How to revive your trees and plants after a flood

If your garden was badly affected by the recent floods, don’t despair. Some plants and trees will survive – if you give them a helping hand

Click to follow
The Independent Online

I started married life on a Thames sailing barge, moored on the banks of the river at Shepperton. She was a magnificent boat, 80ft long, 18ft wide, planked out in wood and complete with all her rigging. The mooring was a fine place generally, though not so handy when it was raining and there was half a mile of footpath between you and the nearest road.

As the level of the river rose, so did the angle of the gangplank that connected boat to bank. The plank was just that – no handrail – and in the last stages of expecting our first baby, I did it on hands and knees. "Very good," said the health visitor, who, one day, followed me on board in the same fashion. "Full marks," I said as, equally efficiently, she negotiated the companionway down from the deck.

Our first 'garden' was on the river bank, a sheaf of wild yellow flag iris which my mother brought from my home in Wales, along with primroses to go by the towpath. But that all disappeared during our first winter on the boat. The river in spate rose higher than we had ever imagined, and the swirling water swept away all our efforts at gardening. The boat was moored bow upstream, and the noise up there in the forecastle was terrifying – not just the force of the water itself, but the tremendous booms and bashes that reverberated through the boat as a tree trunk or some other debris hit the hull. But we never went through anything as bad as the storms and floods that have marked this year.

The worst thing in a flooded garden is the mess that the flood leaves behind. Whether the plants themselves survive will depend on how long they have been submerged. And on the kind of soil they are growing in. Clay soils drain more slowly than sandy or chalky ones. If the soil is waterlogged for a long period, roots can't get the oxygen they need and the plant quite literally drowns.

In a 'good' soil, half the space is taken up by air. This may be replaced, temporarily, by rainwater moving through the soil, but generally, the air returns when the water's gone. In a prolonged flood, it can't. Microbes in the soil suffer as well as plants, because they also need oxygen. Without it, they start producing hydrogen sulphide, which is why waterlogged soil smells so awful.

Trees and shrubs are likely to suffer more than perennial plants. Their roots aren't so quick to make new growth and they may die before they have replaced what they need. You can help them recover by pruning back top growth so the roots have less to do.

It will be easier to coax a shrub through this process than a tree, especially a tree that has been rocked backwards and forwards by the wind, further loosening the grip its roots have on the surrounding soil. But again, the best policy is to wait and see. Trees such as willow, some dogwoods, alder, birch, elder and amelanchier have evolved to take floods in their stride. Others, particularly fruit trees, will not be so forgiving.

You will not have to wait so long to find out if herbaceous plants have survived. As spring progresses, they may not come into growth at all. Or will produce shoots, but then die back as they realise they haven't enough live root to pump up the resources they need to sustain growth.

If a plant is particularly precious, you could try digging it up, washing the roots and re-planting it in ground that is better drained. But generally, with herbaceous stuff which is usually cheap to replace, I'd cut my losses, get rid of the dead plants and take the opportunity to improve the soil, before replanting. Improve, in this context, means adding grit to make the drainage better, and compost.

In the south-west of England, where we are, attention has focused mostly on the flooding of the Somerset Levels and the battering of the Devon and Cornish coasts. But in our particular bit of Dorset, terrifyingly tempestuous winds caused more damage than the rain. We lost far more trees than in the storm of 1987. All the lanes round us were blocked at one time or another by fallen monsters. Unfortunately, many of them were oaks, notable for supporting more wildlife than any other kind of tree.

There's nothing you can do to save a big tree that's split its trunk. But if a smallish tree has blown right out of the ground, root plate and all, you can, after making the crater a bit bigger, tip the whole thing back into place, firm it round with new soil and stake it. As gardeners discovered after the 1987 storm, it may well survive. Much depends on how much of the root plate remains intact. And whether you remember to water it during this coming summer.

For us, many gales came in from the south-west. In our garden, it's the most dangerous direction. On the bank, the Irish yews we planted were pushed right over. They've been in eight years, and had grown to more than two metres, without ever needing stakes. Now we're having to haul them upright, with short stakes low down bashed in at an angle. Their roots will have been loosened underground, so if the soil dries out, they'll have to be watered this summer.

Evergreens had a particularly hard time, as they offered more resistance to the wind. Our young evergreen oak (Quercus ilex) blew clean out of the ground, even though it was staked in the strongest fashion. We replanted it, but first, reduced its top hamper, cutting out two thirds of the growth. That was hard to do – it had such a beautiful crown – but it was the only way we could equip it to cope with such a drastic shock.

Where a tree is still standing but has lost most of its branches, you could consider either coppicing or pollarding. Pollarding has got a bad name because of the hideous 'lopping and topping' to which urban trees are subjected by local authorities. But properly carried out on suitable species (willow, ash, eucalyptus or poplar for instance), it looks good and you may feel that reshaping your tree is less traumatic than losing it altogether. You need to cut down the trunk to about two metres and wait for new shoots to sprout. After that, take out a third of the stems each year to promote fresh growth and keep the shaving-brush effect.