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How to cut back blue mist spirea

Blue mist spirea (Caryopteris x clandenosis) is a wonderful small shrub for Colorado Front Range landscapes. It’s drought tolerant, deer-resistant, and produces beautiful deep blue flowers in late summer. Bees love to visit blue mist spirea when it’s in flower.

Despite its widespread use, blue mist spirea can be puzzling when it comes time for spring cleanup. Should you prune it to ground level, part way, or just to the new growth? Be sure to check out this video about how to cut back or prune blue mist spirea in spring

In order to understand how to prune blue mist spirea, it’s important to understand how it grows. The long flower spike turns brown and dies each winter. This portion of the stem will never regrow once it’s produced seed.

Beneath the flower spikes are the branches.  They produce new leaves and flowers from the existing stems each year. In mild areas of Colorado, new growth may emerge along the entire length of the stems. In colder areas of the state, its stems may dieback part way, leaving some of the upper portion of the stem dead.

Here’s my advice on cutting back blue mist spirea:

Cut back to ground level

Prune blue mist spirea

Cut blue mist spirea stems to ground level

At elevations under 7,000 feet, I prefer to cut all the stems to ground level in late winter. This way, you remove the old woody stems. When it regrows, the plant is more uniform in shape, with more fresh stems originating from the crown. This results in more flower spikes in later summer. It looks tidier and more colorful.

If you have older plants that you would like to rejuvenate, give this method a try. It can be disconcerting if the shrub looks like it’s growing from a single stem at its base, or if the stems are old and thick. It’s scary to risk killing the plant, but you’re likely to be pleased with the results.

If your hand pruner cannot cut through the old branches, you might end up leaving undesirable stubs. Instead, try use a pruning saw to cut off the plant at ground level.

When should you prune blue mist spirea? If you’re going  to the ground, do it before the new leaves start to emerge.  Try to cut it back between March 1 and April 1.

If you’ve planted a blue mist spirea shrub recently and it’s still reaching its full size, wait a few years before you cut it to the ground. You want the root system to be strong before you cut off its top.

Cut back to new growth

Blue mist spirea pruned to new growth

Blue mist spirea pruned to new growth

This is the best method if you want to retain a large portion of the plant. It’s simply taking off the dead lengths of stem that will never regrow and then shaping the plant for symmetry. This method works well for shrubs planted in the past two years that aren’t fully established. It also works well for high elevations locations where the growing season is short, or for plants that are not irrigated. Also, if you’re late cleaning up your yard, this is probably the best option.

Cut off the old growth once you can see the new leaves emerging from the stems. The leaves usually emerge in late April or early May. Use hand pruners to cut each stem right above a newly emerging leaf. Then, step back and notice if you need to touch up the plant to make it more symmetrical. Cut any irregular stems shorter as needed to create a mound-shaped plant.

New leaves on blue mist spirea

New leaves emerging from blue mist spirea

At high elevation, blue mist spirea can become overly woody over time. If you’re considering replacing your shrubs because they’re looking tired, you can try cutting them to the ground every few years. It might take two growing seasons to fully regrow, but it’s worth a try before you replace them.

Cut back part way

This is my least favorite technique for pruning blue mist spirea. It’s okay if you really don’t know what to do, but it’s not an effective method over the long term. You’re not fully rejuvenating the plant, nor are you retaining the maximum amount of growth.

By repeatedly cutting into the thicker stems, the plant produces a lot of branching that can become a thick, messy tangle over time. During the winter when it has no leaves, it looks untidy.

Rather than cutting back part way year after year, try alternating between cutting to the ground and cutting to new growth. In winter, your shrub will have nicer branching structure supporting the beautiful seedheads that catch snow.

Check out this post to review the results of the different pruning methods.

Where does blue mist spirea come from?

This is a pretty interesting horticultural story. Blue mist spirea, also known as bluebeard or Caryopteris x clandenosis, is not found in nature. It is actually a hybrid between two species, Caryopteris incana and Caryopteris mongolica, two shrubby species from the Far East.

Arthur Simmonds, an English plant collector, hybridized the two plants around 1930 to try to combine the longer life of C. incana and deeper blue flowers of C. mongolica. The original hybrid was named Caryopteris x clandenosis ‘Arthur Simmonds.’

Since the original hybridization, there have been other hybrids developed, including ‘Blue Mist,’ ‘First Choice,’ ‘Dark Knight,’ ’Longwood Blue,’ ‘Heavenly Blue,’ and others. I bet Arthur Simmonds had no idea he would be a founding father of one of the most widely-planted shrubs in Colorado.

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One Response to How to cut back blue mist spirea

  1. David February 11, 2016 at 4:58 pm #

    Awesome article; exactly what I needed. I was concerned on how to trim it down in spring, but now I know. I planted this beauty last spring and the tag that came with the plant mentioned to cut down to ground each spring, but some of the stems are almost 1/2 thick. I’ll wait ’til it starts to bloom before I start some trimming.

    Thank you!!!

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