Certain pernicious ones are found in the mulch habitat. Slugs and snails like the moist
conditions beneath, though they are reluctant to move around on top of a dry mulch. Many
bugs are a mixed blessing, such as woodlice and millipedes, which attack crops as well as
processing dead material.
Warm moist conditions may encourage fungal or other diseases, so that the crowns of
herbaceous plants rot or the emergent growths suffer from damping off or a neck rot. To
prevent neck or crown rot in susceptible plants leaves a clear space free of mulch about the
plant. To prevent the mulch encroaching on a valued plant use a ring of wire netting as a
miniature snow fence.
Too thick or un-perforated impermeable mulch may restrict soil aeration; while this may not
damage established plants during dry periods, wet anaerobic (oxygen-starved) conditions
during the dormant season can rapidly cause root die-back and death. Anaerobic conditions
can also change the composition of the soil microlife, leading to decreased beneficial activity
and reduction of fertility.
The most direct damage to plants from mulches generally comes from birds spreading them
on top of low-growing thymes and other susceptible carpeting plants.
A hidden danger is where the sites of dormant plants are not marked and shoots might
unwittingly be trodden on.