[E Annie B]

I have a new polytunnel and lots of seedlings etc growing well in there now, six weeks after it went up. Several of the seed trays and small pots' surfaces have a thin, dark green, growth on top of the compost. Does this matter at all as it doesn't seem to be doing any harm that I can see? Should I be trying to do something to stop it growing please?

[Yorkie]

Welcome to the forums 

This usually indicates that the compost has been kept too wet - could that be the case?

If so, I'd cut back on the watering and/or ensure that the trays are not stood in water for too long.  Seedlings can be prone to damping off, which is a fungal condition that can be attributed to overwatering.

Are you getting any ventilation into the P/T at all, too?

[E Annie B]

Thanks very much for the welcome and your response - much appreciated. Yes, I could have been over-keen with the watering so I will reduce it and see what happens. I am opening both ends of the tunnel as early as I can each day and closing them early evening. So much to learn but I keep telling myself that I am only practising at the moment. Once I retire from work in a year's time I will have so much more time for it!

[Hortic]

Algae. Reduce watering and increase ventilation.