How to revive waterlogged London gardens
Last year was the second wettest recorded in the past hundred years. This winter, as in spring and early summer, we have had heavy and persistent downpours. How has all this torrential rain affected our gardens?
"We will discover the answer in spring," says Matthew Wilson, managing director of Clifton Nurseries, author of Nature's Gardener: How to Garden in a Changing Climate and panellist on BBC Radio 4's Gardeners' Question Time, where the subject of waterlogged gardens has cropped up frequently.
"The wetter the soil, the more rapidly it affects the soil's capacity to hold water. When the ground is saturated, the soil becomes anaerobic, ie has no oxygen, so the roots drown and the plant dies. There are plants that are adapted to waterlogged soil, but most of our more popular garden shrubs and trees aren't equipped to cope."
The damage might not be apparent until the leaves are once more on the trees. "In 2000, I was curator of RHS Hyde Hall, Essex, and during what had then been recorded as the wettest autumn ever, we planted some fastigiate beech trees," says Wilson. "The following spring they all came into leaf, but just as quickly the leaves dropped off and the trees died. The stress of the trees coming into leaf accelerated their demise. When we dug them up, we discovered that the roots had died."
Wilson believes that Londoners might have to be prepared for some damage to their gardens, because the predominant soil type in our gardens is clay, which has poor drainage and holds water.
"When clay is wet, it stays wet — and unfortunately, beneath the surface layers, it tends to be anaerobic. In the past 12 months, the excessive rainfall has caused the anaerobic layer to rise up to a higher level, where it is more likely to affect the roots of plants."
Many of us, unsurprisingly, stayed indoors through the persistent downpours of late spring and early summer last year, and in high summer, either disillusioned or deckchair-bound, didn't catch up. "Last April, May and June we couldn't get into our gardens to plant or replenish our soil, and after that many Londoners gave up. So neglected gardens may pay the price this coming year," says Wilson.
True grit solution
With the only sure knowledge that the future will bring more weather extremes, Wilson says that the best line of defence is to improve the soil's drainage.
"Clay soil is nutritious and once it's in good shape, it's a great growing medium, so it's worth putting in the effort. Grit is the best way to break it up and plenty of organic matter will improve the structure as well."
The message is to put your efforts into the soil before you put in any new plants. "Also, mound your plants," advises Wilson. "Mounding is an ancient technique that fell out of fashion but it's worth doing. Every time you plant, build up the soil with grit and with soil improver, and more soil too, so you create a bump and the water drains away. If your beds are at a lower level than the lawn, the water will run off the lawn and into the beds, leaving the plants to sit in sodden, compacted soil."
Keep the lawn itself from waterlogging by aerating it — but keep off it when the ground is frozen or covered with snow, because your footfall can damage the soil, causing it to compact. "If any snow settles this winter, leave it to melt naturally."
You can never second-guess the weather, says Wilson. "In the 20 years I've been gardening, every climatic record has been broken. Just choose the right plant for the right place, read the labels and get expert advice when you go to the garden centre."