This is a common problem with evergreens. To find the culprits you need to get out a magnifying glass and examine the backs of the leaves, specifically those higher up the plant than the ones covered in “soot”.
On these, you will detect some almost microscopic, virtually colourless limpet-like creatures, probably along the central rib of each leaf. These are sapsuckers, a scale insect, and it is their excretions, falling on to lower leaves and then attracting a blackening mould, that are causing the problem.
Although naturally sought out by hungry blue tits, control of the insects is difficult. Dabbing leaves with neem oil or alcohol (for indoor plants and minimal infestations outside) is impractical for you. Readers are increasingly unenthusiastic, on ecological grounds, about using the only practical spray solution, Bug Clear Ultra (a systemic insecticide absorbed by the leaves).
The spring removal of breeding, vulnerable adults and nymphs by pruning off shoot tips would coincide with pieris’ prime flowering time. So: no easy solution, but at least you now know the nature of the beast.
How long does mulch from pollarded lime trees need to be left before I can put it on my flower and shrub beds? The tree surgeon has, at my request, shredded the spoils and left me a big heap.
Maureen Marshall – via email
You don’t mention the age of this lovely-sounding resource that probably has other readers turning green with envy, but I am assuming that it is at least a few weeks old. Ideally, the pile should be left for three or four months at least before it is distributed around your garden.
Received wisdom on this subject used to be that both commercially produced wood and bark chips as well as woody garden shreddings “leached nitrogen from the soil” as they rotted, adversely affecting plant growth.
But, as a result of research, this opinion seems to have been modified in recent years, and the general advice now is that it is fine to use both bagged products and prunings/shreddings as a surface mulch for moisture retention and weed-smothering, although it should not be dug deep into the soil as a soil bulker and conditioner.
To effectively smother weeds, apply a thick layer, at least 10cm deep. Gardeners are warned, however, not to apply a mulch around delicate annuals, and not to apply it thickly close to shrub bases and tree trunks.