Liver wort on Seed pots

Just wondering if anyone has a sure fire method of stopping liverwort and fern babies from invading my pots of seeds. Just tried, probably very unsuccessfully, to disentangle some tiny (and not growing) Saxifrage seedlings from the scourge. I have some more Saxifrage seeds which need sowing, but there is no point if the babies are going to be totally covered in liverwort etc.

 

I like Liverworts

 

I like liverworts, but not when they grow so well that they stop my seeds from germinating.
Been advised to water only with tap water which should not contain the spores of these things. Trouble is that the seed pots which suffer most are those which need cold stratification and take years for the seed to germinate. They would need to be in a closed frame which is a bind having to open up and remember to water etc.

 

Wonder if a sprinkling of wood ash would help?

 

Never seen liverworts in my garden so cant help, sorry.

I'm guessing its down to dampness?
Maybe acid conditions?

They just dont grow around here.

 

Careful Pete, you'll find your greenhouse full of Pals's seed trays