Spirea nipponica ‘Snowmound’ – Bridle wreath spirea hedge

(Question)

I have a bridle wreath hedge which has grown well and bloomed beautifully for 10 years.  Last year I started to get some dieback on some plants and this year (2014) have few leaves and dead stems.  It seems to be in clumps with 5 healthy shrubs then 3 unhealthy.  There are a few small flies around them but they aren’t excessive.  Causes?  Solutions?

(Answer)

Generally, spireas are hardy, easy care shrubs here in Toronto and Southern Ontario.  There can be many causes for dieback to portions of spriea hedges including, pollution, poor soil conditions, salt spray, or lack of pruning, to name a few.  Spirea nipponica is a dwarf spirea that grows 3 – 5 feet tall, tolerates part shade and is hardy to zone 3.

That being said, since most of the dieback happened this year, my guess is that the past exceptionally cold winter may have impacted your hedge.  You are not alone as gardeners throughout the area are dealing with the devastating effects on so many plants.  There are some steps you may try in the future to minimize the effects over the long term.  Normally, well established shrubs do not require fertilizer but a topdressing of compost or sheep manure to the soil now will give it an extra boost.  Most importantly, spirea can be pruned after flowering – even severely pruned.  This may “even out” the damaged vs. undamaged parts so that it is more esthetically pleasing.  Although it will not look it’s best this year, pruning will help the hedge thrive over the long term.  The key to recovery after this past winter, is patience.

For further reading, here are links that address the care and maintenance of spirea:

https://www.torontomastergardeners.ca/index.php/factsheet/blooming-deciduous-trees-and-shrubs-a-toronto-master-gardeners-guide/

https://www.garden.org/plantguide/?q=show&id=2103