From disease and insect control to plant growth and generative vs. vegetative signals, there are many issues closely tied to controlling your growing environment’s climate. In my region, winter is the most difficult season to deal with as temperatures tend to be cold forcing a lot of difficult decisions.
Problems with venting in cold weather
You may notice that while the outdoor temperature remained cool, we had high temperature spikes inside the greenhouse. If I open the vents several things could go wrong.
- The warm, moist air mixes with the cold, dry air with unpredictable amounts of condensation that may create serious problems with the venting equipment and the real possibly of freezing the vents open (which would be a serious emergency).
- The cold air drops into the greenhouse bay causing some areas to be too cold very quickly. This may damage those crops directly under the vents.
- Venting requires a change away from the aggressive carbon dioxide program I run. (I tend to keep daytime CO2 at 1200ppm in the winter). Venting looses this CO2 to the atmosphere.
- Higher heating costs.
I’ll explore each of these in more detail.
1. Greenhouse vents freezing.
Most of the time, when one vents there is no problem. Open the vents, let the moist and/or hot air out, close the vents. (Often, if opening the vents for dehumidification, the computer is programmed to heat the air among the plants for a few minutes just before opening the vents to get a more effective exodus of the wettest air). This is not a problem in mild temperatures (around freezing, for example). However, I’ve heard horror stories about vents freezing open in colder temperatures and am not excited to experience this.
In a normal February, our temperatures would be about 10°C warmer and we would not worry too much about this problem. This year, however, its been a little more challenging.
It is probably more likely that the vents would freeze closed after having opened them, which is still a problem but much less serious.
2. Cold air drops onto the plants.
The problem with venting is that it is very difficult to do evenly over a large area. It is very likely that the cold air (especially if it’s quite cold outside) would drop quickly and more-or-less straight down.
For some plants (lettuce or kale, perhaps) this would not be a serious problem. But for cold-sensitive crops (basil, tomatoes, eggplant, cucumbers) it could affect them quite a lot and it would be difficult to see the damage until later.
One could disrupt this by partially closing greenhouse curtains and/or by using HAF fans, but it will always be uneven.
3. The CO2 program would need to be changed.
One of the biggest advantages of gardening indoors is that one can add carbon dioxide. The very best greenhouses in the world, IMHO, are closed greenhouses–greenhouses which cannot vent, but rather use aggressive cooling and dehumidification systems. This allows a grower to have high CO2 all year round and can result in incredible yields.
I don’t have the equipment to mechanically dehumidify my greenhouse, but I have an advantage–the air here is cold but also dry. I almost never have to vent in the winter due to humidity levels. This is great news for my CO2 program which can run for much of the late fall/winter/early spring and allow for aggressive plant growth.
4. Higher greenhouse heating costs
Although I use waste wood for heating my greenhouse, which one would think of as inexpensive, there are expenses with processing the wood and also with boiler maintenance. This year, our sources of wood are supplying less as the lower oil, potash and natural gas prices have meant less building in our area.
Last year (which was much colder than this year) we never had to turn on our natural gas boiler due to too little wood. This year, we’ve had natural gas running since about New Years Eve to supplement our dwindling wood supply.
Since heating our space is expensive, I find I am more likely to let the greenhouse get warm as I don’t want to heat the outside air any more than I absolutely have to. Thinking about it now, this probably affects my decision of when to vent disproportionately compared to what I should ideally be doing if I considered everything completely rationally.
Problems with not venting
There are two challenges created by not venting.
- High humidity.
- Too-high temperatures.
I would like to deal with each of these in turn.
1. High Humidity due to not venting enough
Many diseases, perhaps most diseases, in plants can only infect the plant when the humidity is high enough for them to begin infection. Most diseases that are affected by humidity need a humidity in the 85%-100% RH (relative humidity) range in order to damage the plant.
Since diseases are so challenging to control (especially if one is using organic methods), controlling the environment to be below 85% is critical. If the choice is between venting and having a high humidity, I will choose venting every time.
Fortunately for me, our area is quite dry and plants early in their growth have less plant material and therefore transpire less actual water into the air. I almost never face the problem of choosing whether or not to vent when the air is very cold outside. I think I would always choose to bring down the humidity in this situation.
2. Too-high temperatures in the winter greenhouse
Researchers have found that a high 24hr temperature creates the following problems:
- long internodes (the area between where a leaf comes out of the stem and the next place a leaf comes out). In other words the plant looks stretched with long distances between leaves. Usually this means that the plant has put too much energy into growing upwards and not enough in developing bigger leaves, etc.
- thinner leaves and stems.
- small, skinny flowers.
- Weaker plant and flowers.
In other words, a high 24-average temperature results in crop problems that will eventually affect the yield of the crop.
The poorer the light, the more this will be true.
Additionally, there is the problem that a large difference in daytime temperatures over night time temperatures (DIF) creates a more spindly plant with larger distances between internodes and smaller leaves.
I do see these problems in my plants. Long internodes and smaller-than-I’d-like leaves. I think one should have been able to predict this set of problems just by looking at my chart.
While venting does create some problems for me (higher heating costs, possible damage to plants from cold dropping, and the possibility of freezing vents), looking back I should be venting more and sooner than I have been. While writing this paper I changed my program to vent more aggressively and to run a CO2 spring program (700 ppm max and CO2 not running while vents are open).