Waterlogging occurs when the soil is receiving more water than can leave it, either through drainage or evaporation. Even though around 90% of a plant is water, waterlogging is very damaging to most plants. The roots ‘drown’ when there is too much water in the soil.
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What happens to a plant when it is waterlogged?
Under normal conditions, plants create chemical energy for maintaining life functions through aerobic respiration. The root performs aerobic respiration by absorbing oxygen from the gaps between soil particles which is then turned into ATP (Adenosine Triphosphate), the primary carrier of energy in cells.
When water fills spaces between soil particles, oxygen is no longer available in the amounts that roots require. Root cells will then switch from aerobic respiration to a process called glycolysis to meet the plant’s energy needs. Glycolysis is only the first step in the aerobic respiration process. It does not require oxygen but it cannot generate ATP as efficiently as normal aerobic respiration. It only produces 37% of the ATP under aerobic respiration.
Because glycolysis produces less energy, the plant stops growing new root tissue. Shoot and root nutrient content decreases because root nutrient uptake from the soil is limited by its decreased energy. Stomata on the leaves close, reducing transpiration. Plant growth is stunted overall.
Some plants are adapted to waterlogging, such as species that grow in wetlands. Usually, such plants can survive waterlogging by forming aerenchyma. These are air spaces in the stems, roots or leaves. When shoots conduct aerobic respiration, they transport oxygen to the aerenchyma air pockets in the roots. This gives the roots their own reserve supply of oxygen. Rice is the only economic crop that grows optimally in waterlogged soils and this thanks to its ability to form aerenchyma. Other plants adapt to waterlogging is by keeping carbohydrate reserves in roots so that waterlogged roots can transfer starch, a source of energy, to the shoots. Another adaptation to water-logging is the ability to develop new roots on stems to make up for the shortfall in oxygen from roots.
What are the signs of waterlogging?
The symptoms of waterlogging are similar to symptoms of drought. If you have been watering your plant but it is wilting, drooping, yellowing, and has black or brown roots, the soil is likely waterlogged.
How to prevent waterlogging
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It is much easier to over-water than under-water your plants. Many plants – especially succulents and cacti - require that the soil is completely dry before you water. Other plants prefer to be watered more often, when the top few centimeters dry out. There are many apps available that can help you keep on top of your plant water schedule.
Remember that environmental conditions will affect how often the soil dries out. In the summer, a combination of heat and increased plant growth will mean more frequent watering is required. But even in the winters, dry, centrally heated homes can mean soil loses moisture quickly. The best way to check that the soil is completely dry is to stick a bamboo or chopstick into the soil. To check that the top few centimeters are dry, stick your finger into the soil.
The potting medium
Adding large particles like horticultural grit or perlite to soil will increase air pockets around the roots. Soil containing added grit or perlite allows water to flow through more quickly, letting the roots take up the water it needs without becoming saturated. Coconut coir is a good organic material that adds air pockets. Just make sure you take apart any clumps in the coir first.
You can check whether your potting medium is well-draining by filling a plant pot up with the mixture then placing it under a tap. If water immediately starts running through the drainage holes at a fast rate, you’ve got a well-draining potting mixture.
Very compacted soil makes it easier for the soil to become saturated because it eliminates air spaces between particles. Before planting, mix the soil thoroughly with a spade or other instrument to aerate it.
Many succulents and cacti do very well just in just horticultural grit or gravel. They need few of the minerals that are contained in loamy, organic soils and pure grit will keep its roots very dry.
If you have your pots in a saucer, always throw out any water that has collected in it.
If your plant prefers drier soil conditions, use a non-glazed terracotta pot. This material is porous and will dry out quicker.
How to fix waterlogging
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A plant showing signs of waterlogging should be removed from the soil. Rinse off the roots under running water and snip off very damaged areas of the root. Sprinkle the roots with cinnamon powder as it has anti-microbial properties to fend off root rot. If you have to remove a lot of the root system, snip back the plant’s leaves and shoots accordingly. A smaller plant can survive on less energy and water while the root system recovers. Repot the plant in a free-draining potting mix. Water very sparingly until the plant revives.