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Ron's Winter Rules

(Based on experience with a small greenhouse of roughly 8 cubic metres.)

Don't ventilate — Insulate — Humidify

Don't Ventilate
Heresy, you say. Every known work on orchids stresses the necessity of ventilation. But let us consider in more rigorous detail what ventilation actually does for us and our plants. As you would expect, it does some good things and some bad things, and we put up with the bad to get the necessary good. But what if the good things it does could be done another way? Then we wouldn't have to put up with the bad things. which could, all considered, save us a lot of time, money and grief.

Good things brought by ventilation:

  • Plant food - Carbon dioxide is the essential food of plants — apart from water, 99% of a plant is built from it, and all the fertilisers and such account only for the other 1%. Without starch, we starve — without carbon dioxide, plants starve.
  • Fresh air - "Open Air Factor" is the proper term. There is something in fresh air that kills off most of the bacteria, moulds, algae and fungal spores that float around invisibly in the atmosphere, especially near the ground. Research eventually proved that OAF arises from the interaction of ozone and/or ions with terpenes (volatile natural substances, usually with pleasant aromas, that are released by the leaves and fruits of many plants, especially trees). OAF soon disappears in any enclosed space by reacting with the walls and other surfaces.
  • Air movement - This makes sure that OAF and carbon dioxide get to where they are needed — the plants' leaves and shoots.

Bad things brought by ventilation:

  • Contaminants — other folk's bonfire smoke and weed-killer drift, for example: dust, dirt, leaves etc.
  • Crawling pests — ventilators often provide access for these, especially when they are babies. We have all found that prized spike nipped in the bud.
  • Airborne pests — aphids in particular. Lots of chemistry is used to combat them.
  • Viruses — these can come in with the aphids. Your entire collection can be wiped out.
  • Moulds, fungi, yeasts, algae, bacteria — all these have a field day when the house is closed at night and the OAF all used up. Combatting this is a constant battle, easily lost.
  • Humidity loss — when it's under 10%RH outside, it's hard to get to 50+ inside if you ventilate. There is also a great hidden energy cost in the loss of moisture — it takes five times more energy to evaporate water to humidity moisture than it does to bring that water to the boil. The energy lost through ventilated humidity will eventually have to be replaced by heating.
  • Heat loss — warm air out, cold air in — need I say more?
  • Draughts — the cold, dry fresh air is likely to cause draughts and cling to the floor, with adverse effects.
Sure, all the bad effects of ventilation can be managed, and probably most orchidists spend a huge part of their hobby life doing exactly that, often rather expensively. Wouldn't it be much better if we could fairly easily provide the good effects of ventilation while minimising the ventilation itself? Of course it would — we would then be free to spend our time and money on the "nice" aspects of the hobby.

Well, the good news is that we can. Taking them one by one, this is how we can easily do the good things of ventilation — air movement, carbon dioxide food, open air factor (OAF) sterilisation.

  • Air movement is obvious and easy — fans. But try to point them vertically to banish cold spots, and not at plants. See also OAF below.
  • Carbon dioxide is easily provided by combustion. Having done my sums, I burn two small tea-lights (Wilkinsons, £2 per hundred) each day. If they are cleanly lit and protected from draughts they make no smell, and they provide my plants with several times more food than they would get in the wild. Research shows there is no problem with giving plants 10 or even 100 times the natural level of CO2 — they just grow faster, healthier and stronger, but there is a law of diminishing returns so 4 or 5 times is about optimum.
  • OAF. This very important thing is also very intangible, and still not well understood as not much work has been done on it. However, machines operating on the principle are now used in hospitals to kill bacteria and viruses, whether airborne or on surfaces, allegedly with complete success. These machines make ozone and mix it with terpene vapours. Ozone used to be produced as a by- product by some electric motors and room ionisers, but now that ozone is considered toxic, it has been largely eliminated. My approach is therefore hopeful rather than proven. I use a room ioniser as my circulation fan and put a little dish of artists' (natural) turpentine in front of it in the hope that the ions will interact with the turpentine terpenes in a beneficial way similar to ozone. so that the terpenes act as a carrier for the sterilising effect of the ions. The ions are beneficial in themselves, as they neutralise the static charge on the nasties that enables them to float around — this makes them fall out of the air quickly. Also. room ionisers commonly come with a HEPA filter on the intake, which will itself remove all these nasty tiny particles from the airstream. So I don't really know whether the turpentine helps or not, but what I can say is that it gives the greenhouse a lovely fresh smell, a bit like mossy woodland - you breathe deep and it feels good! My personal preference is for Windsor & Newtons turpentine, but I guess that any distilled artists turps will be fine (if a bit pricey!).

