A tall and modest man - about whom you could use the cliche "gentle giant" if you had observed him ducking beneath the doorways in his office building - Toomer has spent nearly five years managing the nation's tree collection and overhauling it.
Chloe Darling, one of the founders of Westonbirt's 22,000-strong friends group, which financially supports many of the projects throughout the garden, comments: "With no disrespect to the previous curator, Simon has come in with his quiet unassuming manner and absolutely transformed the arboretum. He has made difficult decisions about felling trees and opened up the arboretum beautifully. With a fantastic team he is doing the job of four people at once and managing it."
Over the past few years Toomer and his team have been attempting to "reduce the dominance" of older trees in the collection, which covers 250ha. The main planting periods were during the early 1900s and in the 1950s, when Westonbirt came into the ownership of the Forestry Commission, so now some trees are becoming vulnerable.
Toomer displays a refreshingly unsentimental attitude to tree felling: "We don't have enough resources to maintain them into senescence, so we don't prop up trees into old age. We fell vulnerable trees, because if we didn't, we would need to neglect other healthy trees." But if this sounds drastic, be reassured: the team aims to plant 500 trees a year as replacements.
Recently the Maple Loop has been opened up, which is a route through Westonbirt's national collection of Japanese maple cultivars. Larch trees that were planted in the 1970s were thinned and underplanted with acers and yews. Toomer says it is now one of the most popular sections of the arboretum, and a mecca for "leaf-peeping", the craze of coming to see autumn colour, which is so popular in the US and Japan.
And Japan is the country that Toomer and colleagues Mark and Penny are visiting now. They are undertaking a three-week expedition, covering several Japanese provinces, and they have targeted 20 species to gather to enhance the collection. Toomer can't hide his excitement as he likes nothing better than learning about trees in their native habitat. He has fond memories of botanising in New Zealand, "I did a three-day tramp there and didn't see anyone at all, or even the traces of human activities as you would here, such as dry-stone walling," he enthuses.
Despite his practical approach to tree felling, Toomer is not keen on trees being felled unnecessarily, which he believes has happened in towns because of the liability culture.
"How cowardly we are," he says. "People are sometimes guilty of reading too much into legislation. They overestimate the risk of trees. Just because the tree has a fault and it's near a road it doesn't mean it will fall on somebody. Urban arboriculture seems to be all about subsidence and other insurance issues."
However, he is lucky to be able to be more relaxed about possible insurance claims at Westonbirt because the Forestry Commission has crown immunity, which benefits adventurous, young visitors. "Historic gardens can be slightly unwelcoming," Toomer explains. "But we encourage tree climbing. We just ask that (people) be careful. The commission doesn't need to insure anything."
It's not just children who get to play and learn at the national arboretum but adults can too. On a hillside, within Westonbirt's semi-natural woodland of oak and hazel, 30 people are building a barn using greenwood. The group is on a week-long course led by timber framer Henry Russell, from BBC TV's What the Romans Did for Us, and includes students from the Prince's Trust and the Rural Skills College in Cirencester.
The camaraderie is obvious, as the students take a break to make mugs of tea for each other from a teapot that is held over an open fire. The tree-climbing team has been drafted in to make pegs and staff are sitting whittling away. Astride one of the beams sits coppicing apprentice Ruth, who normally works with four others in the semi-natural woodland around the clearing.
Westonbirt's friends group is sponsoring the barn and when it is erected it can be used to hold courses and other events. Toomer says: "It will be something in itself to demonstrate native tree management."
Under Toomer's management Westonbirt is looking ahead. It is increasingly becoming a venue for courses and recently was the centre of a Plant Network conference on climate change, when the effects of changes on the tree collection were discussed. The management team is planning to establish the so-called 2050 glade, to demonstrate the type of trees that might succeed in a different climate.
As Toomer checks that the course tutors have enough beer for the night and chats to his trainee arborists, it sums up the sensitive and sometimes radical way in which the national arboretum is being managed. With this kind of atmosphere it's easy to understand why 350,000 people a year come to Westonbirt.
1978-79: Forestry worker, Fountain Forestry
1979-82: BSc (Hons) in Biological Sciences at Plymouth Polytechnic
1982-83: MSc in Environmental Forestry at University College of North Wales, Bangor
1983-86: Forester, the Parnham Trust, Beaminster, Dorset
1987-89: Woodlands officer, East Cumbria Countryside Project
1999-2003: Self-employed woodlands/arboricultural adviser including work at Westonbirt
2002-03: Treelife course in arboriculture (part-time) Technician's Certificate in Arboriculture (Arb. Assoc.) Professional Diploma in Arboriculture (RFS)
January 2003 to date: Curator, Westonbirt - the National Arboretum
2005: Author of Trees for the Small Garden, published by Timber Press.