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While trees have a greater tolerance to flooding, other garden plants may not do so well if under water for prolonged amounts of time.
As hurricane season gears up, urban gardeners need to protect their vegetables and flowers from heavy rains. Flooding can devastate urban areas and cause a lot of damage to gardens and the landscape.
“A single day of flooding may take many months to repair,” says Jeff Rugg, a horticulturist from the University of Illinois Cooperative Extension. “Flooding in the man-made landscape mimics flooding in the natural landscape, where land that is under water regularly has little vegetation and almost no debris.”
Gardens that experience flooding tend to turn into mud flats that easily erode because most plants don’t survive under water, according to Rugg.
Other materials that prevent erosion, like mulch, are washed up to the high-water mark, so gardeners who experience flooding in their garden will need to replace their mulch.
“If mulch is washed away, it’s a good idea to replace it so the flower beds don’t dry out in the hot weather after the storms and to help prevent weed growth,” he says.
Flood waters will also cause damage to plants.
“Fast-moving water smashes down plants and breaks or cracks their stems and branches, so they cannot straighten up. This is especially true of plants without wooden stems,” Rugg says.
Annual flowers and vegetable crops that are ruined in floods may need to be replaced, Rugg recommends. Perennial plants can be propped up to facilitate growth for the following year, and shrubs and trees can be staked until they regain strength.
When the top of a plant is under water, the plant has a hard time maintaining the proper level of moisture within its leaves and stems. When the plant eventually dries out, some leaves may die off. The longer the plant was under water the worse the damage will be.
Mud that coats the leaves will reduce the plant’s ability to photosynthesize, so wash off any mud-coated leaves.
Soil must have oxygen in it for most terrestrial plant roots to survive. Waterlogged soil doesn’t have enough available oxygen for many dry-land plants. For some big trees, the damage may not appear for several weeks or months. Plants that are native to stream banks and near lakeshores can tolerate low levels of oxygen in the soil. Some maples are especially good at tolerating periodic flooding.
“Because of the way we change the topography of the land, many plants have been placed in areas where they can be harmed by flooding,” he said. “The longer the water is over the roots, the more potential there is for damage.”
Most trees and shrubs will survive under surface water for a few days. However, one to two weeks of water covering the plants’ roots could cause stress, leading to problems in the plant the following year. Longer-standing surface water can kill flood-intolerant plants.
As water is released from retention ponds, water levels may remain artificially high. Trees around the edges of retention ponds should be flood tolerant, but Rugg has seen many instances where this was not the case.
“Overall, there will not be a long-lasting effect on the landscape from localized flooding if it doesn’t last more than a few days and is a rare occurrence,” says Rugg. “Unfortunately, some areas are flooded longer and more often because of land upstream that is being changed from rural to urban. If there is more flooding to come to an area, especially in the few weeks after a previous flood, watch out for drowned plants.”
A simple rule of thumb to remember is that if a plant doesn’t normally grow on the edge of a lake or in a swamp, it won’t do well in a flood. The longer a plant is under water the worse it will do. Slow growers or weakened plants will have a harder time recovering from flood damage.