M. pinnata is possibly not in cultivation in Britain at the present time in its pure state, but some of the finest and most floriferous garden mahonias derive from it by hybridisation with M. aquifolium and M. repens (see further below). It is allied to M. aquifolium, differing in the following characters: leaflets smaller and especially narrower (mostly up to 1 in. wide); lowermost pair of leaflets inserted close to the base of the petiole; inflorescences usually produced from axillary buds as well as terminally; pedicels with two of three bracteoles near the midpoint.
M. pinnata was introduced to the Madrid Botanic Garden by means of seeds (or perhaps plants) collected in September 1791 by the Malaspina expedition. The collection is usually attributed to the chief naturalist Dr Louis Née, but the credit really belongs to his assistant Thaddaeus Haenke, who accompanied the expedition up the coast of western N. America while Née remained behind in Mexico. The Madrid plants were named Berberis pinnata by Lagasca in 1803 and thirteen years later he added a description, though this is so short that it scarcely serves to validate the name. In 1821 de Candolle provided a fuller account under the name Mahonia fascicularis, based on plants deriving from those at Madrid. Furthermore, around 1818 Lagasca sent seeds of his Berberis pinnata to A. B. Lambert of Boyton, Wilts. Two plants raised from these seeds flowered in his greenhouse in February 1823 and a flowering spray from one of them was figured in the Botanical Register for the same year (t. 702), with a detailed description by David Don, Lambert’s librarian-botanist. The two descriptions agree quite well with each other, and a specimen from one of the Boyton plants, preserved in the Kew Herbarium, in turn agrees with wild specimens collected in the Monterey area of California in later years. M. pinnata ranges southward from there in the coastal ranges to San Diego County or slightly beyond. In northern California it is said to intergrade with M. aquifolium. In Mexico closely related plants occur which are now usually considered to be a distinct spccies – M. moranensis (Schult.) I. M. Johnston – but they were included in M. pinnata by Fedde in his monograph, and perhaps rightly so.
By 1838 M. pinnata was in commerce in Britain, though still rare. It was found to be too tender for the open ground and was usually grown against a wall, where it attained 8 or 10 ft (Loudon, Arb. et Frut. Brit., Vol. 1, p. 309). The first hybrid from it was raised accidentally by Messrs Rivers of Sawbridgeworth around 1840, and was thought by Loudon to be a cross with M. repens. In the 1860s and 1870s there are several references in horticultural literature to plants called Berberis fascicularis hybrida, thought to be hybrids between M. aquifolium and M. pinnata (usually referred to in those days as B. fascicularis).
At the present time the form of M. pinnata originally introduced to Britain is perhaps not in cultivation, having been displaced by the hardier hybrids. Of these there are several, all excellent spring-flowering shrubs, growing 6 ft or even more high. Their great merit is that flower-spikes are borne from the axils of the previous leaves as well as terminally and that most bear additional spikes from short spurs on the older wood, so that, on mature plants, several feet of stem may bear flowers. The following are in cultivation or have been described:
1. A mahonia in the trade as M. pinnata has rather short, congested inflorescences, and the flowers are borne on very short stalks, which lack bracteoles. Leaflets dull bluish green where exposed to the sun, but lighter and more glossy on the shaded parts of the plant; lowermost pair very variable in position. Although not the true M. pinnata, this is a fine mahonia, very hardy and attaining 10 ft even in far from ideal conditions.
2. A mahonia in the Berberis Dell at Kew, received under the name B. aquifolium magnifica, is clearly a hybrid of M. pinnata and very floriferous. Indeed no other mahonia in the clump where it grows makes a finer display.
3. A mahonia distributed under the name Berberis fascicularis has been seen which is again a hybrid of M. pinnata, but the leaflets are permanently papillose and dull beneath, which suggests that M. repens may enter into its parentage. The flowers are of a very vivid yellow and the flower-stalks mostly bear bracteoles.
4. Perhaps the finest in this group are the plants called M. aquifolium undulata. These have glossy, undulated leaflets and richly coloured, almost orange-yellow flowers. They are near to M. pinnata, which they resemble in having bracteoles on the pedicels and the lowermost pair of leaflets inserted near to the base of the petiole (at least on the uppermost leaves of the shoot). The origin of this form is not known, but the plants distributed by Messrs Notcutt derive from cuttings taken at Rowallane in Northern Ireland in 1930. The plants there bore the epithet undulata nana, which was scarcely appropriate, considering that even in the dry climate of Suffolk this clone attains 6 ft in height. It is suggested that the plants now called M. aquifolium undulata should be provisionally named M. ‘Undulata’, since they do not belong to M. aquifolium and may eventually prove to be northern forms of M. pinnata.
5. Around 1863 Messrs Simon-Louis Frères of Metz started to propagate a mahonia which they called M. pinnata var. wagneri and this was shortly described by Jouin, manager of their tree and shrub nursery, in 1910. Rehder considered the Simon-Louis plant to be a hybrid between M. pinnata and M. aquifolium and published the binomial M. × wagneri (Jouin) Rehd. in 1919. This would be the valid collective name for hybrids of this parentage, but these combine the characters of the parents in such diverse ways that the name is of little use in garden nomenclature and would not, in any case, be applicable to plants such as No. 3 above, which has M. repens in its ancestry.