Wednesday, March 14, 2012

It’s Time to Prune Your Summer Flowering Shrubs by Elaine Lockey

Rejuvenation pruning on an older Potentilla. The one on the left needs similar work.
Get out your pruners and loppers - it's time to do some pruning. Summer-flowering shrubs bloom on new growth from this year. Some examples are Potentilla, butterflybush, blue mist spirea and Rose of Sharon. They should be thinned or rejuvenated in the late winter or early spring before growth starts.  

Potentilla fruticosa, also known as Shrubby Cinquefoil, is a very common shrub in many gardens.  Many landscape companies include these in their plantings due to their hardiness and adaptability.  They look nice at first, with pretty yellow flowers (other color cultivars also available), but become ugly very quickly when not properly pruned.  They also tend to collect leaves and other debris so can become a maintenance chore.  

If you have one of these in your garden, it may be brought back to its former nice look with some proper and radical pruning. This is referred to as rejuvenation pruning and can be done as often as every 3-5 years. Rejuvenation pruning works well on twiggy multi-stemmed shrubs.  It’s time to do it when a shrub looks woody and leggy. This type of pruning requires cutting the entire shrub down to the ground. The shrub will re-grow from the roots.
A healthy well-shaped blue mist spirea. 
Once you do this then you can get on a regular pruning routine of every year taking out the oldest cane(s) to the ground, called thinning.  You will also want to remove any dead, damaged or diseased canes. Limit the amount removed to no more than 1/3 of the plant at a time.  Removing the larger older canes opens up the plant to sunlight leading to more flowering within the plant instead of just at the tips of the stems and will give the plant a fresher younger look. 
Shrubs that have been un-pruned for several years and have become mostly woody may not respond well to rejuvenation pruning.  In this case, replacement of the plant might be the best option.

A good resource for pruning of all shrubs, both spring and summer flowering is Colorado State Extension's GardenNotes#619.

Blue mist spirea photo courtesy of The Wyoming Plant Company.