The definition of the term ‘microclimate’ at least according to Wikipedia is “A microclimate (or micro-climate) is a local set of atmospheric conditions that differ from those in the surrounding areas, often with a slight difference but sometimes with a substantial one.” But I also like the definition according to Dictionary.com which is “the climate of a small area, as of confined spaces such as caves or houses (cryptoclimate), of plant communities, wooded areas, etc. (Phytoclimate), or of urban communities, which may be different from that in the general region.”
So, in reality, what I will be discussing in this article may be more correctly described as the cryptoclimate of our Zone Zero, but I had never heard the term before and figured neither would have anybody else, so I decided to continue with ‘microclimate’ instead!
Microclimate Aspects
Microclimates are characterised by five environmental factors in general, and also in terms of the microclimates in Zone Zero, that is to say Light, Air circulation, Temperature, Water and Soil. Of these, the most important for the Zone Zero Farm is light so that is where we will start!
1. Light
Plants need light to be able to photosynthesise and make oxygen and energy rich organic compounds that allow them to grow. If you have ever seen the impact of low light levels on seedlings as they grow, you will know it causes them to be long and spindly (referred to as ‘leggy’) and they can’t thrive.
When we grow plants in sunlight, things work pretty well, although sometimes there needs to be adjustments to ensure the microclimate works for the particular plants you want to grow. When we grow in Zone Zero, it is a case of making sure the light microclimate works for our target crops but there is much more detail that needs to be considered to get things right!
While the light hitting the crops not only needs to be of the correct intensity, and the appropriate wavelengths for the plants you are trying to grow they also need to get enough light during each day (although not too much), or in other words, the Daily Light Integral is correct.
There are whole books written about getting this right. I won’t get too technical but give enough detail that you'll have some idea of what you are up against.
Radiant Intensity
This is important to get right if you are growing in a window farm using natural light or inside using artificial light. My original thought after doing some reading was to get hold of a Lux Meter,
- one Lux being one Lumen per square metre,
- a Lumen being ‘unit of luminous flux, equal to the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela’
Man, things got complicated quickly! I just wanted to get an idea of the light coming into the windows and onto the plants using an app on my phone to measure lux! Anyway, it turns out that measuring lux for this purpose isn’t the best way to do it. The correct way is to measure something called PPFD (Photosynthetic Photon Flux Density) and the unit of measurement of PPFD is micromoles per square metre per second (umol/m2/s). It is measured between the wavelengths of 400nm to 700nm, we talk about wavelengths in the next section. While this does seem (and is) somewhat more complex than Lux, what surprised me is that the app which I downloaded onto my phone to measure Lux, also measures PPFD! The app I use is called Photone and I have included a link at the end of the article, but check out what is out there, you’re probably much more computer literate than I am!
Measuring Lux and PPFD on the phone
I don’t want to lead you down a rabbit hole, but just needed you to know how I got where I am with things. While the figure is built around plants under artificial light, I believe it should work for natural light as well. It seems that “around 1200 umol/m2/s is a reasonably safe absolute maximum that we should expose our plants to for a 12 hour lights-on period. Even at this level, it is a good idea to keep an eye on leaf temperature.” According to a gentleman at One Stop Grow Shop UK website.
Now that we know how much radiant intensity of light to allow for, let’s move on to the spectrum we should be looking at.
Spectrum (Wavelengths)
The visible spectrum is comprised of the wavelengths between 400nm and 700nm. These wavelengths also correspond to the PAR (photosynthetically Active Radiation) spectrum. Wavelengths just outside the visible spectrum below 400nm are called ultraviolet (UV), and wavelengths just above 700nm are called infrared (IR). Blue light (450nm to 495nm) stimulates vegetative growth in vegetables and while red light (625nm to 750nm) also does this but it also stimulates flowering and fruiting, which is why you will see blue and red LED lights used in indoor growing situation. It is also said that plants reflect green light (hence they appear green to our eyes) and don’t use it, but this is not correct, and some green light is required for vegetables grown indoors to thrive. This is why a full spectrum grow light is best when growing vegetables indoors.
