Poorly drained Soils
Last night’s book launch of Gabriella Pape’s new book Gebrauchsanweisung fürs Gärtnern was an idyllic event: over 70 people were sitting on the lawn under our beautiful Weymouth pine bathed in glorious evening sunlight by 22° C. One of the guests told me how much she enjoys reading my English blogs, as apparently, they read so much nicer. Thank you for this lovely compliment, it is the language in which I still feel most comfortable, especially when it comes to writing.
This same person suggested I should write about how to deal with waterlogged soil. They have been struggling with a wet heavy clay soil all winter. When they try to dig a hole it instantly fills with water that stinks, a sign of anaerobic conditions where oxygen is lacking, and ethylene and carbon dioxide can build up. This makes for difficult growing conditions and can lead to chlorosis (leaves turning yellow), root rot and eventually the plant’s death. Plants not only breathe through their leaves, they also do so via their roots. No doubt you have experienced a plant in a pot looking thirsty with wilted leaves. You urgently water it in order to rescue it. The next day it still looks wilted, so the plant gets more water. The day after it still looks sad, but by then the water is standing up to the brim of the pot and the plant has actually drowned.
Winter rainfall is important for nature, as water reserves have to be replenished for the coming summer months in lakes, rivers and in the soil. During summer large quantities of water will be transpired into the atmosphere by the leaves and needles of all our plants. Large, mature trees can consume well over 500 litres of water per day during the summer months, depending on size, type and of course weather conditions. As plants come back into growth again after their winter resting period, the water levels should reduce naturally in the soil, the only problem being that these cold, wet soils take longer to warm up, slowing the plants down in their development.
We cannot control the amount of rainfall that comes down each winter. Some years it is not enough, other years it seems to be too much, and especially heavy clay soils become rapidly saturated with water. But sandy soils can also suffer when for example heavy machinery has been used during building works causing compaction.
When soils are regularly waterlogged it is worth investing in a proper drainage system by digging narrow channels to make a herring-bone pattern, 2-3 metres apart, starting at about -20cm dropping to -60 to -80cm by the time they enter the central drainage channel. This should be a wider channel, ideally lined with fleece and filled with gravel that will take the water away to a lower point or a soak-away pit. There are special porous pipes that should be used for this process. It is recommendable to get a garden contractor to do this work for you. Keeping ditches clean so that excess water can drain off the land will also help.
Improving soil structure with compost
Heavy soils can also be improved by adding compost or other organic matter. A yearly application of 3 to 5cm of compost will help to lighten the soil structure. There is no need to dig this in. On the contrary, it will encourage the soil organisms that take care of this organic matter to aerate the soil and hereby improve its drainage capacity. You must be patient. This improvement process takes time, but the long-term rewards are well worth it.
Gardens that are low-lying or near a river can experience high watertable levels which cannot be altered or influenced. No amount of extra drainage will help if the water cannot drain away. In these gardens it pays off choosing trees and shrubs that are tolerant of waterlogging. Alternatively, plant the plants higher: do not dig a hole, but plant them on top of the soil, or only in a very shallow hole and earth up around the base to create a little mound. In the lowlands of Northern Belgium and The Netherlands fields are traditionally lined by drainage ditches and rows of pollarded willows that help to dry the land quicker so the soil warms faster enabling crops to be sown.
Plants that can cope with wet conditions
Some have adapted themselves to these conditions. A wonderful example is the Bald Cypress or Taxodium distichum, which produces aerial roots extensions called Pneumatophores that look like knobbly knees that stick up above ground or water level. Many other woody plants growing in swamp and mangrove regions produce similar aerial root systems. Other genera like Alder and Willow (Alnus and Salix species) can also cope with regularly occurring, prolonged wet conditions, as does the Sweet Gum Liquidambar Stiracyflua. The lovely red-stemmed Dogwoods, Cornus alba, Weigelas and also Hydrangea paniculata will equally cope with wet feet.
Impatience is your biggest enemy. You must wait for your soil to dry and warm up before you can plant and sow. This is little consolation when you are eager to get the season started, but it won’t go any faster, as the plants are just going to sit around and sulk, and seeds will not germinate till the conditions are to their liking. In the meantime they may be discovered by a hungry mouse or eagle-eyed bird and never get the chance to fulfil your expectations.