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Plant Portrait - Ginkgo biloba, The Maidenhair Tree

Ginkgo biloba

Ginkgo is one of the most exciting plants we are growing, with a wide range of uses. This tall but narrow deciduous tree can reach a height of up to 30 metres with a spread of 9 metres. This species is the only surviving member of a family that was believed to be extinct until fairly recent times. It has probably remained virtually unchanged for at least 150 million years and might have been growing when the dinosaurs were roaming the earth. It is exceptional in having motile sperm and fertilization may not take place until after the seed has fallen from the tree. It belongs to a very ancient order and has affinities with tree ferns and cycads. The plant seems to be totally untroubled by pests and diseases and is believed to have evolved a resistance over its many millions of years of existence. I suppose that if it could survive the ravages of dinosaurs, then it is unlikely to be overly concerned about the attention of aphids or whatever.

A very easily grown plant, it succeeds in most soil types so long as they are well-drained, though it prefers a rather dry loam in a position sheltered from strong winds. Some of the best specimens in Britain are growing on soils over chalk or limestone. The plants flower and fruit more reliably after hot summers or when grown in a warm sunny position. Established plants are drought resistant, they also tolerate atmospheric pollution. They can grow in poor hard-packed soil, making them good candidates for street planting. Only the male forms are recommended for this, however, because the fruit from female plants has a nauseous smell.

The ginkgo is usually slow growing, averaging less than 30cm per year with growth taking place from late May to the end of August. Growth is also unpredictable, in some years trees may not put on any new growth whilst in others there may be 1 metre of growth. This variability does not seem to be connected to water or nutrient availability. Trees are probably long-lived in Britain, one of the original plantings (in 1758) is still growing and healthy at Kew Gardens.

Ginkgo is a popular food and medicinal crop in China, the plants are often cultivated for this purpose and are commonly grown in and around temples. The plants are either male or female, one male plant can pollinate up to 5 females. It takes up to 35 years from seed for plants to come into bearing. Prior to maturity the sexes can often be distinguished because female plants tend to have almost horizontal branches and deeply incised leaves whilst males have branches at a sharper angle to the trunk and their leaves are not so deeply lobed. Branches of male trees can be grafted onto female trees in order to fertilize them. When a branch from a female plant was grafted onto a male plant at Kew it fruited prolifically. We have often seen female trees with very good crops of fruit.

The seed can be eaten raw in small quantities, though it is best cooked when it can be used as a staple food. It has a soft and oily texture with a sweet flavour and the books say it tastes somewhat like a large pine nut. We ate some seeds for the first time in the autumn of 1996 and, when baked, they made very pleasant eating. Our verdict was that it tasted rather like a cross between potatoes and sweet chestnuts, and that we would love to be able to eat this food in quantity. The seed can also be boiled and used in soups, porridges etc. It is rich in niacin, is a good source of starch and protein, but is low in fats. These fats are mostly unsaturated or monounsaturated. When cooking the seed, it is best to leave them in their shells. These shells are very thin and it is a simple matter to crack them at the time of eating.

Some care has to be exercised if you want to eat the seed raw. It contains a mildly acrimonious principle, though this is entirely destroyed when the seed is cooked. This acrimonious principle is probably a substance called 4'-methoxypyridoxine, which can destroy vitamin B6 in the body. It is more toxic for children, but the raw nuts would have to be eaten regularly over a period of time for the negative effects to become apparent.

Ginkgo has a long history of medicinal use in traditional Chinese medicine, where the seed is most commonly used. When cooked it is used in the treatment of asthma, coughs with thick phlegm and urinary incontinence. The cooked seeds are also said to stabilize the production of sperm. The raw seed is said to have anticancer activity and also to be antivinous. It should be used with caution, however, due to the reports of toxicity.

Recent research into the plant has discovered a range of medicinally active compounds in the leaves and this has excited a lot of interest in the health-promoting potential of the plant. In particular, the leaves stimulate the blood circulation and have a tonic effect on the brain, reducing lethargy, improving memory and giving an improved sense of well-being. They have also been shown to be effective in improving peripheral arterial circulation and in treating hearing disorders such as tinnitus where these result from poor circulation or damage by free radicals. The leaves contain ginkgolides, these are compounds that are unknown in any other plant species. Ginkgolides inhibit allergic responses and so are of use in treating disorders such as asthma. Eye disorders and senility have also responded to treatment. The leaves are best harvested in the late summer or early autumn just before they begin to change colour. They are dried for later use.

The seed is best sown as soon as it is ripe in the autumn, though it can also be sown in late winter as long as it is not allowed to dry out. There is some disagreement as to whether the seed needs to be exposed to cold weather before it will germinate. In cases such as this we try to ensure that the seed experiences some cold weather, but is protected from the worst extremes of the weather. Sowing in a cold frame or unheated greenhouse usually works well, though make sure the seed is protected from mice, birds and squirrels. The books say that germination is usually good to fair, though we have tried sowing them on many occasions and have yet to get the seed to germinate. Prick out the seedlings into individual pots when they are large enough to handle and grow them on in light shade in the greenhouse for their first year. Plant them out into their permanent positions in the following spring and consider giving them some protection from winter cold for their first winter outdoors.

The seeds are marked by either two or three longitudinal ridges, it is said that those with two ridges produce female plants whilst those with three ridges produce male plants.

Ginkgo can also be propagated by cuttings, and this is a good way of ensuring that you get plants of known sex. Half-ripe wood about 15cm long can be taken during July or August. Put these in a frame, preferably with bottom heat, and make sure they are kept moist. They may not grow away in their first year but usually grow all right after that. Cuttings of mature wood of the current year's growth can also be used. Shoots about 15 - 30cm long are taken in December and placed in a fame. They should root in the spring.

 

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