Managing Flood Damage in Landscape Gardens

BURLINGTON – If recent storms caused a landscape garden to flood, you may be wondering if your plants and trees will rebound, or survive at all.

Full recovery depends on a number of factors including the type of plants, your soil type and length of time that plants were standing in water. The longer that period, the more depletion of oxygen in the soil and time for roots to rot. Clay soils would take much longer to drain than a sandy or loam soil.

Some plants can tolerate “wet feet.” So if you have ferns, sedges, mints, willows, maples, river birches and other ornamental plants that can tolerate wet conditions, chances are that they will survive with little damage. However, plants sensitive to wet conditions, including sedums, crabapples, pines, oaks and tender herbaceous plants, may suffer significant decline or die.

What to do?

For ornamentals, it’s best to take a wait-and-see approach. Start by removing any trash or debris from your gardens and let soils dry. Wash silt from ornamental leaves and stems if needed.

Be on the lookout for Japanese knotweed and other invasives that may have washed into your gardens. Remove and discard, but do not compost.

Many perennials, especially native and well-adapted plants, are tough and will likely survive. The same is not true for shallow-rooted plants, such as yarrow, tickseed and sedum.

Although many perennials will emerge through a few inches of silt, to help them recover, gently rake over them to remove silt and prevent crusting. For deeply silted perennial beds, your best approach is to dig up your choice plants, till the whole bed and start over.

Consider replanting on higher ground, in raised beds or in refurbished soil. The upside is that this gives you a chance to redesign your beds and introduce some new plants to your garden.

For annual flowerbeds, remove as much silt as possible. Since waterlogged soils impede the oxygen needed for root growth, a plant often will wilt since its roots are not able to grow. Wet soils also create good conditions for root diseases to appear. Any damage will be evident within two weeks. 

If your annual beds don’t recover, your best move is to till them under. Then add compost and a cover crop to condition the soil for next year. Consider buckwheat in the summer and oats or winter rye in the fall.

What about trees?

The good news is that many trees are able to withstand up to a week of flooding with little resulting damage. However, it may be several years before they show any flood damage symptoms, and then it will be too late to save them.

Silt and mud can smother shrubs and trees, so scoop or rake away silt deposits from the base of plants and tree trunks. Water and air need to reach roots, and since many of their feeder roots are near the surface, breaking up or removing silt will slow plant decline and eventual death. Never till under trees as this will damage their surface roots.

You can bring the silt you remove from your gardens or around your trees and shrubs to a landfill or use it to fill in eroded spots on your property. Another option is to make it into berms to start new ornamental beds, but do not use for vegetable gardens or berry patches.

For more information, consult “Flood-Recovery Guidance for Lawns and Gardens,” available at go.uvm.edu/lawngardenfloodrecovery. Home gardeners also may contact the University of Vermont Extension Master Gardener Helpline at go.uvm.edu/gardeninghelp with questions.