On 05/03/2016 14:35, David wrote:
It seems a little early for lawn treatment but the lawn seems to be
growing vigorously (as is the moss) so when is it safe to treat?

Once it is growing strongly then there is no problem. If you treat too
soon then the cold dormant weeds and moss are less affected and active
ingredients will wash away in the rain before they take effect.

Unless you are after a bowling green Chemlawn (US tradename) standard
finish then it might be more cost effective to spot weed any dandelions
and other nuisances. Killing moss in a lawn with poor drainage makes it
look a mess and the moss will return unless the drainage is sorted out.

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Regards,
Martin Brown