Mulberries on Campus

We have no native members of the Moraceae in the UK, but a couple of species have been planted fairly widely, one the fig (Ficus carica), and the other the black mulberry, Morus nigra.

Black mulberry at Cliveden. It is about 150 years old, and one of the largest black mulberries in the UK

The black mulberry has a murky history: it was probably a native to what was Persia, it was certainly known to the Greeks and Romans, and became planted throughout the Mediterranean for fruit production.  It was probably introduced to Britain by the Romans.  On campus we have established a small plantation of this species along with the white mulberry (Morus alba) in the experimental grounds, the aim being to produce enough leaf material to rear silkworms, Bombyx mori.  This year we have just started rearing silkworms on mulberry, for the first time.

The campus mulberry plantation in August 2009, about a year after it was established

The two mulberries are rather similar, although they can be told apart by looking at the ‘normal’ leaves: M. nigra has lobes (i.e. is cordate) at the leaf base, whereas M. alba does not.  Both species also produce leaves which resemble those from fig trees, but you cannot tell the species apart from these.  Of course, if they are in fruit you can tell them apart by fruit colour.  I’m not sure there are very many large fruiting white mulberries in the UK; this species, a native to China, is much more frost-sensitive than the black mulberry, and trees in much of the UK seem to die at an early age.

The mulberry plantation today

Mulberries provide an interesting lesson: get your identification correct.  If not there could be serious economic consequences.  For example, in the early 17th century James I instructed his Keeper of the King’s Silkworms (surely bonus points on QI for knowing that such a job existed), William Stallenge, to import mulberry seed from France and start a sericulture (silk production, as opposed to silk spinning) industry in Britain.  Stallenge imported Morus nigra, rather than Morus alba on which silkworms are usually fed.

Black mulberry leaf

Later commentators assumed that he had misidentified the mulberry, working on the erroneous assumption that silkworms will not eat M. nigra.  In fact, black mulberry is a much better choice for the British climate, and thus if a misidentification a lucky one. Still, the industry failed here.

White mulberry leaf

At this time the east coast of North America was being colonised by English settlers who soon noticed that the land contained both silkworms and mulberries. The settlers were keen to start sericulture which they thought could be lucrative; unfortunately it took quite a while for them to realise that neither mulberry nor silkworm were the species they had assumed.  The mulberry native to this coast was neither black nor white, but the red mulberry Morus rubra.

Fig-like mulberry leaf, could be from black or white mulberry

Although Gerard had recognised this species by 1599, and was growing specimens in his London garden, the English settlers were still confusing their mulberries in the 1670s.  Although silkworms can be reared on M. rubra, they much prefer alba or nigra.  Unfortunately, when the settlers realised they would have to import both silkworms and mulberries, they imported alba, which did not thrive in the cold springs.  Needless to say, sericulture did not thrive in the new American colonies.

Attempts at sericulture in North America were periodically revived, the last such being with the discovery of a ‘wonder mulberry’ in China in 1821.  This was called Morus multicaulis, and as its name suggests was a multiple-stemmed mulberry.  It was also quick growing and supposedly cold-tolerant.  During the 1830s a ‘multicaulis mania’ developed in North America; millions of trees were grown, and changed hand for every-increasing sums, all on the basis that they would be ideal for silkworm rearing.  Unfortunately, few people were interested in rearing silkworms (always much trickier than growing the mulberries) and a couple of poor winters in 1837 and 1838 killed many of the trees, and the rest were destroyed by the farmers in disgust.  Morus multicaulis is not a good species, rather it is just a variety of M. alba, and it is no more cold-hardy than any other white mulberry.

Progress of the Reading silkworms, some of which are being reared on black and white mulberry, can be followed on the Exploiters and Exploited blog, and a short history of sericulture can be found in Hatcher, P & Battey, N. (2011)  Biological Diversity: Exploiters and Exploited.  Wiley-Blackwell, Chichester, pp. 73-96.

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