When we talk about heat, humidity and airflow we usually want to look at all three as complimentary aspects of our growing environment. The primary reason for this is because they’re all interlinked, humidity is created and maintained by heat and airflow determines how that humidity is dispersed. Much like the planets natural convection on a grand scale that determines the weather day in and day out, were looking at the same systems at play for gardening.
For outdoor growing were usually focusing on just our growing zones, these are fairly reliable bands of space based on latitude, longitude, and elevation of where our garden is located, we can circumvent our zone through use of greenhouses or cold frames to prolong the growing season or increase humidity, or the use of shade cloth or windbreaks and shade producing features such as landscaping and hardscaping to reduce drying heat for those that prefer a cooler climate or are less drought tolerant.
For indoor growing were looking at man made variables that determine heat, humidity, and airflow for our growing conditions, with ambient and forced air heating and cooling, a variety of home insulation, ceiling, table and floor fans, humidifiers and dehumidifiers – we have a great amount of control over our environment in these settings, but most people want to grow plants in environments that are also favorable to humans, so today let’s look at the average home environment and what that translates to for indoor plants.
As an orchid hobbyist I’m keenly aware of just how at odds my preferred environment is when compared to my plants preferred environment, so we have to make small adaptions to make it work for both of us together. In particular I enjoy several Dendrobium Nobiles that actually require a winter dormancy period with temps as low as 35°f, it’s too cold outside where I live in zone 5/6 to accomplish this task outdoors, so I rely on placing them in different growing areas of my house to help keep them happy and well rested so they can bloom after their rest.
So let’s jump right into it: HEAT
Growing zones, here in the US we’re likely most familiar with USDA hardiness zones, usually representing the possible low temps zone 1 comes in at a low of -60°f whereas zone 13 comes in at a low of 60°f. Most traditional houseplants come from zones 11-13, subtropical and tropical climates therefore we know most houseplants prefer temps over 40-50°f at the lowest – any lower and plants may suffer actual damage. However many cacti and succulents come from desert regions where lows can be quite considerable, these hardy plants can take on the winter cold quite favorably.
Each zone usually increases by about 10°f as you go up the scale, for more specific zones we can also look at if it’s an a or b, which usually increases by about 5°f. These zones only cover the low temps, highs within any given zone can be extreme or mild based of a variety of other factors. As an example my zone 5 garden saw lows of -20 easily throughout winter and highs of 120 the last few years.
PZmaps, CC BY-SA 3.0 via Wikimedia Commons: Monthly Mean T
The above interactive shows the general variation in average temperatures throughout the year. Below is a table of growing zones and a map of the US specifically color coded to each representative zone.
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Zone From To |
Humidity on the other hand is a different beast all together, the hotter the air is the more humidity it can actually hold before reaching the dew point – at which point moisture comes back together in the form of fog, mist, condensation or precipitation. Humidity is measured by a relative percentage 0-100%, based primarily on temperature. Remember the hotter the air the more water it can hold, so the relative humidity of 50% at say 50°f isn’t going to contain the same amount of water as 50% at 90°f. The following chart is based on Dew point and is calculated thus:
Depending on our temperature the humidity reaches total saturation or a dew point at a fairly predictable temperature, for the weather outside we also look at air pressure to determine humidity and dew point, but indoors we’re usually hovering right between 65-75° in a comfortable home with humidity between 20-50%, homes with forced air heating or air conditioning tend to be dryer than homes with other heating sources like radiant heat - including underfloor heating or space heaters such as baseboard heaters or standalone heaters because there is less airflow in these systems to disperse humidity throughout the home and these types of heat tend to not vent out displaced humidity as much. Air conditioners on the other hand work by pulling the humidity from the air itself, condensing it in their coils and returning the air cooler and dryer, unlike fans which just move the air.
So while we like to think of our home heat and AC as creating a comfortable and stable environment indoors, one that arguably places most plants in their ideal growing temperature, by raising the temps throughout winter lows and lowering the temps during summer highs we can see that these systems, particularly when humidity and airflow is involved we are actually creating a space that is completely different than the natural environment outdoors. Drying up humidity inside our homes during the summer heat and active growth can make some plants downright unhappy, and conversely those radiant heating systems can create a major displacement of humidity, evaporating much of it into the air quite thoroughly only to have it deposited as dew in the coolest spaces of our homes, those also most prone to mold – home perimeter walls and windows which tend to be colder during the cooler seasons, and where we tend to place our plants.