All I can say is that non-ventilation works for me — my heating bills are kept in check and the plants seem to love it. In the days when I ventilated, I was forever having trouble with black spots and moulds and rots, whereas now I have hardly any (and those are probably my own fault as I almost never sterilise anything). My use of insecticides and fungicides is near zero, and my fertiliser use is very low, but my plants mostly grow OK. Rather than forever trying to heal sickly plants, I now have time to titivate and admire them.

I still have the essential automatic roof vent. but, as there is no through flow, only the hottest air escapes and is replaced with cool air from (importantly) a decent distance above the ground (away from the coldest and most contaminated air).


Obvious, but how well do most of us manage to do it?

First, as any home energy adviser will tell you, it is essential to eliminate all draughts. as the smallest leak can carry away a lot of heat, especially when it's windy. You also should seal all gaps just to keep the bugs out (I failed to properly seal my house as it was being built, and have regretted it ever since!). Sadly. many greenhouse designs are almost impossible to seal effectively, but it's important to do your very best on this.

I used to use horticultural bubble-thene, the one with two skins, but it was a lot of hassle and not that good. On the sections where I need light to get in. I have now put secondary glazing of polycarbonate twinwall, and on the sections where I don't need light I have put one-inch polystyrene insulation board (could be better, but chosen for ease of handling, cutting and installation, and minimum loss of useful space). Polycarbonate glazing is a boon here, as you can easily put screws and so on through it for fixing — glass glazing would provide quite a few challenges!


We all know that humidity is vital, but providing it in the right way and at the right time is also vital.

Since I run my hobby on a shoe-string (as an engineer I have always enjoyed the challenge of doing the maximum with the minimum, and much as I might admire the products of Simply Controls, I can't see me ever shopping there), most things come from chain-stores. A fountain pump drives a home-made cascade with an ultrasonic mister in it and I have two more ultrasonic misters in the form of personal room humidifiers all are turned on by an ordinary room thermostat wired to come on with rising temperature and placed where it catches the sun. Effectively. I am humidifying whenever the sun is shining, but not otherwise, The trace from my data-logger tells me that this system works well, and often the house gets misty enough that it looks like a real cloud forest! On a hot sunny day the cascade will get through 5 to 10 litres of rain water (quite a lot is used up by splashes) and the misters will use 2 to 3 litres, but my RH stays above 50% and is more usually 70, 80 or more. Perhaps surprisingly, condensation doesn't seem to be a problem.

A word here about rain and rain water. The two are not the same. Rain (after the first bit) is mostly pure and clean and sterile, and maybe still retains some slight sterilising properties it does a good job of cleaning plants and keeping them healthy.

Rain water, in contrast, is contaminated with all the rubbish and dirt and spores that have fallen on your roof and gutters, and dissolved or bred in your rain-butt. No surprise, then (if you think about it) that spraying or misting your plants with rain water is a pretty sure way to infect them with all the moulds, rots and bugs you can imagine. Also, if you leave rainwater lying around, as in a sprayer or a mister reservoir, it will quickly grow films of algae, bacteria and the like.

My own solution to this problem is to add a biocide to any rain water that is going to reach the green parts of plants (I don't add it to water for the compost, as I might kill off good bacteria arid fungi there). I use Corsodyl Original mouthwash. about a teaspoon to a bucket and left a while to do its work. This product was recommended to me (as a mouthwash! !!) by my doctor decades ago, and has been my personal general purpose antiseptic of choice ever since (they use the same stuff to ''preclean surgery). The water in my sprayers and misters stays clear, and any residual sterilant no doubt helps to keep my plants free of nasties.


© Ronald Lamont November 2011

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