All plants except aquatics (Water Lilies), marginals (Flag Iris) or water tolerant plants (Hornbeam or Willow) will succumb to waterlogging…
Waterlogging threatens plants in two stages:
Stage 1 – Drowning
When roots are waterlogged, they can no longer do their job. The excessive water restricts (or even cuts off) the oxygen supply to the roots (waterlogged soil has no air pockets) and prevents their diffusion of carbon dioxide. Consequently, and this sounds counter-intuitive, the roots cannot draw water and nutrients from the soil. The plant is effectively drowning and, above ground, will suffer from lack of water.
Incidentally, waterlogging occurs when water collects, does not drain away and the ground becomes saturated. The short-term increase in water levels from, say, a heavy downpour will not cause waterlogging if the ground has been properly prepared and the water can – and does – drain away.
Stage 2 – Rotting
Waterlogging does not kill plants instantaneously and there is a short window when affected plants can be rescued and restored. But, left unchecked, the roots will start to rot and once rot and opportunistic disease have taken hold, the plant is probably beyond redemption.
Plants in containers are more likely to suffer from a lack of water than waterlogging – but drainage holes can become blocked. Therefore, should a container plant display the symptoms detailed below, check to see if the problem is too much or too little water. If the container is too large or heavy to check the drainage holes, a simple inspection of the compost will tell you.
The very first symptoms of waterlogging are similar (as explained above) to those of under-watering:
▫ Leaves show yellowing or decay between the veins; the base and/or centre of the leaves soften and parts of the leaf may turn brown. This last symptom is particularly prevalent with evergreens
As waterlogging takes hold, further symptoms appear:
▫ Shoots die back
▫ Stunted growth
The final symptom is rotting roots but, frankly, if the problem has been left unchecked this long, it is probably too late.
Waterlogging can occur throughout the year but has a far greater impact during the growing seasons of spring and summer when plants need more water and nutrients.
Greater vigilance is required in autumn and winter as early symptoms will be less evident – especially on deciduous and semi-evergreen plants like Beech.
Plants – be they hedges or specimens – in level, free-draining soil, well away from ponds, stream or rivers are unlikely to fall victim to waterlogging unless there is cataclysmic flooding like we experienced a few years ago.
If, though, you are planting in a dip, on heavy (clay) soil or near to water that may overflow a few simple preventative measures will offer added protection that are well worth the effort:
▫ Improve drainage by working grit and organic matter into the planting area. If you know your planting area is especially prone, dig a good 50% deeper (and even wider) than recommended and fill the additional space with more grit and organic matter
▫ Ensure that holes dug in heavy soil do not have smooth, flattened sides as this will turn the hole into a collecting vessel. Work the sides of the hole with a garden fork to ensure they are rough – this will improve drainage
If your garden is at extreme risk of flooding, then extra measure can be taken:
▫ Consider raising the planting area by creating a mound or ridge – specimen plants can be planted in raised beds
▫ Ensure any paths or driveways are permeable – don’t even think about concrete, tarmac or paving slabs
▫ Dig a ditch or a pond at the lowest part of the garden so water has somewhere to go
▫ Choose hedging plants best suited to wet conditions e.g. Hornbeam (Carpinus betulus), or Common Box (Buxus sempervirens). Cherry Laurel (Prunus laurocerasus ‘Rotundifolia‘) will tolerate wet soil, but not prolonged flooding. Click here for a previous post that explores this further.
AFTER PROLONGED RAIN OR FLOODING
If your garden has experienced a period of greater than usual rainfall or temporary flooding, here are a few tips to help your plants get back to health:
▫ Avoid compacting the wet soil as this will worsen conditions and delay recovery – tread lightly, if at all
▫ Check the affected plants and remove any damaged shoots
▫ Apply a general-purpose, well-balanced fertiliser in the following spring and apply a mulch
▫ Apply a foliar feed once the next growing season starts if the plant’s leaves are paler than they should be. This will also help encourage root growth
▫ Water thoroughly during any subsequent dry periods as plants water-stressed plants are more prone to drought stress
NB: If you are unlucky enough to have suffered cataclysmic flooding, follow the health and safety advice about clearing up that will be provided by your insurers and local authorities.