The impact of too much water on plants

By David Sprague

OSU master gardener volunteer

The forecast called for rain, and more rain. In June we had rain just about every day on top of already wet soil from April and May. For farmers, the impact of all this rain could be seen on their stunted cops. For the rest of us, we could see the result in our gardens and lawns. Some effects are still showing up and will continue to be recognized into the fall season.

All plants need water, of course, but too much can be as bad or worse than too little. Too much stops root growth, inhibits nutrient uptake, (especially nitrogen) and contributes to disease. Plant roots need oxygen to survive, and the roots find this oxygen in tiny spaces in the soil called macropores. When the soil is wet, these tiny spaces fill with water, thereby cutting off oxygen to the roots. If the soil stays wet for long periods, the roots stop growing and begin to die. In the garden, symptoms of too much rain may appear as wilted or yellow leaves; stunted growth; loss of flower, fruit and vegetable productivity; and even death of certain plants.

In the yard, the water stress may have thinned out or killed parts of the lawn. This loss makes room for opportunistic weeds to get a start. The stress also makes remaining grass plants more susceptible to disease.

The wet conditions this spring were perfect incubators for plant diseases, especially fungal pathogens. Fungi grow best in warm (70 to 90 degrees F) conditions with limited light and lots of moisture. These large populations of fungi will attack plants stressed by excessive rain. Bacterial and viral disease also can take advantage of these stressed plants. Underground, the wet roots of many plants succumb to root rot caused mainly by various types of fungi.

We can’t control how much it rains, but we can take steps to mitigate some of the damage from too much water. If your soil has dried out by now, you can fertilize your garden with a balanced fertilizer. This will replace nutrients washed out by the rains and help your plants recover from the stressful spring and summer.

Prune any dead or diseased parts of plants. Follow good cleanup practices this fall to remove plant parts where disease can overwinter. Repair or create ways to drain water away from gardens, flower beds or lawns. This could involve making new raised beds where plants were affected or where water was standing most of June and July. Keeping the soil from being water-logged is the best way to control root rot. For the lawn, wait until fall to fertilize. Almost all turf grasses found in our area are cool season grasses and are dormant during hot weather, making applications this time of year ineffective and a waste of your money. If you fertilized now, it would benefit the weeds more than the turf. Weed control in the yard is also best done in the fall and next spring, with the exception of controlling wild violets.

We need to remember this wet spring later this year and into next year. Larger plants, such as trees and shrubs, will take longer to show the effects of saturated soil created by June and July’s heavy rains and need to be monitored for problems until next summer.