How do I stop plants rotting in wet soil over winter?

Once you accept that wet rather than cold is what kills most borderline plants here, the interventions change completely. Nothing on this list involves insulation, and all of it is about getting water away from the neck and the roots of the plant.

Safety

What you'll need

• Coarse grit or sharp sand
• Gloves
• Pot feet, or bricks or tiles (optional)
• A pane of glass or clear rigid plastic and bricks
• A spade and a fork

Step by step

1. Plant on a low mound in the first place.Set the plant so its crown sits above the surrounding soil level and mound the soil up around the root ball. Surface water then runs away from the neck rather than towards it. This is by far the highest-value thing on the list and it costs nothing at planting time.
2. Work grit into the planting area, not just the hole.Grit in a hole in clay makes a sump that fills with water. Grit forked through a wider area, or better still a raised bed or a whole improved border, actually drains.
3. Get the planting depth right.A plant set deeper than it grew has soil and water permanently against a stem that was never meant to be buried. If something has sunk over the years, lift and reset it in autumn or spring.
4. Make mulch a collar, not a plug.Heaped over and around the crown, hollow at the stem. Coarse, open material rather than fine compost, which packs into a wet mat.
5. Leave the right things standing.Many herbaceous plants — grasses, sedums, and most silver-leaved subjects — are better with last year's stems left on over winter. The standing growth sheds water off the crown and the hollow stems are habitat. Cut in spring instead.
6. Raise every container.Pot feet, bricks, tiles, anything that lifts the base clear so water leaves through the drainage holes. Remove saucers entirely for winter, and move pots out from under a gutter that overflows.
7. Keep rain off the few plants that need it kept off.Alpines, many silver-leaved plants, some bulbs with a dry dormancy. A pane of glass or rigid clear plastic on bricks over the plant, open on all four sides, sheds the rain without trapping air or heat.
8. Fix the ground where losses repeat.If the same corner takes plants every winter, dig a hole a spade deep, fill it with water, and see how long it stands. That answers whether you have a drainage problem to solve or a planting choice to change.

Tips

• Plant borderline things in spring, not autumn. A full growing season of root establishment before the first wet winter changes the odds considerably.
• Improving structure with organic matter is slow but permanent; grit and mounding are fast and local. Do both, for different reasons.
• Do not feed with nitrogen late in the season. Soft unripened growth going into a wet winter rots readily, and the plant would have been better off going dormant hard.
• Wind helps. A plant in a still, damp, enclosed corner stays wet for far longer than the same plant somewhere air moves, which is why crowded planting suffers more.
• If you are growing something genuinely intolerant of wet, a pot you can move under an overhang for the winter is often more reliable than any ground treatment.

Common mistakes

• Digging a deep hole in clay, filling it with grit and compost and planting into it — It behaves as a sump. Water flows into it from the surrounding clay and cannot leave, so you have built a container with no drainage hole and planted into it.
• Cutting everything to the ground in autumn for tidiness — It exposes bare crowns to months of rain, removes the standing growth that would have shed water off them, and destroys overwintering habitat. Some plants genuinely need cutting back; most do not.
• Adding more mulch when a plant looks like it is struggling in a wet winter — Mulch on saturated ground holds the water in. If the problem is wet, the answer is drainage and airflow, not another insulating layer.

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