Nitrogen-Fixing Trees For the UK

Nitrogen is the most common element in the earth's atmosphere and is also an essential element for plant growth that is found in the soil. Non-organic gardeners often add nitrogen rich chemical fertilisers to the soil to aid leafy growth. Organic gardeners, on the other hand, go for more natural methods to maintain soil health and improve plant health and yield. Nitrogen-fixers are a group of plants and organisms that play a key role in the nitrogen cycle on this planet. They are an important part of any healthy garden ecosystem.

If you have a medium to large garden then you will have enough space to incorporate some trees into your garden design. Trees can help you to make the most of the space in your garden through vertical growth, can provide shade and other yields and can generally improve the look and feel of your garden. Above and beyond all that, some trees also act as nitrogen-fixers – taking nitrogen from the air and, through a number of complex mechanisms, making it available in the soil for the use of other nearby plants. These nitrogen-fixing trees could be a key component in a food forest or edible garden in the UK:

AlderAlder (Alnus glutinosa)

Common alder provides a high yield of nitrogen. It is a native tree which makes it a wonderful choice for those who wish to attract native wildlife to their gardens. Alder has a symbiotic relationship with a nitrogen-fixing bacterium called Frankia alni, which is found in its root nodules. Therefore, the alder is able to improve the soil where it grows and is a useful pioneer species. Other alders, such as the Italian Alder (Alnus cordata) and other Alder species, though non-native, may also be useful nitrogen fixers in gardens in some parts of the UK.

LaburnumLaburnum Alpinum (Scottish Laburnum) or hybrid, Laburnum X Watereri

In more northern reaches of the UK, this is the largest nitrogen fixing tree that you are likely to see. Well known Scottish forest gardens, for example, use these colourful yellow trees to provide a constant stream of cuttings which will distribute nitrogen around the whole system. Note, however, that while it can be useful to provide nitrogen to an edible garden, all parts of the tree are highly poisonous and should never be eaten.

Caragana arborescensSiberian Pea Tree (Caragana arborescens)

While it is non-native, this small nitrogen-fixing tree has proved useful for temperature climate forest gardens. It has an extensive root system and edible seeds (which should be cooked before they are eaten) which are said to resemble lentils. The flowers are also edible and can be used in salads.

For other examples of small trees/ shrubs which are nitrogen-fixers, please see the article on Nitrogen-fixing shrubs for the UK.