Fan to lessen chance of botrytis?

[quanglewangle]

Last year was our first with a greenhouse. On the whole it was quite successful: tomatoes, aubergines and melons all worked ok. We did commit the rookie error of overcrowding though and lost some melons to (I guess) botrytis mould.

I am thinking of installing a fan: an axial one up in the ceiling ridge with a length of duct to bring the warm air down of ground level, and generally stir the air up.

I will also reduce the overcrowding a bit, but want to work the greenhouse hard so it earns its keep.

What do folks think?

[Bren In Pots]

I've never tried anything like that QW all I have are louvres at the back then auto vents in the roof windows. Be interesting to see how this works out don't think I've seen anyone else mention having a system like this but if they have I hope they post a reply.

[Small pumpkin]

I intend on setting up a couple of fans in my polytunnel. Had the fans a while just not got round to getting electric to the garden yet. No point now for a couple of years. But it will happen one day.

[quanglewangle]

Looking around, fans seem to be used very widely in commercial glasshouses - for air circulation within the space, not just for ventilation.

[Babru]

I'm always puzzled by the advice you get for greenhouses: on the one hand, keep the air moist by damping down the floor, red spider loves dry conditions etc; on the other hand, damp conditions cause mould, disease etc. Can't win!

I have a heater in my summerhouse that blows hot air or cold air, and have wondered whether it would be useful in the greenhouse on cold when my tomatoes are growing.

[Small pumpkin]

Looking around, fans seem to be used very widely in commercial glasshouses - for air circulation within the space, not just for ventilation.

Air circulation is what I’ll be using them for.

[quanglewangle]

I'm always puzzled by the advice you get for greenhouses: on the one hand, keep the air moist by damping down the floor, red spider loves dry conditions etc; on the other hand, damp conditions cause mould, disease etc. Can't win!

Actually, I think both can be true. Consider a sunny morning in a greenhouse damped down the previous evening: the vents aren't open yet so the only air movement is from gentle convection currents. The topside of sunlit foliage and the ground in the sun could be quite warm. Warm air can carry much more moisture than cool air and so is "drier" even though may be carrying the same about of water. On the other hand the lower side of leaves, and any shaded parts of plants, will be cooler and so much more humid.

So I guess one could get pesky red spider mite on the top and botrytis on the bottom of leaves!

The point of increased circulation is to make the air more homogeneous - all the same humidity (and implicitly, all the correct humidity).

Another point is that the products of metabolism and transpiration are most concentrated near the surface of the leaves - just where they are expelled. Chemical reactions often proceed more quickly if the products of reaction are quickly removed and air movement over the leaf surface will do this.

Phew. That is enough heavy lifting for one evening.

[Starch]

Also had problems with melons, squash etc. with white powdery leaves. The new polytunnel cover recently fitted has roll up vents 4 along each side so hopefully this will alleviate the problem.

[Scarlet]

I have a heater in my summerhouse that blows hot air or cold air, and have wondered whether it would be useful in the greenhouse on cold when my tomatoes are growing.

I use one of those! And my doors and windows are left open all the time.

But space between plants is essential - if you grow them too close, there can't be airflow between them and as soon as the bottom leaves die off and leaves touch each other the problem can start.

No plant thrives without enough space.

[greenishfing]

I think this is a really interesting thread and would like to know how much success everyone has.
On our allotment site we have no electricity, however thinking about those holiday hats with a mini fan and even smaller solar plaque there must be a way....
Maybe the type of fans that we used to build into desktop computers. I'm pretty sure they were low voltage.
Somebody ought to invent a prebuilt system of a couple of fans (one for air in, one for air out which would give circulation) connected to a small solar plaque. Maybe this would not be feasible. I don't know.
As with most other gardeners I get tempted to just add another plant to the greenhouse....and another....and another....

[quanglewangle]

Don't confuse circulation with ventilation. I think greenhouses can be in need of circulation, mixing the air up, at times when ventilation is not needed. Think of mornings when it is still chilly but the place is dripping with condensation.

[greenishfing]

Confusing. Does it do any good to circulate damp air?

[quanglewangle]

Confusing. Does it do any good to circulate damp air?

Warning: what follows is not based on experience or (much in the way of) theory.

1. It can't be worse than leaving it still
2. Some parts of the air, especially in among foliage, will be damper than others, such as in the body of the space. Mixing those two up must be good
3. As soon as it gets light (more so if/when it gets sunny), some parts of the greenhouse will be locally quite dry (ie lower RH %) even in a damp greenhouse. EG tops of leaves, ground in sun. Mixing those up must be good.
4. A general old wives tale feeling that mold doesn't like moving air

Remember warm air is drier than cool air even if it contains the same mass of water.

[ESBkevin]

Things that cost nothing (except time) and will decrease botrytis.

Keep the soil surface clear of any organic matter that is not fully composted (like yellow/fallen leaves and fruit).
Remove lower leaves from plants up to the current ripening fruit (typical on tomatos etc).
Only water onto the soil surface and it's best if that surface is heavily mulched with fully composted material to absorb the water fully.
Venting during the day every day. It's fine having a dew/condensation, but let it dry naturally ASAP and the risk is low.

[Scarlet]

Think of mornings when it is still chilly but the place is dripping with condensation.

I try to keep condensation down by only watering into plastic bottles sunk into the ground. All the surfaces is covered with card and newspapers to keep the moisture in.
I also open up early in the Spring. At the moment I have lots of seedlings in the greenhouse and my doors have been left open a little which help.

#should read the whole thread! I follow the same rules as EBKevin goes by....and I don't cram in extra plants.