Too Much Water Can Be Harmful to Plants

As the summer sun arrives full strength, watering becomes a primary concern to Los Angeles gardeners. This is the first summer in many years without drought-imposed restrictions on watering. Yet, too much water can be just as bad for plants as too little.
The majority of Los Angeles back yards are plagued with clay soil that does not drain well. After all, the ground upon which our city stands was once the ocean floor. The earth we valiantly try to cultivate is actually a layer of ancient marine sediment or compacted muck, not the best soil for growing plants.
Some of the most commonly grown plants in our area – gazania, iceplant and zonal geranium, as well as many California native plants – are extremely sensitive to overwatering during warm weather. These plants go dormant during the summer, the same time of year when lethal water molds (soil fungi) are most active.
When you have disease-causing soil fungi in the root zone of overwatered,
dormant plants, you are asking for trouble. The roots of dormant plants are not actively growing or taking up water; this results in water standing around roots and, in warm weather, the rapid growth of pathogenic fungi that eventually enter the roots and kill the plant.
Rosemary, for example, is extremely sensitive in this respect. In the hottest weather, it should not be watered more than two or three times a month. It simply cannot survive in soil where any excess moisture exists.
Because of the extremely rainy season we experienced this year, the soil remained wet, throughout the root zone, for an unusually long time. As a result, gardeners all over town saw plantings of rosemary die where soil drainage was inadequate.
Planting in the summer can be a precarious enterprise, especially with summer-dormant plants or plants susceptible to soil fungus attack, such as annual periwinkle and such California natives as ceanothus and manzanita. In general, plants should be watered every day for the first 10 days after they are planted in summer months.
However, in hot weather, the daily watering schedule that is critical to establishing a young plant is also an encouragement to fungus attack.
If you insist on planting fungus-sensitive plants, such as petunias, this time of the year, it would make more sense to plant them in a large container, filled with a sterile planting mix. Such a mix might contain equal parts of perlite, vermiculte and peat moss, all available at any nursery. Or, you might consider planting in a hanging wire basket filled with coarse, green sphagnum. Pathogenic soil fungi are less likely to be a problem when plants are grown in a soil-less environment.

originally published 7/24/93

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