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I built a large propagator to overwinter plants and use next year to harden off.

i occasionally open the lids if weather permits. 

Today I lifted a few things around in there and found a lot of mould beneath the pots and trays.

the lids are wooden frames with something a bit more substantial than fleece. More nylon I would say but not too thick. Has this happened because there is a lack of ventilation? everything is sitting on a good layer of grit. 

i gave it a good rake and left the plants out all day but they're back in now. Will the plants be okay and can I prevent this by more ventilation or moving everything around. I would have thought the materials I have used would have been ideal to let water through and help ventilation. Jings, crivvens, he'll ma boab.   far too many plants in there too loose.


Hi Heather, can you describe or take a photo of the mould?  Without knowing exactly what it is, it's difficult to advise.  Removing all of the plants and watering the gravel with diluted Jeyes Fluid might help if it's some form of fungus but probably not if it's an algae like Nostoc commune.


Too late, I raked it in through the grit. I will take all the plants out tomorrow again and give it a good raking and airing again. The soil in the pots was saturated.

i should have said it was a cold frame and not propagator.

i will google both your suggestions and see if I can locate something similar. Thanks

not great at putting pictures up on the forum. 



I think you have said it yourself 'pots are saturated' plants in coldframes this time of year should be almost dry, the more water in them, the more chance they will freeze solid.

Could you put them somewhere else, greenhouse? Until they dry out.

heather, mould only grows when its too warm. Cool it down, kill the mould spores as Lyn has said and increase the ventilation.


flowering rose

bad ventilation and pots with  water in or leaks gives mould a good place to grow . empty any water filled pots and don't water to often (they don't need it ,the plants)


I cannot think of a time when I've ever had my cold frames totally closed.

Thank you everyone, there are three lids that cover one cold frame and I plan to re-cover two with polythene. Keep one with mesh for ventilation. We are very exposed here so the wind would cause damage if I was leaving the cold frames open. 

i think the mould under the pots were from the slug pellets I put down before I put the pots in. 

You learn something new every day. Especially from you guys on the forum. 


Can anyone help me with mould developing on the leaves of my Camellia cuttings in my new Propagator?  It is a Stewarts and cannot be heat controlled manually.

The vents are fully open, but the lid condenses up still.


No expert rose but maybe try taking the lid off.  Ive taken quite a few cuttings for the first time this and have not covered them and so far they seem to have taken.  That said they are in a cold greenhouse.  Hopefully someone more qualified will come along and help.


Mould usually develops where there isn't enough ventilation. Do you have the propagator inside the house as well? As Yvie says - if they're in a greenhouse or even indoors on a windowsill they shouldn't need any extra help. I definitely wouldn't have heat under them, just shelter from the cold weather.  


I agree, remove the lid.  Cuttings take well with bottom heat (ie warm soil in the pot) but the humid, warm and still air inside a closed propagator lid will only encourage moulds and other diseases.


OOh I think I got something right.


Yvie -

Fairy is right as well, once the cuttings have roots they don't need the heat any more, just sheltered conditions with plenty of light.

Can Bob The Gardener advise any product that will kill 'Nostoc Commune'?   This cyanobacteria is spreading everywhere in my garden  -  drive, flowerbeds, grass and even concrete paths.   Because we're on solid clay ground. I imagine that it's proliferating because of the poor drainage but I can find no information as to eradicating it.


Hi i openend my propogator this morning to find areas of mould on the ground. there r doors at both ends with very large windows approx half the size of the doors is my problem also lack of ventilation i have cabbages broccoli lettuce etc in the ground will i loose them or can i save them and if so how many thanks
Mel M

Propogators do condense up. It is a natural process (heat/water.) Every morning you should remove the lid and drain off as much liquid as you can. I have never had mould in any of my units. Slug pellets always go mouldy if they get wet.

I am a numpty its a polytunnel i have Sorry Mel does the same apply
Steve 309

Ventilate, ventilate, ventilate.  Then ventilate some more.  Whatever's in there it won't get too cold at this time of year.  Another month or two and you'll have to at least partially close it up at night (assuming you have tender plants in there).

And don't over-water