Why do my plants die after heavy rain, and how do I tell drowning from drought?

Roots need oxygen, and they get it from the air in the spaces between soil particles. Fill those spaces with water for long enough and the roots die of suffocation, in soil that is visibly wet. That single fact explains why drowning and drought look the same from above and why the wrong response is so easy to reach for.

Step by step

1. Dig an inspection hole and watch it.Half a metre deep and a spade wide, near the affected plants. If water seeps in and stands, or if the sides are grey, sticky and smell of drains, the ground is waterlogged. This is the check that settles it and almost nobody does it.
2. Compare the two symptom sets.Drought: soil dry and cracked or shrunk from the edges, leaves crisp and brown at the tips and margins, plant recovers within hours of a proper soak. Waterlogging: soil wet and heavy, leaves yellowing from the bottom upwards and dropping, growth soft, plant wilts and does not respond to watering, sour smell from the soil, and often a black rot at the stem base.
3. Look at the colour of the soil.Permanently waterlogged soil goes grey or blue-grey with orange mottling, because iron in it has been chemically changed in the absence of oxygen. That colour a spade's depth down means the problem is chronic rather than a one-off downpour.
4. Understand what happens to nutrients too.Prolonged rain leaches nitrogen out of the soil, particularly on light ground and in pots, so plants often go pale and yellow after weeks of wet weather. That is a genuine nutrient effect and it corrects itself with a feed once the ground has drained and growth restarts.
5. Do not walk on it.Walking on saturated soil destroys its structure and compacts it, which makes the drainage permanently worse. Use a board to spread your weight if you must cross a bed, and stay off it entirely if you can.
6. Do not dig it while it is wet.For the same reason. Wet soil worked or dug smears and forms a pan. Wait until it is workable, which on clay may be weeks.
7. Help pots and containers immediately.Lift them onto feet or bricks so water runs out of the base, empty every saucer, move them under shelter if possible, and tip a waterlogged pot on its side to drain. This is the one place you can act at once and it works.
8. Give plants in the ground time before deciding.Many recover once the water table falls, particularly if the flooding was brief. Do not lift, feed or cut back while the ground is still wet. Wait until the soil is workable and judge by whether new growth appears.
9. Fix the drainage rather than the plants.Break up compaction and any pan, add organic matter over years, build raised beds, install a French drain or a soakaway where there is somewhere for the water to go, and check that downpipes and surface water are not being discharged onto the bed.
10. Change what you grow in the wet spot.There is a substantial list of plants that tolerate or enjoy wet ground — willow, dogwood, alder, astilbe, hosta, ligularia, iris, primula, rodgersia. Working with a wet corner is far more successful than fighting it every winter.

Tips

• Newly planted trees and shrubs are the most vulnerable, because their root systems are shallow and concentrated in the planting hole, which acts as a sump in clay.
• A hole of good compost dug in heavy clay collects water from the surrounding soil. Improve a whole area or none of it.
• Evergreens suffer more than deciduous plants in a wet winter, because they are still losing water through their leaves while the roots are drowning.
• Grass on a waterlogged lawn recovers surprisingly well. Keep off it until it is firm, then spike and topdress rather than trying to reseed into wet ground.

Common mistakes

• Watering a wilting plant in wet ground — The roots have suffocated and cannot take water up. Adding more is the fastest way to finish the plant.
• Feeding immediately to help recovery — Roots that are damaged or dying cannot take up nutrients, and concentrated feed on them causes further injury. Wait until growth restarts.
• Digging over a wet bed to let the water out — It smears the structure and creates a pan, making the drainage permanently worse. Wait until it is workable.
• Assuming a flooded plant is dead because it looks dead — Many woody plants defoliate and reshoot. Scratch a stem for green and give it a full season.

Questions people ask