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If you are growing a Mahonia shrub it’s important that you keep it healthy and well-maintained. One of the things you should do to help it remain healthy and grow well is to prune it.
Pruning Mahonia aquifolium (Oregon grape/Mountain grape)
Firstly there are two different classes of Mahonia has. The first is called Mahonia aquifolium and these are the lower growing evergreens that flower in the spring. These smaller Mahonia has typically don’t grow more than one meter in height and spread.
They require very limited pruning unless you plant them in the wrong place and they no longer fit. Because they flower later in the season you want to prune them right after flowering if necessary. Otherwise, just a light trim will do.
If you’re growing them in a shrub border you can prune them by way of just deadheading the flowers. In some situations, you might want to leave the spent flowers so that fruit develops for self-seeding or for birds to eat. If your plant is getting too big for its space, as mentioned, you can prune it in stages by cutting back the longer branches down to ground level but never removing more than a third of the plant at one time.
Pruning Mahonia X media
The Mahonia X media are more popular in landscape because they are taller and wider specimens. They tend to grow in upright clumps and they have longer canes. They flower at the beginning of winter usually in November and December. The flower heads will turn into seeds very quickly and fall off. But if you don’t prune them they can grow to be large in size upwards of 2 meters to as tall as 4 meters.
If you have room to accommodate this size obviously you don’t have to prune to keep it in check but if you don’t have that much space you should cut one out of every three stems each year at the beginning of Spring as soon as it’s done flowering. You can cut it down to within 60cm of the ground.
With either Mahonia type, it is important that you prune them as soon as they are done flowering. This will allow the plant time to develop new shoots and to mature so that you get flowers the following season. And of course, make sure you give it a quick treatment of extra water and possibly fertilizer depending on the conditions after you have finished pruning. This becomes particularly important if you were pruning one-third of the shrub.