Why the right tree matters on a riverbank

Trees along a waterway are doing several jobs at once. Their roots knit the bank together, their canopies shade the channel, their leaves feed the insects that feed the fish, and their branches give kingfishers somewhere to sit while they wait. Get the species right and the whole stretch starts to look after itself. Get it wrong, or plant the same tree everywhere, and you can end up with a dark, starved channel and a bank that still slumps after every spate.

Britain has a small group of genuinely riverside natives — alder, willow and oak chief among them — and each one earns its place in a slightly different way.

Alder: the riverside workhorse

Alder (Alnus glutinosa) is the tree most of us picture when we think of a riverbank, and for good reason. It tolerates wet feet, survives being flooded for weeks at a time, and its roots run deep and wide enough to hold a bank through winter spates. It also fixes nitrogen through nodules on its roots, so it quietly improves the soil for whatever grows beside it.

Wildlife-wise, alder punches well above its weight. The catkins open early, offering pollen when little else is about, and the seeds — small cones that stay on the tree through winter — feed siskins and redpolls. Alder is also a favourite of the alder beetle and dozens of moths, and its roots shelter the kind of invertebrate life that brown trout and juvenile salmon depend on.

Plant alder where the bank is wet, exposed or newly repaired. It grows quickly, copes with wind, and looks well coppiced if you need to keep it lower.

Willow: fast, flexible and full of life

Willows are the great improvisers of the waterside. Crack willow, white willow, osier and goat willow all belong here, and they will root from a cutting pushed into damp ground, which makes them wonderfully cheap and forgiving to establish.

• Osier and common willow suit wet hollows and can be coppiced on a short rotation for stakes, hurdles and woven bank revetment.
• White and crack willow grow into big, arching trees that shade the water and drop branches into the channel, creating cover for fish and perches for birds.
• Goat willow is smaller and happier on drier margins, and its early catkins are a lifeline for queen bumblebees coming out of hibernation.

Willow is brilliant but greedy. Plant it where you want quick cover, not where you want a long-lived, tidy tree, and expect to manage it. A willow left to its own devices can lean right across a small river within twenty years.

Oak and the wider mix

Pedunculate oak (Quercus robur) is the slowest of the three, but it is the one that will still be standing in a hundred years. Oak supports more invertebrate species than any other native tree in Britain — hundreds of them — and a mature oak leaning over a river is a small ecosystem in itself, shedding leaf litter that shrimp, caddis larvae and stoneflies shred into fish food.

Oak wants drier ground than alder or willow, so set it back from the water's edge on the upper bank or the floodplain terrace. Fill the gaps around your main trees with a working understorey: hawthorn and blackthorn for nesting birds and blossom, hazel for coppice and nuts, guelder rose and dogwood for colour and cover. A mixed bank is a resilient bank — if one species suffers, the others carry on.

Reading the bank before you plant

Spend an hour walking your stretch before you buy a single tree. Look at where the bank is undercut, where the water slows, where gravel shows through, and where the sun already reaches the water.

• Sunlight matters. Aim for dappled shade over roughly half the channel. Too little shade and the water warms; too much and in-channel plants and insects disappear.
• Keep spawning gravel clear. Don't plant a dense block directly over clean gravel riffles — silt and shade both harm it.
• Check permissions first. On a main river, a Site of Special Scientific Interest or land with flood defence structures, speak to your local flood or environmental regulator before you dig.
• Work with the flow. Plant on the inside of bends and along eroding banks; leave the natural deposition zones to do their job.

Getting them through the first few years

Bare-root whips of 60–90cm planted between November and March are far cheaper and establish better than potted stock. Plant when the ground isn't frozen or standing in water, and firm the soil well around the roots.

Space alder and willow at 2–3 metres if you want quick cover, and oaks at 5–6 metres to give them room. Mulch with bark chip or a woven mat, keeping it clear of the trunk, and fit a spiral guard against rabbits and voles — it is the single best thing you can do for survival rates. Water newly planted stock through dry spells in the first two summers.

Then be patient and watch. Within three years you should see more insects over the water, more birds feeding along the edge, and a bank that holds a little firmer after heavy rain. That is the quiet reward of riverside planting: you put in a few hundred whips, and the river does the rest.