Effects of Flooding on Woody Landscape Plants

Overview

The question often asked following a flood is, “How long will my plants survive with their roots under water?” The answer depends on a variety of factors including time of year, how long the roots remain submerged, soil type, plant species, and age and health of the plant.

Some plants can recover from flooding injury in as little as one growing season; others may never recover. Plants that survive a flood should be given special care to help them recover.

Will my plants survive?

The likelihood of woody landscape plants surviving a flood depends on a variety of factors:

• Time of year. Dormant plants are more tolerant of flooding than are actively growing plants.
• How long roots remain submerged. Most plants can tolerate a few days of flooding during the growing season. Some can survive a week or more of standing water (some species can survive months of standing water).
• Sensitivity of the species to excess water. For sensitive tree and shrub species (listed in table 1), anything more than a few days can start to cause severe injury and death.
• Soil type. Sandy soils drain much faster than predominantly clay-based soils, which hold water and remain wet for longer periods.
• Age and health of plant. Established, healthy trees and shrubs will be more tolerant of flooding than very old, stressed, or young plants of the same species.

Symptoms of flood stress

Besides the obvious damage to submerged branches and foliage, few plants can tolerate having their roots submerged for long periods of time. (Refer to tables 1 and 2 for plants likely to be injured by flooding and those that can tolerate wet soils.)

Symptoms of plants experiencing flooding stress include yellowing or browning of leaves, leaves curling and pointing downward, leaf wilt and drop, reduced size of new leaves, early fall color, branch dieback, formation of sprouts along stems or trunk, and, in extreme cases, gradual plant decline and death.

Roots need oxygen for growth and respiration, so the longer they’re submerged, the more difficult it is for the roots—and plant—to survive. Trees that have suffered a substantial amount of root injury are prone to “windthrow” (being uprooted in heavy wind) and should be monitored closely or removed. Trees that begin to lean severely should be removed as soon as possible.

Stressed trees are more susceptible to secondary organisms such as canker fungi and insects that bore into phloem and wood.

Why do plants die in water-soaked soils?

Roots need oxygen for growth and respiration. Flooding reduces the amount of oxygen in the soil, impeding root respiration. As a result, carbon dioxide, methane, hydrogen and nitrogen gas levels around the roots increase sharply. The roots then suffocate and die. Toxic compounds, such as ethanol and hydrogen sulfide, can build up in saturated soils. As root function becomes impaired, photosynthesis in the leaves 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 affect many species and prefer wet soil conditions.

Even when standing water is not present, poorly drained soils can reduce plant growth and long-term survival in the landscape due to low oxygen levels in the soil.

Caring for flood-stressed plants

Immediate plant care needs

Once floodwaters have receded, inspect the soil around your plants. Flooding may have deposited significant amounts of new soil and rocks over plant roots—or it may have eroded soil, leaving roots exposed. If more than 3 inches of debris has been deposited, remove it to allow oxygen movement from the atmosphere to tree and shrub roots. Conversely, carefully cover any exposed roots with soil back to the original depth to prevent drying or damage.

Long-term plant care strategy

Plant health care is an important component to improve plant vigor and further reduce plant decline.

• Remove any dead, damaged, or diseased branches.
• Water plants thoroughly (1 inch per week) during extended droughts.
• Aerate the soil around the tree’s roots using vertical or radial mulching (should be performed by a professional arborist to protect roots).
• Properly mulch trees. Spread 2–4 inches of shredded bark (not wood chips) in a donut shape around the base of your plants, keeping mulch away from the trunk and off any low-growing branches.
• Do not fertilize trees and shrubs for at least a year following a flood due to root injury. Nutrient uptake is an energy-requiring process that will further stress the plant, especially during times of flooding (and drought).

Modifying the landscape

If water routinely stands in an area following heavy rains, you may wish to consider improving drainage and aeration, and replanting the area with plants that are able to tolerate wet conditions.

Before making any changes, evaluate the drainage situation. Is your area permanently wet, somewhat wet, or wet for only a few days at a time? For areas that are permanently wet, you may be best off planting species that are tolerant of wet soils.

Bottomland 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. Woody species that can tolerate wet soils are listed in table 2.

In areas prone to staying wet for only a few days at a time, you can improve soil porosity before planting by adding loose organic material, such as composted leaves, pine bark, and peat moss.

Other options for improving drainage include planting on raised beds or berms, and installing swales, waterways, and drain tiles to divert excess water away from trees and shrubs.

Plants likely to be injured by flooding

Table 1

Common name(s)	Botanical name
Norway maple	Acer platanoides
Sugar maple	Acer saccharum
Buckeyes, horsechestnuts	Aesculus spp.
Paper birch, canoe birch	Betula papyrifera
Whitespire Senior gray birch	Betula populifolia ‘Whitespire Senior’
Hickories	Carya spp.
Eastern redbud	Cercis canadensis
American yellowwood	Cladrastis kentukea
Cotoneasters	Cotoneaster spp.
Washington hawthorn	Crataegus phaenopyrum
Daphnes	Daphne spp.
Euonymus	Euonymus spp.
Beeches	Fagus spp.
Black walnut	Juglans nigra
Junipers	Juniperus spp.
Tuliptree, tulip-poplar	Liriodendron tulipifera
Magnolias	Magnolia spp.
Crabapple	Malus spp.
Siberian cypress, Russian-arborvitae	Microbiota decussata
White mulberry	Morus alba
American hophornbeam, ironwood	Ostrya virginiana
Norway spruce	Picea abies
Serbian spruce	Picea omorika
Colorado spruce	Picea pungens
Pines	Pinus spp.
Cherries, plums, peaches, apricots	Prunus spp.
White oak	Quercus alba
Chinkapin oak	Quercus muehlenbergii
English oak	Quercus robur
Northern red oak	Quercus rubra
Rhododendrons	Rhododendron spp.
Black locust	Robinia pseudoacacia
Mountain ashes	Sorbus spp.
Japanese spirea	Spiraea japonica
Lilacs	Syringa spp.
Yews	Taxus spp.
Lindens	Tilia spp.
Canadian hemlock, Eastern hemlock	Tsuga canadensis
Weigelas	Weigela spp.
Yuccas	Yucca spp.

Woody landscape plants able to tolerate wet conditions

Table 2

Trees – Deciduous

Trees – Evergreen

Shrubs – Deciduous

Shrubs – Evergreen

Vines and Groundcovers