Growing plants in wet, poorly drained soil can be quite difficult. To the extreme, very heavy rainfall followed by flooding can not only cause tremendous damage to buildings and homes, but also can kill woody and herbaceous plants, while other plants remain unaffected. The question is often raised, “How long can my plants tolerate their roots being submerged? It would depend on the time of year the flood event occurs, duration of the flood event, species sensitivity to flooding, and type of soil the plants are growing in. Dormant plants are more tolerant than actively growing plants to flooding. Most plants can tolerate a couple of days of flooding during the growing season, but for some plants, a week or more of flooding can cause severe injury and death, particularly for sensitive tree and shrub species like: lindens, Norway and sugar maples, beech, northern red, white, and chinkapin oaks, hickories, black walnut, black locust, buckeyes, tuliptree, white-barked birches, American hophornbeam, Siberian elm, mulberry, yellowwood, cherries and plums, eastern redbud, magnolias, crabapples, mountainash, Washington hawthorn, lilacs, rhododendrons, privets, cotoneaster, spirea, euonymus, daphne, weigela, and evergreens like pines, Norway and Colorado blue spruces, Canadian hemlock, eastern red-cedar, Siberian cypress, yucca, and yews. Soil type is also an important factor to keep in mind with regard to drainage patterns. Sandy soil drains much faster than predominantly clay-based soils, which remain wet for longer durations.
Are there differences in a plant’s ability to tolerate flooding? Established, healthy trees and shrubs will be more tolerant to flooding than very old trees, stressed trees, or young trees and seedlings of the same species. Symptoms of plants under excessive water stress include yellowing or browning of leaves, leaf curling and pointing downward, leaf wilting, reduced new leaf size, early fall color, defoliation, branch dieback and in extreme cases, gradual plant decline and death over the next couple of years. There are plants that can recover from flooding injury in as little as one growing season while others do not recover at all. However, these stressed trees are more susceptible to secondary organisms such as cankers fungi and wood boring insects. Trees that had a substantial amount of root injury and death are more subject to wind throw and should be monitored closely or removed entirely.
What causes plants to die in water-soaked soils? Besides the obvious killing of submerged branches and foliage, many plants are intolerant to having their roots submerged for long periods of time. Excessive moisture in the soil causes oxygen levels in the soil to decrease, impeding proper root respiration. As a result, carbon dioxide, methane, hydrogen and nitrogen gas levels around the roots increase sharply, thus, roots can suffocate and die. Toxic compounds, such as ethanol and hydrogen sulfide, as well as numerous other harmful compounds, can build up in saturated soils. Photosynthesis is inhibited and growth slows or even stops. Excessively wet soils also favor soil-borne, root and crown rot organisms including Fusarium spp., Phytophthora spp., Pythium spp., and Rhizoctonia solani. These organisms have wide host ranges and prefer wet soil conditions. Even when standing water is not present, poorly drained soil can reduce plant growth and long-term survival in the landscape.
Another thing to be concerned about is the deposition of excess soil and rocks over tree roots following floodwater recession. Excess soil greater than 3” may impede oxygen transport from the atmosphere to tree and shrub roots, especially on smaller growing plants. This excess sediment should be removed after the water recedes. In contrast, tree roots may also become exposed due to soil erosion following flooding. These roots should be covered with soil to prevent drying out and damage of exposed roots. Improving drainage and aeration is critical to prevent future root injury. Finally, tree fertilization is not a cure for root injury and can make the problem worse.
How can I alleviate poorly drained soils in the future? If possible, avoid planting in areas that drain slowly after rain or are flooded consistently after a very heavy rainfall. The next step is to improve the site’s drainage. Addition of loose organic material, such as composted leaves, pine bark, and peat moss can improve porosity in the soil. Plant on raised beds or berms, install swales, waterways, and drain tiles to divert excess water away from trees and shrubs. Finally, plant health care is an important step to reduce further plant decline. Remove dead or diseased branches, water plants during extended droughts, aerate the soil around the tree’s roots, and properly mulch trees with no more than 3-4” of shredded bark, avoiding mulch from touching the bark on the trunk and branches, all key to improving plant vigor.
If your soil is subject to standing water after a heavy rainfall, it is best to plant species that are tolerant to wet soils. Bottomland plants, plants that naturally grow in lowland areas along riverbanks subject to fluctuating water tables, are able to tolerate wet soils better than upland species that grow at higher elevations. Also, different plants tolerate different degrees of wetness. Is your area permanently wet, somewhat wet, or wet for only a few days at a time? Roots need oxygen for growth and respiration and the longer the roots stay submerged, the more difficult it is for the plant to survive.
Originally Posted on: uwex.edu By: Laura G. Jull