As Benmore Botanic Garden undertakes critical work to help safeguard its historic Giant Redwood (Sequoiadendron giganteum) Avenue, we are taking a moment to reflect on what has been achieved so far, and the support that has made this possible.
Described as one of the finest entrances to a botanical garden anywhere in the world, the magnificent Giant Redwood Avenue is in desperate need of innovative actions or risks being lost to the nation through climate change and disease.
The Avenue was planted in 1863 by the wealthy American James Piers Patrick, and lined the original driveway to Benmore House. These trees, native to California, were among the earliest to reach British shores when the species had only just been introduced to the outside world. This makes Benmore’s 49 redwoods some of Europe’s oldest and tallest, reaching up more than 50m high. They are still young at 158 years old, and have the potential to grow for millenia like their counterparts in the Sierra Nevada mountains.
However, due to a combination of soil compaction and increased rainfall in recent years, trees are now showing signs that they are now existing rather than growing. Benmore’s Curator Peter Baxter observed, ‘Their crowns are thinning and many lower branches are completely defoliated’. The situation is expected to worsen, with models from the UK Met Office anticipating increased rainfall and more intense storms in the region in future years, so work has begun to urgently address these issues.
Thanks in part to the support of the Younger Benmore Trust, work began in 2019 to assess the damage caused by waterlogging and compaction, and a thorough consultation process determined that improving and accelerating soil drainage would be the best way to reduce pathogen pressure and prevent waterlogging around these delicate giants’ roots.
‘Carrying out the programme of geo injection and air lance work requires a massive team effort from the Benmore horticulturists,’ explains Benmore Botanic Garden’s Curator, Peter Baxter. ‘With increased and heavier rainfall in recent years, this work is all the more vital.’
Following this guidance, the Garden is now undertaking a multi-year geo injection and air lancing project, which will be followed by the construction of a new central drainage raft along the entire grassed length of the avenue – all to benefit the long-term health of this unique avenue of trees. These practical and critical steps can be taken thanks to the combined support of community fundraisers, individual donors, and grants from the Loch Lomond & Trossachs National Park Authority and Brown Forbes Memorial Fund.
Thank you to the Younger Benmore Trust, Loch Lomond & Trossachs National Park Authority, Brown Forbes Memorial Fund, and everyone who has donated to support this vital work.
If you would like to make a donation to help safeguard the Giant Sequoias for future generations to enjoy, please visit rbge.org.uk/benmoregiants. If you would like to learn more about how to support Apprenticeships at the Garden, please email email@example.com.Donate to the Benmore Sequoia Appeal
‘Benmore Botanic Garden is very special place that our family has enjoyed for many years, with the most beautiful collection of plants and trees. Today, the Redwood Avenue, planted over 150 years ago is threatened and we risk losing these magnificent trees to the effects of climate change. All is not lost though, and with the help of science we can save this precious part of our natural heritage. By donating to the Benmore Avenue Project, you can help support vital horticultural interventions that will improve the health of these endangered trees, and ensure that they stand for many more centuries to come’.
- Emma Thompson & Greg Wise
‘Carrying out the programme of geo injection and air lance work requires a massive team effort from the Benmore horticulturists. With increased and heavier rainfall in recent years, this work is all the more vital.’
- Peter Baxter, Curator, Benmore Botanic GardenDonate to the Benmore Sequoia Appeal
Find out more about the Geo-Injection Program
- What is Geo Injection?
The aims of the geo injection programme are to achieve decompaction of soils, improve drainage and stimulate root growth with the benefit of greater nutrient uptake.
Phase one of the project was having consultants give a tree and soil health report followed by demonstrating the geo injecting of approximately four hundred square meters of a section of the avenue. Air spade work was demonstrated and used to show the extent of tree roots across a central portion of the avenue. A much abridged version of the consultants report is outlined below: -
Consultants Tree and Soil Health Report
Soil samples were collected from various locations along the entire length of the avenue. The samples were for laboratory analysis of macro and micro nutrient content, bulk density, moisture content, pH and organic matter content. Bark, twig and foliage samples were also taken.
Comparisons were made in connection with the growing conditions at Benmore and their native habitats in California
- Soil analysis results were variable across the site but generally showed to be high in Iron, optimal in Phosphorous and either deficient or moderate in Potassium, Magnesium, Calcium, Manganese, Copper, Zinc and Boron
- Soil bulk density results showed the soils to be compacted leading to limited oxygen availability as well as nutrient availability and increasing the resistance to root growth. A likely contributing factor in the visually apparent physiological stress
- Soil moisture content was found to be at field capacity which is creating anaerobic conditions for the tree roots
- PH average of 4.4 ranging from 4.9 to 4.1 at Benmore with averages in the groves in California being pH 6.5. It is observed the trees have grown in the Benmore pH range from young and as a result Benmore staff don’t see raising the pH to be a priority
- Organic matter averaged 4.4% which is within the ideal range of between 3 to 5%
Part of phase one was to ascertain the most economic process to geo injecting the avenue. The results highlighted the purchasing of equipment and using the Benmore team trained to use the equipment being the most cost effective solution in the long term.
Items purchased included a compressor, a geo injector, an injector air and product mixing unit and an air lance. Existing tractor and trailers would be used to support the programme.
Phase two was to purchase the necessary materials required and to carry out the injection programme.
Materials required to carry out phase two of the project are estimated to be: -
- 650 cubic meters of composted fine bark = 10 large articulated truck loads
- 54,000 litres of Perlite = 54 bags
- 800 kilograms of Bio stimulant = 40 bags
- 6,000 kilograms of Rootzone dried sand topdressing = 240 bags
The system we have chosen to use at Benmore, is to geo inject at one-meter centres, to a depth of 500mm, injecting two blasts of compressed air at 100psi followed by two injections of perlite.
The perlite keeps the void open allowing free drainage. Perlite does not absorb water also improving the free drainage situation. After the injection probe is removed, the hole is backfilled with rootzone dry sand.
Over 7,250 square meters of injection is to take place, this equates to the entire area of the length of the avenue, plus outside each side of the avenue to approximately two meters beyond the drip line.
This programme is followed by air lancing all the areas to be mulched. The air lance fires a continuous blast of compressed air at 100psi to the top 200mm of soil or turf. Turf areas to be mulched will previously have been sprayed out. The air lancing de compacts the surface layers and helps create an open tilth.
Mulching to a depth of 100mm follows, the mulched areas are then given a light air lance to help mix a small portion of the mulch into the upper soil layer. Over 6,300 square meters are to be air lanced and then mulched.
The final part of the process is to add an organic bio stimulant to the mulched areas and then lightly rake in. This is carried out once the soil temperatures are above 10 degrees C. The bio stimulant helps the microbial activity in the soil, which will in turn help release existing nutrients for uptake by the tree roots.
Browse through our diverse range of formal and informal education programmes for people of all ages and at all levels
Books at the Botanics
RBGE publishes a range of books inspired by the Garden's work and collections
Knowledge Exchange links the research community with others.
Searchable Resource Centres
View our selection of searchable resource centres.
Check our latest news and connect with our experts
Find the ideal venue for your corporate event
Find out how you can support our work at the Royal Botanic Garden Edinburgh.