Why does the same plant keep dying in the same spot?

A spot that kills three plants in a row is giving you information, and the temptation is to answer it with a fourth plant. The useful sequence is to eliminate the physical causes first, because they are far more common, then the family-specific ones, then the small number of genuinely serious soil-borne diseases.

Step by step

1. Dig a proper inspection hole, deeper than you think.At least 45cm, wider than a planting hole, and go down until you are certain. You are looking for builders' rubble, a buried path or slab, a concrete footing, a plastic sheet, a band of hard grey compacted subsoil, or a change to something that is not soil.
2. Fill the hole with water and time it.Pour in a couple of buckets and come back in a few hours. If water is still standing, roots there are in airless conditions for days after every downpour, and everything except a genuinely wet-tolerant plant will slowly rot. That is the answer more often than any disease.
3. Feel for a pan.Push a metal rod or a long screwdriver into the ground in several places. A sudden hard layer at a consistent depth is compaction, usually from machinery during building work or from digging in the wet. Roots stop at it and water sits on it.
4. Look up and around before blaming the soil.A drip line from a roof edge or a gutter joint, the rain shadow of a wall or an overhang, a downpipe discharging nearby, the dry zone under a large tree, a frost pocket at the bottom of a slope, a channel where the wind funnels. All of these kill repeatedly in one small area.
5. Consider what was there before.A path, a shed base, a bonfire site, a spot where cement was mixed, a place where a previous owner emptied something. Cement washings leave the soil strongly alkaline for years. A bonfire site is sterile and salty. Neither shows on the surface.
6. Suspect replant disease if it is the same family each time.Rose after rose is the classic — a new rose in the position of an old one grows poorly for years while an unrelated plant thrives. It is a soil biology effect rather than a single organism. The reliable answer is to plant something unrelated there, or to change a large volume of soil, at least a cubic metre, and use mycorrhizal fungi at planting.
7. Consider the family-specific soil pathogens.Clubroot kills brassicas and wallflowers and persists for many years. White rot kills onions, garlic and leeks. Both are family-specific, permanent in practical terms, and mean growing that family elsewhere or in containers. Neither harms an unrelated plant.
8. Rule out honey fungus if woody plants keep dying.Look for creamy-white sheets of fungal growth under the bark at the base, smelling of mushroom, and flat black bootlace strands in the soil. This changes the plan entirely, because it spreads through the soil to other woody plants and the response is resistant planting rather than treatment.
9. Change the plant before you change the soil again.The cheapest experiment is to put something completely different, tough and unrelated in that spot. If it thrives, the problem was family-specific. If it also fails, the problem is the site, and the site is what you fix.

Tips

• Improving one planting hole in bad ground can make things worse — a hole of good compost in clay becomes a sump. Improve a whole area or none of it.
• A raised bed or a large container over a bad spot is a legitimate answer and sometimes the only practical one.
• Keep a note of what has failed where and when. Three failures over five years is a pattern that is easy to miss without a record.
• Mycorrhizal fungi at planting are genuinely useful for rose replant disease and for trees and shrubs on poor ground. They are not a general tonic.

Common mistakes

• Replacing with the same plant to see if it was bad luck — It rarely was. You have spent another plant and another year to confirm what the first two already told you.
• Adding compost and feed to a waterlogged spot — Neither addresses the missing air. Organic matter in permanently wet soil goes anaerobic and makes the root environment worse.
• Assuming it must be a disease — Physical causes — water, compaction, rubble, a rain shadow, a root-filled zone under a tree — account for most repeat failures, and all of them are visible if you dig.

Questions people ask