Looking good now
The multitude of golden black-eyed Susan is setting the Autumn Garden aglow.
The multitude of golden black-eyed Susan is setting the Autumn Garden aglow.
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Lake to be dredged for the first time in the Garden’s history

Cambridge University Botanic Garden’s lake, constructed in 1857–58, is to be dredged for the first time in the Garden’s history
The last time the Lake was empty - 1976 when the drought dried out the Lake completely. Photo courtesy Cambridge NewspapersThe last time the Lake was empty - 1976 when the drought dried out the Lake completely. Photo courtesy Cambridge Newspapers
The Lake, which was originally created around an old gravel pit using water diverted from Hobson’s Conduit, covers about three quarters of an acre. It provides an important habitat in the Garden, supporting an array of wildlife and a natural habitat for many species of plants such as the swamp cypress, Taxodium distichum. It aesthetically forms a mirror-like, reflective surface enhancing many of the Garden’s spectacular trees - as has been seen this autumn with, for example, the fiery-red tones from the Liquidambar styraciflua. It also gives the Garden team an opportunity to grow many different water plants, such as rushes, sedges and reeds, all of which add to the diversity of species growing within the Garden’s living collection.

The Garden’s Director, Beverley Glover, explains:
“The Garden’s lake is not just a beautiful part of the Garden but also an important ecological habitat which we want to conserve to sustain the diversity of plant and animal life that lives in and around it. However, over the last 150 or so years, it has become very full of silt, to the point that water and oxygen levels are very low in some parts, making it hard to sustain pond life adequately. This ultimately means the lake needs some major ‘TLC’, which involves dredging it.”

The decision to dredge the lake is a result of a series of professional, analytical studies, culminating in a report confirming that the lake is heavily silted, with a water depth of just 0.3m in places, and stating that its biodiversity and ecology is suffering due to silt depth and oxygen depletion.

Beverley continues: “High levels of silt are impeding plant development and also mean that particular plants such as waterlilies and reeds are dominating the lake. Silt is also building up on root mass in and around the lake and affecting the shrubs and trees both in the lake and on its edges. It’s important that the lake provides a thriving habitat for wildlife and plants as well as a safe area for staff and visitors. We know the lake’s condition will never naturally improve and, without intervention, the lake would eventually silt over entirely and become a swamp.”

The process of clearing and dredging the lake is estimated to take twelve weeks, beginning at the end of November. Once complete, the lake will have an enhanced ecosystem, the removal of dominating plants will provide the opportunity to open up vistas from a horticultural perspective and the process will also ease the management of the lake.

The process will involve:

• plant protection: exclusion zones will be put in place around trees which are key features of the lake e.g. Taxodium distichum, Nyssa sylvatica, Cotinus coggygria x obovatus ‘Flame’, Liquidambar styraciflua ‘Worplesdon’ & Salix sepulcralis var. chrysocoma. These exclusion zones will allow the retention of silt in some parts of the lake, providing habitat for invertebrate animals that like to burrow
• site enclosure: establishing a safe working area around the lake and blocking the water course into the Garden from Hobson’s Conduit
• safe removal of wildlife: pond life will be carefully removed by specialists for safe-keeping and subsequent re-establishment
• water removal and filtering
• access to lake bed: with the safe removal of wildlife and water, the process of removing the silt will take place using specialist machinery. The silt will be loaded into a filter to remove bulk matter and excess water before being passed into geotextile membrane tubes
• silt settlement: these membrane tubes will remain on the Garden’s Main Walk as water will be allowed to drain naturally from them. It is estimated that all water will be drained within twelve weeks, leaving a solid mass which will be transported from the lake for disposal on the University farm
• lake repairs: once the silt has been removed, repairs to the banks will be made including smoothing out of the lake bottom to “pinch out” any minor leaks, required tree works and additional planting. It is anticipated that this will also take twelve weeks to complete
• re-introduction of wildlife: the lake will be re-filled and wildlife will be re-introduced and any temporary fencing collapsed

The last time the lake was empty was a result of the drought in 1976; it also reached very low levels in the 1980s but it has never been dredged in its history. Beverley emphasises: “We take the view that this is a ‘once-in-a-lifetime’ process and will be part of the lake’s and the Garden’s story. While we recognise the Garden won’t be quite as peaceful as normal, we hope it will provide an opportunity for visitors to come and observe history in the making while also leaving plenty of the Garden still to explore or find tranquillity.”

Visitor access to the Garden itself will not be affected by the lake work and outside the restricted lake area itself, there will be little or no disruption to Garden access or activities. Staff anticipate that the main areas affected by the works, other than the lake, will include parts of the Main Walk and around the New Pinetum, which will be the main throughway for works vehicles, with the intention being for work to be as minimally intrusive to the Garden as is possible.

From a horticultural perspective, this is also the prime time for carrying out this vital work. Beverley continues: “Another reason why this work is happening in the winter is that trees and plants will not be unduly affected at this time of year. Water intake by the plants will be slowing down and there should be sufficient moisture in the soil to sustain plants during the dredging period.”

As for the ‘new lake’, Beverley anticipates a natural-looking waterscape, very similar to the lake as it is now but with smaller populations of reeds, rushes and lilies which have been becoming ever-more dominant. And importantly, a lake thriving and teeming with healthy wildlife and plant species for hundreds of years to come.
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