Daily Light Integral (DLI) and photoperiod
Photoperiod is the length of time a plant is exposed to light in a day, Daily light Integral is the total amount of light a plant receives in a day, measured in mol/m²/d. DLI is calculated by multiplying the photosynthetic photon flux density (PPFD) by the photoperiod. (Just so you know)
This is also a most important concept when using grow lights, because when you are working with sunlight coming in your window, unless you are in somewhere like Svalbard in the middle of summer, you will not approach 24 hours of sunlight. Grow lights can be left on 24 hours a day, but is that a good idea?
Measuring the Daily Light Integral (DLI) on the phone
Well, as usual, it depends! It turns out that there are two types of plants in terms of how long they need light – short day plants and long day plants. This concept is important when you want to stimulate plants to flower, or to avoid flowering. For example, if you want your tomatoes or capsicums to flower under lights it is important to provide what they need, but if want your veg such as lettuce or herbs to avoid flowering, it is also important.
In general terms, grow lights should be left off for 8 to 10 hours for mature plants (6 hours+ for seedlings) with an absolute limit of 14 to 16 hours of light per day. Leafy vegetables and root vegetables require lights to be on for 12 to 14 hours per day, fruiting plants for 16 hours per day.
2. Air circulation
Outdoors, there is almost always air movement of some description but within our Zone Zero, there can be areas with no air movement for long periods. Air circulation is important for plant growth for in the Zone Zero farm, but also to help maintain our Zone Zero itself.
In terms of the veggies, plants need some air movement for the following reasons –
- Gas exchange – Plants need carbon dioxide to photosynthesise and produce oxygen as a waste product (lucky for us!) so air movement is required to provide a supply of fresh CO2 and carry away the oxygen waste product.
- Temperature regulation – air movement removes water vapour transpired by plants through their leaves, allowing them to reduce their temperature by evaporation. The plants can also be heated up if using grow lights and removal of hot air while introducing cooler air will reduce temperature stress on plants.
- Disease prevention – still areas around growing vegetables that are high in humidity favour the spread of fungal and bacterial diseases.
- Stronger plants – plants exposed to air movement release hormones that stimulate cell growth, making them stronger.
Lack of air movement when growing our food plants in Zone Zero can result in still, humid air causing mould growth on surfaces within Zone Zero such as walls, fabrics such as curtains, books and other areas.
Zone Zero crops grown on balconies, patios or decks, depending on their aspect, are likely to have access to air movement from the macroenvironment. Modification of the air movement microclimate may be necessary for an upper floor balcony. Excess wind can damage and dry out crops growing there. Consideration should be given to staking and tying taller crops like tomatoes, fixing pots shelves or trellises to walls for support and/or attaching pot hangers to ceiling or walls.
So, it can be seen that providing air movement is important for not only our Zone Zero crops to survive but to prevent damage to our Zone Zero Itself. There are a number of ways that air movement can be improved in our Zone Zero microclimate –
- Cross ventilation – by growing our Zone Zero crops near open windows, the natural air movement of wind from outdoors can work for us, and this is improved if we have windows or vents open on both sides of our Zone Zero.
A box fan is cheap to buy and run (consuming only 45 watts) and can introduce air circulation to an otherwise still room
- Fans – Making use of the wind to increase air movement is not always practical so fans can be used in a number of ways –
o A fan set up on the inside of an open window to draw air in during times when there is low or no wind
o Ceiling fans can circulate air within a room.
o Extractor fans can draw fresh air into a room from other parts of Zone Zero while expelling stale air outdoors or into the roof area. Obviously not all rooms will have extractor fans built in but bathrooms/toilets, kitchens and possibly laundry areas can be taken advantage of in this manner.
o Portable fans – can be set up in any number of configurations to keep the air around our crops moving.
