Gardening for heavy rain and flooding: draining, slowing and planting for wet

The same warming that brings hotter, drier summers also brings heavier rain. Warmer air holds more moisture, and the IPCC reports with high confidence that downpours have grown more intense. A garden that is dry in August can be waterlogged in winter or after a single summer storm. Too much water kills a plant as surely as too little, and it can do so faster.

Rain, heat and drought belong together. Gardening for extreme heat and drought-tolerant gardening cover dry extremes, and gardening in a changing climate covers the wider pattern. The same garden may need to handle drought and flooding, so plan for both ends of the range rather than the mild average between them.

What too much water does

Roots need air as much as they need water. Healthy soil has spaces between its particles, and roots draw oxygen from those spaces. When heavy rain fills them with water, the air is pushed out. Roots then suffocate, die and rot. The plant above wilts and yellows, much like a plant dying of thirst, so worried gardeners often water a waterlogged plant again.

Dead and dying roots are then open to disease. Root rot and phytophthora both take hold in wet soil and kill plants that waterlogging has weakened. Standing water and long-wet foliage also favour leaf diseases such as downy mildew. On top of all that, a downpour leaches nutrients out of reach and scours bare soil away, so a garden loses fertility and structure in the same storm that drowns its roots.

Open the soil and raise the planting

Start with the soil. Organic matter dug in or laid on top opens heavy ground so water can move through it instead of sitting on it. The same improved soil can also hold more water during drought, so this work helps in both extremes. Avoid working or walking on soil while it is wet, since that compacts it and closes the spaces that let it drain.

Where water still lingers, lift the planting above it. Raised beds put roots into free-draining soil above the wet, and breaking up a compacted layer below the surface, or digging a soakaway on very heavy ground, gives trapped water somewhere to go. Containers should stand on pot feet so they never sit in a saucer of rain. Keep roots out of standing water, because plants grown for dry-to-average conditions do not survive long with their roots submerged.

Slow the water down

Sending storm water away as fast as possible moves the problem downstream and wastes water you may need later. Slow the water and let it soak into the ground where you can. A rain garden, a shallow planted hollow fed by a downpipe or a path, holds a burst of rain and lets it soak away over hours. A swale, a level shallow channel along a slope, does the same across a larger area.

Permeable surfaces also help. Gravel, planting and open paving let rain enter the soil where hard paving would send it to the drain. A water butt on each downpipe catches roof runoff for the dry weeks to come. Every measure that holds water on the plot reduces flooding now and stores more water for later.

Protect the soil from the storm

Downpours damage bare soil most. Rain hitting open ground compacts the surface, washes away the finest and most fertile particles and carries dissolved nutrients out of reach. Keep the ground covered with mulch, low planting or a green manure crop. Cover takes the force of the rain and holds the soil in place. Covered soil keeps more of its structure and fertility through wet spells.

Plant for wet feet, within reason

Some plants take wet ground by nature, from riverbanks, damp meadows and pond margins. Hosta, astilbe, rhubarb, primrose and many ferns thrive in reliably moist soil, and trees and shrubs such as dogwood, alder, willow and hornbeam cope with ground that floods from time to time.

Separate soil that is occasionally wet from soil that is permanently waterlogged. Many plants survive short flooding if the water drains away within a day or two. Far fewer survive ground that never drains, and even bog and marginal plants have limits. Match the plant to how long the water stands, not to how wet the ground looks after a single storm.

Storms bring wind as well as water

Heavy rain often arrives with strong wind, and the combination is worse than either problem alone. Saturated soil holds roots less firmly just when wind is pushing hardest, so trees and tall plants can rock loose or blow over in a wet storm that they would have survived when dry. Firm and stake new plantings, check tree ties, and inspect established trees for loose footing before the stormy season. Plan for the wettest week as deliberately as for the driest.