Top Tips for Tree Care - The Kew Way

Last week Threadspider and I went to Bath University Gardening Club and were treated to Tony Kirkham's talk on 250 Years of Trees at Kew. Those of you who've seen Tony on the TV programmes made about Kew or The Trees That Made Britain, know he's very knowledgeable with a great sense of humour.

His talk had the lot: history, royalty, people (especially his predecessors who shaped Kew's Arboretum) and lots of top tips re tree care. It's the latter I'm concentrating on today. Tree care has changed a lot over Kew's 250+ years of history where they still have a few of the trees originally planted (part of "The Old Lions") when Kew began.

The notorious Great Storm of 1987 (when 15 million trees were lost overnight in England, including 500 at Kew) and the learning from that one event has helped to shape the way Tony and his team look after Kew's trees today.

After that event they realised their tree care could be improved greatly by taking care of soil compaction. To this end their treatment for this problem comprises:

• Removal of any grass around a tree, ideally to the canopy boundary. Grass is the trees' biggest competitor for nutrients and water
• Regular applications of mulch around the tree within the circle of grass removed
• Blasting of air through the soil, so air gets to the roots. It also encourages an increase of beneficial myccorhizal fungi associated with the trees' roots
• Planting 6 million bulbs per year (it takes 15 people 1 week) - flowering bulbs such as crocus beneath the trees discourages visitors from walking there at a time when soil structure is at its most vulnerable

Tony also told us their tree planting techniques have changed, exploding the myth in the process that a tree's root depth extends downwards to the same depth as the tree's height. One metre is the average depth of roots as below this soils tend to be anaerobic and lacking in moisture. Instead, roots tend to extend outwards to the same length as the tree's canopy (logical when you think about it).

Apparently many of us tend to plant our trees too deeply. An inch or too either way can make quite a difference to root development and thus the establishment and overall health of a tree. Tony's top tips for planting are:

• Excavate a soil pit to just one spit spade depth
• The planting pit should be square, not round - this encourages the tree's roots to break out at the corners of the pit instead of spiralling round
• Don't plant too deep - if the tree is seen to rock on a windy day after planting, it's too deep
• Don't stake for support - trees need to flex after planting (= seismorphogenesis and is different to rocking) which encourages both root development and trunk flare. The latter will develop into buttresses as the tree matures. These are needed so the tree can support itself
• Don't fertilise or add bonemeal at planting. Trees don't need additional nutrients in their first year as the tree is more intent on putting roots down and finding water rather than the uptake of nutrients

The team at Kew have also changed the way in which they prune trees. Instead of flush cutting which was practised there until the 1980s, they now carry out target pruning, a technique pioneered by Tony's predecessor William Dallimore. A correctly target pruned cut is much smaller than a flush cut, rounded and looks like an archer's target due to the exposure of the branches growth rings.

I was also relieved to hear the team don't rake leaves in the autumn. Instead they cut them up finely and let the worms take the leaves down in to the soil, pretty much overnight apparently. Tony added that when the weather gets cold, the worms burrow even more deeply into the soil, thus ensuring the organic matter goes with them. My approach of letting the trees automatically mulch my borders rather than gathering all the leaves up seems to be a good one then :)

The talk was a fascinating insight into Kew's history and how tree care is still being developed today. You can catch up with Tony and his team via the Arboretum Team Blog :)

The picture is of a Golden Ash Tree at Kew and was taken by David Hawgood who has licensed image use via the Creative Commons Attribition-ShareAlike 2.0 licence. I obtained it via Wikimedia.