Trees and flooding Part 2: how to treat trees following flooding


Dr Glynn Percival

Dr Glynn Percival believes Biochar – a form of charcoal – is beneficial in returning nutrition to soil following a flood

Soil is often lacking in oxygen after prolonged flooding.  Decompaction of the soil (aerating it), is important. 


This involves using an air spade (a jet similar to those used for pressure jetting cars and patios, except that here, air is forced through the jet at high pressure, breaking up soil).  Care is needed to ensure that roots within the soil are not damaged during this process, nor roots loosened.  However, the benefits are evident, including creating conditions that make the spread of pathogens such as Armillaria and Phythophtora difficult.


λ Adding mulch, biochar or compost is beneficial.

λ Mulch helps to retain soil moisture (this main sound counter-intuitive, but it does help), aerates the soil, and encourages soil organisms to flourish, especially during drought conditions.

λ Biochar, which is a form of charcoal, is beneficial.  Trials have shown that it encourages good vigour in plants.  It also helps to restrict the impact of some of the compounds that hinder photosynthesis, making it harder for the plant to absorb these, and aid the retention of nutrients in the soil.  It helps with moisture retention, which is important in droughts, and can aid the proliferation of beneficial soil microbes.  Biochar can be applied at 5% of soil volume.

λ Plant foods, such as sugars, can be applied to the soil to encourage growth.  This can be beneficial when the root to shoot ratio is imbalanced.  This is useful in the short term but is not necessarily a long-term solution.


λ Mycorrhizae, beneficial fungi often found around the roots of trees, are recognised as being important organisms in assisting trees with increased vigour, and are easily damaged by flooding.  They are widely available commercially, and it can be tempting to add them to the soil.  However, studies show, as with any ecological interaction, there are many variables determining the success of such inoculations, and their impact on plant growth.  Current thinking is that further research is needed.

In part three, we will explore the potential of ‘flood-proofing’ the tree population

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