Using plants with a purpose

One of the more interesting challenges facing natural gardeners is taking our good, basic principles - such as the use of nitrogen fixation and harnessing natures balance of pests and predators - and feeding these in to the complex and diverse landscapes that increasingly give greater hope for a more sustainable future. Geoff Hamilton saw the advantage of this complexity in his rejuvenation of the ideas of the kitchen and cottage gardens, and permaculturists with their plant guilds and forest edge gardens are also part of this trend. From the gardening world, we see Noel Kingsbury bringing in the complex mix of perennials reminiscent of North American prairies and the continental drift styles of planting, and Peter Thompson describes his combining of many plants to form self-sustaining matrices.

What can also be inspirational are the new urban hedgerows – those mixtures of shrubs and trees that line inner city highways and their central reservations, and edge pedestrian walkways and car parks. Often these shine with blossom and then berries, and their foliage glows autumnally, showing that they have been chosen by landscape architects for appearance as much as survival. These plantings, with their rowan, hazel, birch, buddleia, willow and dog wood; and their dog roses, guelder rose, hawthorn and June Berry (Amelanchier ssp) offer a haven to the natural world which is often lacking in urban areas. These planting schemes also contain another range of plant families that while they are all different, have something very much in common - and that is nitrogen fixation.

I’m not talking here about the odd laburnum or broom that may find its way into these planting schemes, but the shrubs and trees that have a symbiotic association with a different bacteria, called Frankia, and which form no part of the LEGUMINOSAE. Probably the most common example of these are the alders (Alnus, BETULACEAE) long known for growing in any situation, including damp conditions, and making very effective windbreaks while delighting us with cones and catkins. A shrub in these urban plantings that has resistance to maritime exposure is the spikey sea buckthorn (Hippophae rhamnoides, ELAEAGNACEAE) which when planted with both sexes will produce orange berries in autumn. They are acrid in taste.

Others used in the same family also fix nitrogen and are also known for use in shelter belts, but have fruits more palatable for human consumption. These are the deciduous elaeagnus (E. angustifolia and E. umbelata, ELAEAGNACEAE) which flower fragrantly in June and the fruit is a refreshing delight in autumn. Their deciduous cousin, the Buffalo Berry (Shepherdia argentea, ELAEAGNACEAE) flowers earlier, in March, and needs both sexes to fruit. The evergreen elaeagnus (E. x ebbingei or E. pungens, ELAEAGNACEAE) has all-season use in shelter belts and can tolerate some shade, but the very fragrant flowering in autumn is too late in Northern climes for fruit production to occur by the following spring – stick to the deciduous!

The California Lilac (Ceanothus, RHAMNACEAE) is another member of these nitrogen-fixing plantings used in urban schemes, but in my Northern clime it is the low-growing (C. thyrsiflorus repens) or mounded (C. ‘Blue Mound’) varieties used in shrub borders or on banks that survive through the winter and reward with fragrant blue flowers in early summer. Occasionally, you may also see another of these nitrogen fixers, the carpet forming Mountain Avens (Dryas octopetala, ROSACEAE) growing over rock features with its white, yellow-centred flowers – like little dog roses – covering the carpet in June. If there were bogs in the centres of cities, we would have the perfect nitrogen fixing shrub to plant there too - Bog Myrtle (Myrica gale, MYRICACEAE). This wonderfully aromatic shrub thrives in acidic boggy swamps and the leaves can be used for making gale beer and a smelly midge repellent.

It has been fascinating for me to learn that many of the plants which I knew could be used for a purpose, also have this extra property of nitrogen fixation adding to their value. Two more families are known to have this association with Frankia - the CORIARIACEAE and CASUARINACEAE -  and there are good reasons to believe that others will eventually turn up, quite often through observing the presence of nitrogen-fixing root nodules when the plants are dug up! In propagating some of these known species – alder from seed and the deciduous elaeagnus and sea buckthorn from hardwood heel cuttings – I have found that the nodulation appears to occur quite readily without the need for inoculants. These Frankia  nodules are much more persistent and robust than those found with rhizobium in the LEGUMINOSAE, and it is likely that they may contribute more in the way of nitrogen fertility. Either way, as I design and plant more schemes to include greater diversity and purpose, I find myself adding these Frankia symbionts and revelling in their uses.

Mark Fisher, 8th April 1998

For further reading, the Agroforestry Research Trust (46 Hunters Moon, Dartington, Totnes, Devon, TQ9 6JT) publish a directory of nitrogen-fixing plants for temperate climates which includes the non-leguminous plants.