- Air conditioners – these will aid air circulation as well as helping to modify the temperature microclimate but will also impact on humidity.
An aircon can be set to just reduce humidity and provide air movement but not impact temperature
3. Temperature
When working with temperature microclimates outdoors, generally we are looking to keep the hot places cooler and cool places warmer. The thing is, we generally do this within out Zone Zero anyway, to ensure our own thermal comfort, let alone that of any food plants we are growing.
At temperature extremes which we would want to avoid in our zone zero ie below 15°C and above 35°C, photosynthesis slows down and can come to a halt, meaning the plants will grow very slowly, if at all.
There are a couple of considerations to be borne in mind when designing or modifying a microclimate based around temperature:
Warm season vegetables vs cool season vegetables
Some vegetables referred to as ‘warm season’ crops prefer higher temperatures, meaning a soil temperature above 21°C and an air temperature between 18°C and 30°C. Examples of warm season crops include tomatoes, capsicum, eggplants, cucumber and beans. In contrast, ‘cool season’ crops prefer soil temperatures between 10°C and 21°C and an air temperature of 15°C and 29°C but the optimum air temperature for cool season crops is 21°C. Examples of cool season crops include baby pak choi, lettuce, onions, parsley, radishes and spinach.
Thermoperiod
In their natural environment, plants are used to cooler temperatures at night and warmer temperatures during the day. Most plants do best when the nighttime temperature is 2°C to 6°C cooler than the daytime temperatures. This may vary for specific plants, but it is worth keeping in mind as a general rule.
With a bit of luck, modifying the temperature microclimate to avoid temperature extremes and thus improve our own comfort, (eg using heaters or air conditioning) will also create a microclimate that is beneficial for growing food plants. It is worth investing in a maximum/minimum thermometer or two to provide hard data on what temperatures we are achieving in our zone zero, therefore removing any guesswork. This is especially useful if we are growing plants in bedrooms which may not be visited as regularly as other parts of the house and closed off to make more efficient use of heating and cooling.
If the temperature of your plants is a bit low for them, it can be tickled up by use of gear such as a heat pad, and if they are getting a bit warm, increasing air movement by using fans may increase evaporation, helping the plants drop their temperature that way.
Zone Zero crops grown on balconies, patios or decks, that is to say close to the thermal mass of the dwelling, will already have a more crop friendly microclimate. It the structures face north or west there may be a need to modify the microclimate further by providing 50% shade cloth covers to take the sting out of high summer temperatures.
4. Water & Humidity
Unlike growing crops outdoors where areas of damp soil due to accumulation of rainwater are possible, there is unlikely to be a damp area in your Zone Zero growing areas (if there is you have a real problem!) but there will be areas of low and high humidity. The bathroom, laundry and kitchen are the areas of your Zone Zero that are most likely to have high humidity, depending on the time of day and activities being carried out. Areas that are air conditioned will most likely be areas of low humidity.
If you want to go old school on measuring humidity, get a sling psychrometer
Just to be clear, when I use the term ‘humidity’ I am referring to relative humidity, which may be defined as the ratio of the amount of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature, expressed as a percentage. The air can hold more water at higher temperatures such that if the temperature decreases and the amount of water vapour in the air remains the same, the relative humidity will increase.
In general terms, plants will do best in a microclimate of 50% humidity although mushrooms prefer a much higher humidity of 85% to 95%, whereas we humans prefer a humidity of 30% to 50%.
Mushroom fruiting chamber to maintain required humidity
Having a means of measuring humidity such as a hygrometer, either digital or analogue will make keeping track of the humidity in your growing areas much easier so that it will be obvious if changes to the microclimate are required.
A cheap weather station can show you humidity inside and outside your Zone Zero
Increasing humidity either to help plants like mushrooms be more productive or to reduce the impact of air conditioners, particularly on young plants like seedlings or microgreens, it is a case of isolating them from the dry environment and increasing the humidity in the plants growing area.
