In keeping with the general ease of maintaining and growing good Mahonia shrubs, the pruning aspect is well laid back and not always necessary. That's in ideal circumstances of course and experience has taught me that ideal situations do not always exist in gardening.
There are basically two different classes of Mahonias. The Mahonia aquifolium types are the lower growing spring flowering evergreens that find their way into public planting schemes because of their ultra hardy nature. No less hardy are the Mahonia x media types Mahonia Charity being a popular example. These are used in larger landscape schemes and more popular in larger gardens or woodland settings being taller growing specimens than the lower growing Mahonias.
Pruning Mahonia aquifolium; Mahonia repens; Mahonia wagneri types and all the lower growing Mahonias, which are sometimes referred to as Oregon Grape Mahonias.
(In reality the Oregon Grape is the common name for the Mahonia aquifolium only) This group normally grows to around 1m tall though some can reach up to 1.5m over time with a similar spread.
These Mahonias require the minimum of pruning attention unless of course they were planted in the wrong place and have outgrown their home. This group flower later that the taller varieties which tend to flower in early winter, so should ideally be pruned right after flowering if requires. If grown as a ground cover mass, a light trimming will normally do the trick.
If grown as specimens in the shrub border, then pruning should simply be taking off the dead flowers unless you want the fruit to develop either for birds or for self seeding. If these are getting too large, pruning can take place in stages by cutting some of the longer branches or shoots down to near ground level where they will soon develop new growth which will develop and flower the following year providing any pruning is carried out right after flowering.
This group encompasses those that normally grow upright with clumps of long canes having the long pinnate leaves forming a top canopy. Flowering generally takes place in early winter November/December.
Mahonia x media types including Charity Buckland, Winter Sun, Lionel Fortescue and others together with japonica, bealei, lomariifolia, are virtually self-pruning. The flower heads soon develop into racemes of seeds which invariably fall off as the new leaves and growth near the top pushes out and upwards.
However, left un-pruned some of the varieties will grow to 2m over time. A splendid sight if you have the room to accommodate one. The best way forward is to cut out one in three stems each year as soon as possible in early spring after flowering. Some of the canes can be cut back half way or to within 60cm of the ground, or alternatively down nearer to ground level.
With both the lower and taller growing types it is essential that any required pruning should take place immediately after flowering. This then allows time for the new shoots to re-grow and mature during the summer producing flower bud for the next flowering season.
Most gardeners either love or hate Mahonias. I most certainly fall into the former category for growing and photographing this range of plants has always given me great pleasure – especially on cold November days!
Mahonias are in the family Berberidaceae and as you might surmise from that, are related not too distantly to Berberis. Some of the tell-tale similarities include the make-up of the individual small flowers and in particular, the fact that just under their bark there is a bright yellow cambium layer or heartwood even.
Mahonia shrubs basically fall into one of two groups – conveniently tall or short! The taller varieties are typified by Mahonia x media ‘Charity’ types and the shorter, more compact group by the Mahonia aquifolium groups.
Image of Mahonia x media 'Lionel Fortescue' © David Hughes
All the Mahonias we discuss here are flowering evergreen shrubs with yellow flowers – normally scented – which appear either in the winter or spring months. The flowers of all Mahonia plants are followed by dark purple or black berries (fruits) which are soon dispatched by hungry birds. Whether or not they are edible is a matter of taste – and uncertainty!
If they taste anything like the related Berberis berries that I tried as a youngster they are best avoided, though there are recipes available for various jams. Some are listed as being poisonous to the effect of vomiting, lowering blood pressure etc if eaten in large quantities. I would suggest that they could certainly cause stomach upset if eaten raw in quantity. Best avoided!
The evergreen foliage is a gardener’s plus – sometimes striking, but always welcome. The taller types of Mahonia have long striking pinnate leaves atop the upright stems, whilst the shorter group – whilst attractive foliage and flowers have shorter individual pinnate leaves.
Once you have chosen the correct planting position. It is very much a ‘plant and forget’ situation as far as immediate aftercare is concerned. That is not to say that general neglect is the order of the day, but Mahonias really are low maintenance shrubs.
The best planting position would be in semi shaded areas – dappled shade and full shade will also be acceptable for they are basically rocky or woodland plants from areas as disparate as the Himalayas, Eastern Asia – including the colder climates – and central to North America. The shorter Mahonias will tolerate sunny positions providing the soil does not totally dry out. Otherwise in the ideal shaded areas they will rarely if ever require additional watering – or feeding – after settling in during planting.
Be aware –as you soon will be – that the leaves can be rather spiky and be uncomfortable to brush against. This is particularly so with the taller growing types. That is not to say that the shorter varieties are cuddle. The leaves on all types are tough and leathery.
Early spring is the best time to plant, though summer will also be acceptable with container grown plants.
Dependent upon variety – do give space for them to grow either into tall specimens, shrub border inhabitants, specimen shrubs – or low groundcover plantations.
Mahonia aquifolium Apollo - Mahonia lomariifolia - Mahonia x media 'Charity' © David Hughes
Most light to medium fertile soils including loams will suit. Heavy clay soil best avoided unless regularly dressed in the vicinity with copious annual mulches. Either slightly acid, neutral or even alkaline soils seem to be ok for all. I have grown them in woodland situations with a slightly acid mix and also on the chalk downs of Kent with no ill effects.
There are virtually no insect pests that affect Mahonias – mainly on account of their tough, leathery foliage being unpalatable for any sap suckers.
The main diseases – which only seem to affect the shorter Mahonia aquifolium types are varied leaf spots, Rust and powdery mildew. I have never encountered such problems with the taller Mahonia x media types. (after growing several thousand on a nursery, and planting many hundreds in public and private planting schemes.
All types will grow quite readily from seeds sown in a cold frame as soon as they are ripe. Germination tends to happen later in the year. Stratification for a month in damp sand seems to aid germination.
All can be struck by various types of cuttings.
The larger types of Mahonia x media can be rooted by way of a short section of stem inserted into a pot and left over winter. Reduce the length of the long leaved down to just 4 leaflets.
The shrubby Mahonia aquifolium - Oregon Grape -can be best rooted by tip cuttings in the late summer placed in a cold frame.
Pruning Mahonia different types
Cornus alba, stolonifera and sanguinea