Aquatic Plant Physiology and Over Watering

water lily hybrid (Nymphaea 'Colorado')

water lily hybrid (Nymphaea ‘Colorado’)

My neighbors’ sprinklers throw water into the street and down into the rain gutters.
Isn’t there an ordinance against this — and don’t we water too much anyway? Please explain why overwatering is not good for plants. I think we should plant more drought resistant shrubs and trees like ceanothus, with its beautiful blue flowers and spreading tendency.
— Bette Simons,
Sherman Oaks
To the best of my knowledge, Santa Monica is the only municipality in the Los Angeles area where you can be fined if your sprinklers overspray your landscape and water sidewalk and street instead of plants. However, many ordinances discouraging wasteful irrigation practices have been instituted throughout the United States in recent years, especially in Western cities, and we are bound to see more of them.
Overwatering is not good for plants because of roots’ need for oxygen, which is no less critical than that of animals or humans.
When the micropores between soil particles are constantly full of water, oxygen cannot get through, and roots suffocate. Even before suffocation occurs, water molds can develop in constantly wet soil and kill plants.
Two questions invariably arise whenever the subject of overwatering and its danger to plants are discussed. First, if plant roots need oxygen, how do you explain the phenomenon of aquatic plants? Second, if overwatering is not good for plants, how come plants can be heavily soaked during the rainy season without suffering any ill effects?
The physiology of aquatic plants, which have highly fibrous roots designed to absorb the limited oxygen present in water, differs from that of land plants. All plants have stomata or leaf pores. These pores open up in order to take in carbon dioxide, from which plants make carbohydrate, the energy source that sustains them. In land plants, however, these pores open and close. If their leaf pores were open all the time, land plants would quickly dehydrate and die, as all of the water entering through their roots would go straight up and out into the atmosphere. In water plants, however, leaf pores are constantly open, since there is no danger of dehydration. Unlike land plants, water plants also have specialized cells that transport oxygen that enters their leaves down to their roots.
Thus aquatic plants are not only flush with oxygen but are purposely placed in ponds as oxygenators, to the benefit of fish and pond health in general, since they expel oxygen through their roots.
In our climate, overwatering is more of a problem during the warm growing season than in the winter, when it rains.  In the warm season, lethal soil fungi come to life when there is excess water in the soil.  In the winter, these fungi are dormant.
I certainly agree with your opinion about ceanthous, the California lilac. No garden should be without its fragrant spikes of baby- to deep-royal-blue flowers. The Chumash Indians utilized ceanothus flowers as a soap, and you will see why when you rub the soft inflorescences between your palms. Any day now, you will see arboreal ceanothus specimens begin to bloom along Mulholland Drive, as well as in the Sepulveda Pass.
These flower mostly in white — and occasionally in pale blue. As you get closer to the coast, ceanothus tends to get shrubbier until it nearly flattens out in ground-cover form.
After their first year in the garden, ceanothus requires no water other than winter rain.
APPLES AND CUKES: Responding to last week’s column on fruit trees for the Valley, Terri Taylor recommended the crisp apple variety known as `Fuji,’ which has produced nicely for her. She also enthused about the favorable Valley soil and climate for cultivation of pickling cucumbers. From a few plants, she harvested more than 700 of these specialty cucumbers last summer.

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