I pruned my spirea bushes (Magic Carpet, Spiraea bumalda)
in April before they leafed out. Now, a month later they are about a foot tall and filling in with lovely reddish leaves. However on each bush, there are several branches that have just one or two or no leaves or leaf buds at all. The confusing part is that if you snip off a piece of these branches they are still green inside. My question is whether I should prune off these branches now or wait a while longer. They bloom in late June.
Thank you for contacting Toronto Master Gardeners.
When pruning shrubs it is important know if it blooms on old wood from the previous year or new growth in the spring. This decides the time of year you prune.
Your Spiraea bumalda blooms on new wood so you are correct pruning in the spring. When pruning always prune out dead and broken baranches. The next step is to remove the oldest branches that will not give you as many blooms. I am guessing the branches you are talking about are the older branches. They are still alive but getting leggy. Pruning these out will help maintain the shape of the plant and promote better blooming on the newer branches.
When pruning out a branch follow the branch back to where it meet a mainstem and cut it off there. This will maintain the shape of the plant and thin out the centre of the bush increasing airflow and a heathier more resiliant plant. If you cut off just the end of the branch and do not follow it back to the main branch you may get growth at the end of that branch that will not follow the natural line of the shrub; making it look untidy. If you cut out the older branches each year you will have a happy healthy plant.
It may take a couple of years to get your Spirea the way you want, depending on how much older wood you have. If it is a good portion of the shrub you may want to take the oldest branches the first year and take out more the following year. Older shrubs can take a few years to rejuvenate.
Here are some articles for further reading.