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Mahonias are easy to grow and they tolerate most conditions. The most probable reason that your mahonia is turning yellow is over-watering. If they are positioned in full sun, they can also turn yellow is the soil is not kept moist. However, it may also be due to a nutrition deficiency or even sunburn. Read the rest of my guide below to learn more about these causes of yellowing leaves on Mahonia shrubs.
Mahonias are not pot plants and should be grown in the open ground. If you do so, they’re extremely tolerant to drought conditions. Only water them in extremely dry conditions. You may be tempted to water them when you water the rest of your garden plants, but that’s just too much. This kind of contradicts the advice to keep the soil moist if they are grown in sunny positions. However, you will find that overwatering then usually becomes a problem instead, and this can be much more of a problem than being a little too dry.
What to do
When the plant is established, only water it during hot dry spells and be careful not to waterlog it. Check that any excess water that goes into the soil is draining away.
The one exception to this is when you first plant the mahonia. It needs to establish itself and requires water for this. During its first year, water the plant regularly and deeply. But still, watch out for the soil staying really wet after the watering. If this happens, add some perlite, wood bark or other organic matter to break up the soil and let the water drain away.
This problem is a difficult one to pin down. Mahonias don’t need a lot of looking after and that includes fertilising. Your mahonia turning yellow could be a deficiency of nitrogen, potash or magnesium.
What to do
If you use rich compost with fish and bone meal added to plant the mahonia in, that should be enough. Especially if you then use an annual slow-release fertiliser in the spring. Adding a rich layer of mulch around the plant in the winter as protection also adds needed nutrients to the soil.
Transpiration in the winter
If your mahonia has yellow leaves along with a smaller than usual crop of berries, it could be because of transpiration in the winter. If the roots of the plant become frozen in the winter, it can’t send enough water up the plant to replace that leaving the leaves by transpiration. Rather than dehydrate the entire plant, the leaves don’t receive the water they need, become yellow and wither.
What to do
If your mahonia turning yellow starts in late autumn or winter, apply a layer of mulch around the base of the plant as warmth for the roots underneath. But keep the much away from the trunk of the plant so as not to encourage pests to curl up in the warmth there and infest the plant.
Note: this problem can also occur in the summer if the ground is parched because of an extremely dry spell and no watering.
The yellowing leaves can be due to sunburn or windburn. Mahonias like to be in partial or full shade in sheltered conditions. Too much (direct) sun results in the leaves turning yellow and brown, becoming dry and crisp and then falling off. Exposure to the drying effects of wind has much the same effect.
What to do
As you’re pretty much stuck with where your mahonia is, place a barrier between it and the sun or wind. This could be another (tall) plant or an artificial barrier such as a screen. And make sure to keep checking on how dried out the soil becomes in those conditions and water it appropriately.
Finally, something to be aware of is that Mahonia are very hardy plants, and sometimes yellowing leaves is something you just need to learn to live with.