’m sitting back and observing for the time being.
What Is Humidity?
Humidity is the amount of water vapour held in the air. There are several measurements of humidity. For us fern growers we are concerned with relative humidity, which in simple terms is the ratio of how much water vapour is held in the air to how much water vapour the air could potentially hold at a given temperature and pressure. Relative humidity therefore varies with the temperature, meaning a change in temperature will alter the relative humidity, even if the total amount of water vapour in the air remains constant. Colder air can hold less water vapour than warmer air.
Relative humidity is expressed as a percentage (%RH). At 100 %RH, the air is said to be ‘saturated’ and is at its ‘dew point’. The dew point is the temperature to which air must be cooled to become saturated with water vapour, assuming constant air pressure and water content. When cooled below the dew point, the air’s capacity to hold water vapour is reduced and airborne water vapour will begin to condense to form liquid water known as ‘dew’. When this occurs, usually via contact with a colder surface, dew will form on that surface. Interestingly, relative humidity is only a measurement of the invisible water vapour. Mists, sprays, fogs and aerosols of water do not count towards the measure of relative humidity of the air and won’t necessarily directly affect the relative humidity. However, the presence of mists and fogs may indicate a body of air may be close to its dew point. Warming a body of air that contains a fog may cause that fog to evaporate, as the air between the water droplets becomes more able to hold water vapour.
Relative humidity affects our own perception of temperature. This is because a high relative humidity may hinder the body's ability to cool itself as it can slow down the evaporation of perspiration from the skin.
Why Is Humidity Important To Ferns?
The main factor that has led to ferns and their allies evolving in humid environments is that their lifecycle has a reliance on moist habitats. In ferns, the egg, produced by a structure called an archegonium, remains static while the flagellated sperm, produced by a structure called an antheridium, have to swim in a thin film of water to an egg. Therefore, although the adult fern sporophyte (the general fern plant with leaves and such), can live in fairly dry soil, the sperm need a film of water to swim to the egg. Therefore, ferns are mostly found in areas that are at least seasonally wet. Many ferns therefore grow in moist areas as it is a more conducive environment for their reproduction. Consequently, this is also why ferns often grow in shady areas, as many moist areas are in shade.
There are, of course, exceptions to this, and many ferns have adaptations, included changes to their morphology and reproductive behaviour, that allow them to survive in places where they can grow for extended periods of time with very little moisture—these are often referred to as xeric ferns.
By evolving in moist environments, ferns have made other adaptations, that have yoked them further to relying on higher levels of humidity. The extra moisture in the air means there is less potential for transpiration—less water from the plant evaporates. It is harder for water to evaporate into more saturated air than into dryer air. Therefore ferns, and other plants that live in moist environments, are generally able to keep their stomata (the tiny holes in their leaves which allow gas exchange) open longer and wider because there is less risk of losing excessive amounts of water. In turn, this increase in CO2 absorption means more photosynthesis and more growth. Another way of framing this is that the rate of photosynthesis increases as the humidity increases. More photosynthesis is particularly welcome if you are living in an environment with reduced
light levels. If a fern species which has evolved and adapted to living in a humid environment finds itself in drier conditions, then transpiration rates will be too high and fronds may quickly desiccate.
What Is VPD?
Without getting overly complicated, vapour pressure deficit (VPD) is a concept that explores the difference between the maximum possible water vapour pressure (saturation vapour pressure) and the actual amount of water vapour pressure (actual vapour pressure). In essence it’s a measurement that relates to the plant’s ability to transpire, open its stomata and thrive. At a low VPD the stomata will open but there may be no room in the air for more water vapour and therefore transpiration may not occur. As VPD increases the stomata will get smaller and close, meaning CO2 uptake will reduce. Under these conditions too much water could evaporate from the leaf due to a larger difference in vapour pressure between the leaf and the air. An ideal VPD means the stomata will open and there will be just enough room in the air for the correct level of transpiration. It’s a balancing act. Most ferns are adapted to low VPD environments and will suffer when the VPD is high. It’s worth also considering that VPD requirements may change during different stages of growth.
A study of Nephrolepis cultivars (Dawson, King and Staay, 1991) shows, despite these ferns being among the most sun-tolerant of ferns, they all exhibit best growth when very low VPD levels were maintained.
The following table shows the basic relationship between environmental
factors and VPD.
Techniques To Increase Humidity
Adding more water to the environment. This may be in the form of standing vats of water or ponds. The water will evaporate and increase the humidity. The higher the temperature of the water, the more evaporation will occur. Using damp capillary matting, expanded clay balls or gravel will have a similar effect.
Terracotta pots. By using terracotta pots, water is wicked away from the soil into the pot and evaporation will occur from the surface of the pot into the environment.
Wetting the floor. In environments such as greenhouses, you will be able to wet the floor. The water will evaporate and increase humidity.
Lower the temperature. Excessive heat will potentially reduce humidity, lowering the temperature may help. Providing shade to your greenhouse might work. Increasing ventilation may lower the the temperature but it could potentially lower the humidity further if the outside air has a lower humidity level.
Use a humidifier, fogger or misting system. There are several types available which use different mechanisms to add water vapour or mists to the air. Commercial foggers generally use high pressure to create a fine mist through nozzles. There are smaller ultrasonic systems available which would be suitable for smaller greenhouses, conservatories and plant rooms. Very small ultrasonic systems and misting systems are also available for terrariums. The latter being available from reptile and amphibian specialists. Be aware that if you are using tap water in a hard
water area, excessive use of these systems may result in calcium deposits on your plants which, as well as being unsightly, may impede photosynthesis and affect the health of your plants.
Using a handheld mister or spray. The effect will probably do very little to adjust the relative humidity, any adjustment is likely to be only temporary.
Grouping plants. Keeping plants closer together will increase relative humidity locally around the plants.
Positioning your plants. You may find there are corners or areas of your greenhouse that may have greater localised humidity. The temperature differential from the bottom of the greenhouse to the top, will equally mean there is also a differential of humidity too. If the top shelf of your greenhouse is 20 °C with a relative humidity of 62 %RH, and the floor of your greenhouse is 16 °C, the corresponding relative humidity at the floor would be around 80 %RH.
Measuring Humidity
Hygrometers are used to measure humidity. There are two main kinds. Mechanical hygrometers make use of organic substances (typically a piece of hair) that will contract and expand in response to the humidity. Contraction and expansion of the hair element in a mechanical hygrometer causes the spring to move the needle on the dial. Electronic hygrometers measure the change in electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the humidity changes. Other hygrometers sense changes in weight, volume, or transparency of various substances that react to humidity.
I’ve experimented with a few brands of electronic hygrometers in my greenhouses. It’s worth noting that most of them are not recommended for use outside or in environments over 90 %RH. However, I’ve had most success with the Govee H5179 Smart Thermo-Hygrometer. This little device can hang in the hothouse and sends data to your smartphone via Bluetooth and WiFi. It’s battery powered, with a working humidity range of 0–99 %RH and working temperature range of -20–60 °C. It also allows you to export your data. I did chance one outside, but it got rained on, and promptly stopped working. I replaced it but in a new position under the eaves of the garage, so it remains away from any rain.
MATTHEW REEVE
Matthew is an amateur pteridomaniac with a passion of tropical, tender and aquatic ferns. He works in theatre as a musical director and composer. He recently joined the BPS committee and co-leads the Tropical and Indoor Fern Group with Peter Blake.
Related Posts
