After the Flood … How to Deal with the Impact of Excess Water in the Landscape

While many landscape plants will survive short periods of flooding, extended periods of standing water are often detrimental because of declining levels of oxygen in the soil.

As soon as possible, remove flood debris from the landscape. If additional soil or sediment has been deposited on lawns or over tree root systems, you may need to remove it. One inch of silt or soil is sufficient to kill a lawn. Silt deposits of 3 inches or more can be injurious to trees.

Other than removing debris, try not to walk on waterlogged soil. If you must walk in the garden, use a plank of wood to spread your weight and help avoid compaction.

In the vegetable garden, leafy vegetables, such as lettuce and spinach, should be pulled and discarded. Flood waters may contain potentially harmful microorganisms and leafy vegetables are difficult to wash completely. Vegetables that produce fruit (tomatoes, peppers, eggplant, cucumbers, etc.) should be safe to eat by the time they are mature. For additional safety wash the fruits thoroughly before eating.

You should know if vegetable and flowering annuals survive within one to two weeks. These plants do not have large storage reserves, therefore, they will show damage first. With woody plants, the wait may be much longer. Survivability depends greatly on the species and the site. Symptoms of stress for trees and/or shrubs can seem relatively minor with leaf yellowing, reduced leaf size and shoot growth, and sprouts along the stem or trunk, to fairly obvious stress symptoms such as crown dieback, early fall coloration and/or leaf drop, and large seed crops in years to come.

Climatologists are suggesting that massive rainfalls such as that experienced in Toronto August 7 will become more frequent as a result of climate change. Take this into consideration when selecting plants for your garden.

Alders (Alnus species), Willow (Salix species), and River Birch (Betula nigra), will easily survive standing water and are often planted along edges of ponds. Other trees that should tolerate brief periods of saturated or flooded soils without injury are Red Maple (Acer rubrum), Boxelder (Acer negundo), Sycamore (Platanus occidentalis), and Cottonwood (Populus deltoides). A few herbaceous perennials that prefer moist or wet soils include: Yellow Flag (Iris pseudocorus), Joe-Pye Weed (Eupatorium purpureum), Spiderwort (Tradescantia species), Cardinal Flower (Lobelia cardinalis), Ligularia (Ligularia species), Hardy Hibiscus (Hibiscus species), and tender perennials such as Canna (Canna xgeneralis) and Calla Lily (Zantedeschia hybrids).

Plants that are most likely to be damaged by flooding are: many species of evergreens such as Pine (Pinus), Spruce (Picea), and Fir (Abies), many Oak (Quercus) species, Sugar Maple (Acer saccarhum), Crabapples (Malus species), Redbud (Cercis canadensis) and Magnolias (Magnolia species).

A few herbaceous perennials that do not tolerate wet soils include: Lamb’s Ear (Stachys byzantina), Butterfly Weed (Asclepias tuberosa), Russian Sage (Physostegia atriplicifolia), Stonecrop (Sedum species), and many spring flowering bulbs like Daffodils (Narcissus), Tulips (Tulipa), Crocus (Crocus), and Hyacinth (Hyacinthus).

For more advice, click on the Ask a Master Gardener link at the bottom of this page.


Source: Iowa State University Department of Agriculture
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