All the recent flooding, especially around Brechin, has made me wonder which trees could tolerate this, and how we should choose trees that are resilient. Frequent and increasingly damaging winter gales demonstrate that climate change is already under way and conditions will get worse.

How does flooding damage trees? Firstly and most importantly, it reduces the oxygen that’s available in the soil for tree roots: like us they must “breathe”. Water enters soil pores, driving out this vital oxygen. Aerobic bacteria also play an important part in breaking down plant material which then feeds trees.

The Herald: Trees in a sea of flood waterTrees in a sea of flood water (Image: unknown)

With little oxygen in the ground, anaerobic bacteria take over and they deny trees essential nutrition and release methane, a much more powerful greenhouse gas than CO2. When heavy rains cause the water table to rise, trees with deep root systems are further damaged.This makes the tree less stable so more likely to blow over during any gales.

When and where flooding occurs is critical for the trees but it also helps us identify those that have managed to cope with it.

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Flooding is obviously most damaging during the growing season.

Late spring and early summer floods cause fast river flows with resulting soil erosion.

The grey alder, Alnus incana, has managed to deal with this. Like peas and beans, it has little nitrogen-fixing nodes on its roots. The tree has a symbiotic relationship with specialist bacteria which create the nitrogen that lets it grow in pretty poor soil.

This nitrogen-fixing allows alders to survive and even thrive in wet conditions and flooded areas. Although they’re not considered ornamental, I do find alders fine, statuesque specimens and am delighted to have a fair few that self-seed on wilder parts of my ground. If you have a large garden, I’d thoroughly recommend an alder.

Alders can also develop fresh roots further up the trunk to utilise any fresh silt swept down in a flood. Willows (Salix), like alder thrive in wet, sometimes flooded areas and may also develop an “emergency” root system on the trunk.

Willows are up to lots of tricks to get much-needed oxygen. Though not fully proved, it seems that willows, together with lodgepole pine and possibly other species, can absorb oxygen though their leaves and transport this down to the roots for general distribution throughout the tree.

So, whatever the size of your garden, you’ll find an attractive willow. Salix caprea ‘Silberglanz’ is a great choice for smaller gardens. It grows quickly to 4-7m, with a truly magnificent display of large yellow catkins in spring.

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And if you’ve the space, you shouldn’t be without S daphnoides, European violet willow. It grows to around 8m, is upright, multi-stemmed, with slightly weeping shoots on the crown periphery.

New shoots are shiny reddish brown, and a fine show of catkins is guaranteed.

As a final, if exotic, thought, why not try an American sweetgum, Liquidambar styraciflua; especially if you’ve a sheltered spot. It has spectacular autumn colour: a brilliant orange/red show on this 20m beauty. The ‘Burgundy’ cultivar is a little smaller with foliage to match the name.


Plant of the week:

KALE ‘MIDNIGHT SUN’ is a form of curly kale that has bright pink ribs to the leaves making it an attractive plant in the veg plot. It also tastes good and young leaves are striking shredded in salads. Plants are as hardy as you would expect of a kale and crop from autumn to the following spring.