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Your mahonia turning red is an unusual occurrence. These are hardy evergreen plants, tolerant to a wide variety of growing conditions. Some fungal diseases cause the plant to turn red in parts, but fully red leaves are rare. However, I tracked down some problems that may cause your discoloured leaves. Just be aware that in Autumn, parts of the leaves can turn purple which is a natural autumn colour of some mahonias.
Variety of mahonia
With over 70 varieties, some mahonia cultivars have red on their leaves. Check out what type of mahonia you have and research as to whether you should expect red or reddish leaves.
In addition, some types of this plant may have red flowers instead of yellow. Is this the case with yours?
There’s a condition that plants sometimes have if they experience water or heat stress. Part of its effect is to partially or fully stop photosynthesis. This may result in discoloured leaves, including yellow which I discussed in thus guide and, even red ones. If you haven’t given your mahonia any fertiliser in the spring and its soil is poor and doesn’t have many nutrients, give it a shot of fertiliser now. Mulching around the base in autumn us also a good idea.
In the autumn, some mahonia naturally turn red in parts.
If only part of your mahonia is turning red, it may be due to the fungal disease called rust. Rust grows reddish or brownish spots on the leaves. If left untreated, these spots can join up and cover more of the leaf. Brown boil-like pustules appear on the underside of the leaf.
Remove the affected leaves and any stems that have these brown and red spots. (Use a sterilised cutting tool.) You can learn more about rust on mahonias in this guide.
If you have one of the taller varieties of mahonia (Mahonia x media), it may turn red under more forms of stress. I’ve covered heat and water stress, but what about over-pruning? Did you cut your tree back by more than one-third of its size? That causes stress that may turn the leaves red. Or is it in a wind tunnel with especially cold winds in winter? Or has the mahonia encountered some form of physical damage. Maybe it was run into by a lawn mower? Any stress can severely affect the health of the plant.
Age of leaves
Sometimes during the second half of the year, older leaves will turn red (mostly partially) and fall off the tree. This is just the tree’s way of getting rid of used up leaves to expend its energy on new growth.