It happened – I turned on the hose, walked away, and forgot about it. Now the soil is squishy, with puddles of water around my newly planted shrub. In horticulture terms, we’d say the soil is waterlogged. In gardener’s terms, we say the plant is drowning. ‘Drowning’ is actually the more appropriate term in this case.
Plants have two processes to make energy for themselves and to grow: photosynthesis and respiration. Most of us learned about photosynthesis in school – how the green chlorophyll in the leaves converts sunlight and water into sugar. Respiration then combines that sugar, or glucose, with oxygen which creates the chemical adenosine triphosphate (ATP). ATP is essential for all kinds of cellular functions including DNA replication, protein synthesis, and ion movement.
Respiration happens throughout the plant: in the leaves, stems, and the roots. The respiration that occurs in the roots facilitates nutrient uptake from the soil. Through photosynthesis, the plant sends sugars down to the roots. Through root respiration, the plant sends nutrients up to the leaves.
In healthy soil, both air and water are held between the particles of soil in spaces called pores. When the soil is water-logged, all of the pore space is taken up by water molecules. Without air in the soil, there is reduced water permeability, which means the water can not move across the cell membranes into the roots. Often the leaves wilt. Remember the guard cells controlling the stoma openings from last month’s blog post? When roots can not bring water in, the guard cells become flaccid and the leaves wilt.
Waterlogged soil creates all kinds of problems: anaerobic microbial activity (which smells bad), increases in fungi, and an increased threat of pathogens. When a plant is waterlogged for too long, it can get root rot and die. In the case of leaving the hose on for too long, I can just leave it to dry out for a while and the plant will likely recover. In the case of a container plant or houseplant that is waterlogged, I would consider removing the plant from the pot and giving it fresh soil. In the case of an entire field being flooded by Mother Nature, not much can be done besides wait for it to dry out.
Some plants are able to process the oxygen in the water for root respiration, provided the water is well oxygenated. Hydroponics is a method of growing plants entirely in water with soluble nutrients. Some plants are adapted to growing entirely in water (such as Water Lilies) or in areas where the water table is very high (such as Mangrove).
As you can see, overwatering is bad for the plant. It’s bad for the water bill, too, and I feel bad about leaving the hose running. Next time, I’ll set a timer on my phone!