Even a double pane window or a well insulated wall is susceptible to these drastic differences in humidity as it seeks out cooler spaces to condense. A 70-degree inside temperature and 40% indoor relative humidity creates a 44-degree dew point, or condensation point. Once the window surface hits 44 degrees the water vapor in the air will turn to liquid, the specific temperature that happens depends greatly on interior humidity and temperature, a running shower can saturate the air with enough humidity to fog up a window very well and that humidity isn’t going to just stop there- even with a fan venting the humidity it can creep through the entire house seeking out lower temperatures. This is because water molecules are bent so the hydrogen atoms are to one side of the oxygen atom. When it come to molecules opposites attract so a bond forms between the negative side of one water molecule and the positive side of another. That is what gives water its stickiness – as the temperature of water changes so do the bonds formed by water molecules themselves:
Humidity varies greatly based on climate zones – this is where we want to look for figuring out the humidity our plants prefer, much like checking the growing zone the plant originated in or it’s natural habitat range, we want to match the climate zone. In general we like to think of the tropics as being humid and wet, however maps that only take account of growing zones such as the one above fail to really catch the whole picture, this is because deserts or savannahs occur in places we would usually assume to be wet and humid just because they are tropical or temperate, and microclimates exist everywhere based on elevation, latitude and longitude as well as proximity to natural water sources. These microclimates make up the various Lifezones of our planet.
Peter Halasz, CC BY-SA 2.5 via Wikimedia Commons:Lifezones Pengo 
For that reason probably the most popular maps on climate zones are based on Köppen’s climate classification as follows:
Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F., CC BY 4.0 via Wikimedia Commons: ”Köppen-Geiger Climate Classification Map”
So the factors of heat and humidity in the various climate zones combine into what we perceive as apparent temperature – humidity deeply influences how we perceive temperatures, as much as the dehumidifying effect of AC can lower the actual temperature the increase in humidity can effect how we perceive the temperatures were experiencing. Sometimes making them downright intolerable.
Apparent temperatures are what it feels like even when the actual temperature is different. Plants like humans feel apparent temperatures, the combination of heat and humidity effect a plants overall health, too much or too little heat or humidity at the wrong time or over a prolonged period can subject plants to unnecessary stress. Plant temperature and air temperature are not equal because plants are able to cool off through evaporation and warm up through irradiance – but keeping plants in improper conditions creates excess stress for certain. Heat and humidity when used responsibly the subtle changes in apparent temperature can communicate with plants variation that they otherwise miss indoors, seasonal changes indoors are particularly different than outdoors – our dependence on home heating and cooling to counteract the natural seasons can be particularly confusing for indoor plants.
Up to now we’ve focused our discussion on heat and humidity – but there’s a third factor in calculating apparent temperatures that is often greatly overlooked indoors, and that’s Airflow. We touched lightly on the subject here so far, but we’re going to look closer at how airflow actually effects plant growth.
Plants actually respond to soft and strong gusts of wind by producing growth hormones – chemical signals that tell the plant to toughen up, indoor plants are sensitive to both hot and cold drafts because of what that communicates in terms of them growing, conserving energy and utilizing evaporation and irradiance. So stability is key, the best airflow is air that moves but doesn’t create a dramatic change in heat or humidity outside of a plants preferred climate range and natural seasonal shifts. So with that in mind let’s look at circulation verses ventilation.
Circulation means air that moves but does not displace heat, create a cold draft or effect humidity
At minimum all plants need air circulation, the movement of air throughout any given space to prevent stagnation, and in the case of younger seedlings damping off, which occurs when otherwise healthy seedlings experience excess moisture and stagnant air which allows pathogenic fungi to bloom within the soil. However if we just keep recirculating the same air we can run into several problems as well - remember circulation does not increase or decrease heat and humidity, it just moves it around.
Ventilation on the other hand means the displacement or exchange of air, and may effect heat and humidity.
The second issue concerning airflow relates specifically to the growth of the plants themselves, plants produce auxens or growth hormones in response to abiotic stresses such as the wind, rain and other biotic stresses such as touch from animals and insects through the process of thigmomorphogenesis, the best way to support this process indoors is through the use of fans to circulate air, and through proper ventilation – additional ways we can help the process is through shaking plants gently, and cleaning their leaves with a quick shower of water, or by manually wiping leaves clean.
Plants actually like touch and movement of their foliage, it helps them grow. Roots are likewise stimulated during repotting and up-potting, when we repot we’re usually replacing soil which roots equate to underground critters and natural shifts in the Earth’s surface soil, when we up pot were simply removing the plant from one pot and placing it soil and all into the next size up, even this action alerts plants to produce more auxens.
So what about ventilation? Well, it really helps plants as much as people to keep a fresh environment, stale stagnant air couples with heat and humidity can be a recipe for all sorts of molds and fungi that can be damaging to plants and humans (some are beneficial, but it’s best to err on the side of caution in our own homes). These stresses and pathogens can be enough to kill a plant and make us sick, so ventilation is the answer here. In addition, subtle variations in temperature and humidity brought about by good ventilation coupled with the average hours of daylight that we discussed in our last series help plants stay on track with their natural circadian rhythms.
On that note we will close out our How Does Your Garden Grow series next time with a discussion on soil: planting media/soil structure, microbiome, and nutrients.