Oregon grape, Mahonia aquifolium

(Question)

I have this plant on the north side of the house where it gets very little light, but, grows happily. It has been there for years and years. Two questions. Each winter some  upper branches die back and I prune them off in the spring. Would wrapping the plant in burlap help it survive? If part of the plant was moved to a half-sun, halfshade  location, would it grow differently? (It is in zone 6) Thanks.

(Answer)

I love Mahonia aquifolium or Oregon grape. My Mum used to have two on either side of the steps leading up to the front porch. The flowers, berries and seasonal changes in the colour of the foliage are lovely. The plant is native to British Columbia, is hardy to zone 6 but considered an exotic in Ontario. However, it grows quite well in urban areas of Southern Ontario, including Toronto.

With respect to your first question: because it is a broad-leaf evergreen (its leaves stay on all winter), Mahonia is very susceptible to foliar winter desiccation injury, so your experience is a common one and not too worrisome. Pruning back the foliage in the spring, as you’ve been doing, is the perfect thing to do. Mahonia is very tolerant of pruning so don’t be afraid to do this including pruning out some of the older branches right back to the ground. This will improve air circulation throughout the shrub and reduce the opportunity for disease and insect damage.

Your suggestion of wrapping evergreens to protect them during the winter months is a good one, I think, but perhaps somewhat impractical for Mahonia given the prickliness of its foliage. Instead, I would suggest erecting a wind fence using perhaps a fabric, such as burlap, around the shrub and as close to it as possible for winter wind protection. Ideally, Mahonia should be planted in a protected spot out of prevailing or channelling winds and direct sunshine in the wintertime. But protecting an exposed shrub with a fabric fence of some kind will minimize any potential damage from wind and sun.

You may still get some winter burn but pruning will take care of that easily enough and it is nothing to be overly concerned about. One disadvantage of hiding it like this is that you will not be able to enjoy the deep burgundy foliage in the winter.

Your second question seems to indicate that you are considering transplanting part of the plant to perhaps a sunnier location in your garden. Mahonia prefers partial shade to shade but will tolerate some sun. The seasonal colour changes in the foliage of the Mahonia tend to be more distinct in plants grown in shade than those grown in sunnier locations.

Given the age and ‘happiness’ of your resident Mahonia, I would be reluctant to move it. However, if you would like to try growing it elsewhere, you could try dividing it or taking some root cuttings. Due to the plant’s rhizomatic growth habits, propagation through either of these means would likely be successful. Each method has its advantages and disadvantages.

Again, given the shrub’s age, division may be the more difficult technique. Depending on how it is done, it might also require a more severe pruning than usual to attain the desired shape, size and overall appearance over a few years time.

On the other hand, plantings from root cuttings, taken in the spring, may be more easily accomplished. However, they will take longer to establish themselves and grow to a sizable plant.

Hope this is helps. Let us know how your Mahonia survives the coming winter.