Home grown mushroom with fruiting chanber in the background
We did this when we built a fruiting chamber to grow mushrooms in. We used a small, commercially available mini-greenhouse, installed a fogger in a cat litter tray of water on the top shelf, and then punched some holes through the sides and covered them with micropore tape to allow CO2 to diffuse out while retaining the water vapour and thereby maintaining humidity. It worked very well. Details on how I put it together are available here.
Zone Zero crops grown on balconies, patios or decks, depending on how things are put together, may have direct access to rainwater or can be placed in trays where rainwater is allowed to collect and keep pots watered by capillary action.
Harvesting water direct from the downpipe
While we are discussing water, it is worthwhile giving some consideration to harvesting and re-using water in the Zone Zero microfarm, to reduce the environmental impact of Zone Zero farming even further. If you have access to downpipes, you can harvest rainwater directly from them to be stored in a container for later use, or just leaving containers out in the rain. Also, gathering water from the shower or taps while waiting for it to warm up, or saving water from the kitchen such as that used to wash veggies prior to cooking can be used to water Zone Zero crop plants.
There are other ways to make the most of water while keeping the crops hydrated. These include mulching pots and other growing containers, using self watering containers either bought in or home made, installing small to medium ollas as water reservoirs in growing containers or installing bottle and wick irrigation in pots which also provide a water reservoir. For microgreens or shoots, a capillary bed installed underneath the growing tray, with or without a bottle reservoir will ensure that they stay hydrated.
5. Soil
When looking at the impact of soil on microclimates outside our Zone Zero, there are four aspects to be considered: Thermal conductivity; Albedo (solar radiation reflectivity); Texture and structure and Water content. In our Zone Zero farm we have much more control over our growing medium so that these may be less of an issue. There are however still problems that can impact the microclimate of our growing medium.
Due to the relatively small amounts of soil used to grow crops in Zone Zero, thermal conductivity and albedo are unlikely to have much of an impact on the soil microclimate. Soil texture and structure, however, can still have impact in terms of the factors below -
Soil Compaction - The soil in our Zone Zero farm containers may become compacted due to several issues including overwatering, shallow watering or the pressure of the top growing medium on lower levels in larger containers. This is likely to reduce the amount of water in the growing medium, reduce gas exchange and make the roots work harder to grow down into the growing medium resulting in stunted, unhealthy plants and a poor harvest. In terms of the microclimate, compacted soil will impact soil structure and water content.
Compaction may be reduced by adding compost, vermiculite and/or perlite to your growing medium and mixing thoroughly. Other techniques to reduce soil compaction include repotting, watering correctly or installing any of the techniques mentioned under ‘water’ above such as ollas, bottle and wick or self-watering containers to ensure overwatering doesn’t occur.
A small olla installed in a pot can prevent overwatering
Soil becoming hydrophobic - When soil in containers dries out, the microbe population that breaks down the waxy compounds dies off, allowing the oils to build up resulting in the soil becoming hydrophobic or in other words, shedding water applied to the soil without allowing it to soak in. This can be dealt with in a number of ways including soaking the affected pots totally in water to rehydrate them, adding some surfactant designed for the purpose or prevented by using the watering techniques suggested above to prevent the growing medium from drying out. Hydrophobic soil will also impact the water content.
While any of these issues can affect the growing medium of Zone Zero crops grown on balconies, patios or decks, mulching crops with an organic mulch such as compost, hay, straw or sugarcane mulch will help to prevent problems occurring.
Conclusion
During the process of planning the Zone Zero farm it is worth considering and researching the aspects of microclimate that will have an impact in your particular farm. In that way your plan can be designed take advantage and maximise the positive microclimate aspects while mitigating and avoiding the negative aspects of existing microclimates.
Next in the series: Developing and Implementing your Zone Zero Farm Plan
Link
