Delving deep into Cambridge University’s Botanic Garden with Sally Petitt

It is enough to make any small boy’s eyes light up – talk of a giant conker. Imagine turning up in the playground with a shiny horse chestnut on a piece of string three times larger than any other.

But it is not a small boy talking about the giant horse chestnut, Aesculus wangii, the latest acquisition of the Cambridge University Botanic Garden, but the diminutive head of horticulture, Sally Petitt. She has the enthusiasm of the Victorian plant hunter about her; very soon she hopes the new tree will be growing in her 40-acre patch in the middle of the city.

Currently the giant conkers, from which she hopes to propagate this new specimen, are in quarantine having been brought home with other new species by Alex Summers, the glasshouse supervisor, who has been on an expedition to Vietnam.

The tree, which is a normal size horse chestnut with giant seeds, will first be carefully grown in a pot and then transferred to the garden, so it will be a few years before small boys will be able to forage for a potential king conker.

Sally's team are involved in a number collaborative ventures with places that once could not have been visited for political reasons but are now keen to share both specimens and knowledge.

Sally has a staff of 23. She has the job of providing scientists at the university with plant specimens, aiding the scientific research that goes on within the botanical gardens, and accommodating the ever increasing number of local people and tourist visitors.

In the midst of an ever-changing series of different themed gardens her current biggest project is the lake, arguably the focal point for visitors. Alarm bells about the state of the lake have been ringing for some time, not least because the water, once more than a metre deep, became so shallow that herons could wade across it and many of the aquatic plants were running out of water.

Diggers are currently lifting 150 years of silt out of the lake. The silt is being sifted for artifacts, and then being dried before it is moved. Eventually the silt will be deposited on the land at the university's Laundry Farm, in Barton Road, Cambridge, no doubt improving its fertility.

Sally said:"So far nothing has come up by way of bones or treasure but we are very keen to see what is down there. As far as we know no silt has been taken out since 1858 when the lake was built. We had to do something, fish numbers were dwindling, plants were being choked. The lake will have a new lease of life. We are hoping to get it back to its original depth which was more than a metre."

The lake will eventually be refilled from Hobson's Conduit, the watercourse dating from 1610 that brings clean water into the city from the springs four miles away at the Gog Magog hills. The conduit water which has always fed the lake is clean so the silt has come from the leaves of trees and the other dense vegetation falling into the water.

The importance of the Botanic Garden to Cambridge appears to be increasing with 250,000 visitors this year, a record. This is partly because close by there have been much new development and the gardens provide an oasis for people who might not otherwise have a garden. It is also attracting more tourists.

Visitors provide much-needed revenue to keep the gardens going and helping to pay for the 50 staff, but the principle role of the garden is scientific. The garden was opened in 1848, following a lot of work and political effort by John Stevens Henslow, professor of botany at Cambridge, who persuaded the university that a large site was needed to properly study plants. He was clearly an inspiring teacher - one of his students was Charles Darwin.

The gardens remain today as a sub-department of the Department of Plant Scientists and many plots are still used for scientific research. The university is backing the lake renovations.

Sally, who began work at the garden as trainee technician, has been there 28 years and finds herself much in demand for advice. She sits on the Merlin Trust that gives grants to students to travel to places distant places to study horticulture. She knows how important this experience can be because one of her most memorable early trips was to Armenia made possible by an award from the trust. She is also on the advisory committee of Chelsea Physic Garden among others.

But while she loves plants, and is clearly excited by the prospect of new species, she says she also has a ruthless side.

"There are certain things we cannot grow here because the soil is not right,she said."We have an alkali soil so no rhododendrons. They must have acid soil and won't thrive here and so we do not try.

"If other plants do not do well they simply have to go. The public do not want to see sickly plants.

It's not just plants

Reeves' muntjac are small, stocky and russet brown deer which have taken up residence in the Botanic gardens and have proved impossible to shift.

These Chinese deer, sometimes called barking deer, from the sound they make, are destructive and chew the bark off growing trees and eat some of the delicate and rare plants that the gardens are famous for.

Muntjacs were imported in the 19th century by the Duke of Bedford to join his deer collection at Woburn Abbey, Bedfordshire and escaped into the countryside sometime in the 1920s. Now they are all over the country and inhabit many cities, and are probably the most common deer in Britain.

So exterminating them from the Botanic Garden, which was the original plan, is probably not possible because more would sneak in to replace them. After all 40 acres of good grazing with thickets to hide in is a perfect habitat.

So along with all the other mammals that inhabit the gardens - foxes, badgers and grey squirrels - the deer are tolerated despite the damage they do.

The gardens are also a haven for amphibians, notably grass snakes, which are rare elsewhere. The water features, including the lake, stream and fenland habitats, are the home of frogs, toads and smooth newts as well as the snakes.

More than 100 species of birds have been recorded so not surprisingly the gardens have become a happy hunting ground for a pair of sparrowhawks. So as well as a garden this is a wildlife reserve in the middle of the city.