Why do my greenhouse crops get worse every year in the same border?
Greenhouse soil sickness — what actually accumulates in a border grown with the same crop, how to recognise it against the things it is mistaken for, and the four real remedies.
- Difficulty
- intermediate
- Time
- 2 hr
- Read
- 4 min
Short answer
Repeated cropping builds up root pathogens, eelworm and salt, and depletes specific nutrients. The plants get smaller and yellower each year despite identical care. Replace the top 30cm of soil, switch to ring culture or pots, rotate the crop, or grow a resistant grafted plant.
A greenhouse border is a small volume of soil that never gets rained on, never gets frozen, and grows the same crop every year. Every condition that makes crop rotation necessary outdoors is intensified. Soil sickness is the general name for the resulting decline, and it is a combination of causes rather than one disease.
Step by step
- Confirm it before acting.Lift a struggling plant at the end of the season and look at the roots. Sparse, brown, rotted or galled roots indicate a soil problem. Healthy white roots on a poor plant point to feeding, light, water or temperature instead.
- Rule out the simpler explanations.Insufficient feeding, irregular watering, poor light from dirty glass, an unusually hot or cold season, or a different seed batch. Soil sickness is a diagnosis of exclusion and a progressive one over several years.
- Replace the top 30cm of border soil.The most direct remedy. Dig out to about 30cm, remove the soil from the greenhouse entirely, and refill with good topsoil mixed with garden compost. Do a whole border, not a spot, or the problem simply returns from the sides.
- Or switch to ring culture.Lay a 100mm bed of gravel over the old soil with a membrane between, stand bottomless rings on it, and plant into the rings. The feeding roots are in fresh compost and the water roots are in inert gravel. The traditional and very effective answer.
- Or move to containers entirely.Large pots or growbags on top of the border, or on a paved floor. Complete isolation, at the cost of daily watering.
- Rotate the border in future.Alternate tomatoes with cucumbers, peppers, salads or an overwintering green manure. Even a single year's break makes a substantial difference to the build-up.
- Consider grafted plants.Tomatoes and cucumbers grafted onto vigorous, disease-resistant rootstocks are widely sold and are specifically intended for exactly this problem. They cost more per plant and crop heavily.
- Refresh the border every autumn.Clear all roots and debris, add garden compost, and flood the soil thoroughly to leach salts. A border that is looked after annually lasts far longer before it tires.
Tips
- Take the excavated soil right out of the greenhouse and spread it thinly on an ornamental border a long way away, not on a vegetable bed. Do not compost it.
- The old practice of sterilising border soil with chemicals is no longer available to amateurs and was never pleasant. Replacement or isolation are the practical options.
- Green manures such as grazing rye or field beans sown in the empty border over winter improve structure and give the soil a genuine break from the crop.
If it doesn't work
Plants smaller and paler each successive year, same variety, same care
Cause: Cumulative build-up of soil-borne root pathogens and depletion of specific nutrients in a soil that is never rotated or leached by rain. — Fix: Excavate and replace the top 30cm with fresh topsoil and garden compost, or move to ring culture over a gravel bed.
Plants wilt in sun and recover overnight, though the soil is moist
Cause: Root damage — root rot fungi or root-knot eelworm — reducing water uptake. — Fix: Lift a plant and examine the roots. Brown, sparse or galled roots confirm it. Replace the soil or move to containers; there is no domestic soil treatment.
White or yellowish crust on the border soil surface
Cause: Fertiliser and hard-water salts accumulating because the border is never flushed by rain. — Fix: Flood the border thoroughly two or three times over a few days in winter to leach the salts, and reduce feeding rates.
Roots have knobbly swellings along them
Cause: Root-knot eelworm, a classic greenhouse border problem once established. — Fix: Soil replacement or a permanent move to containers. Do not move the excavated soil to a vegetable bed.
Base of the stem darkens and the plant collapses at fruiting
Cause: Foot and root rot organisms living in the border soil, favoured by wet, cold soil early in the season. — Fix: Replace the soil, plant slightly proud, warm the border before planting, and avoid overwatering young plants.
Poor growth but roots look healthy and abundant
Cause: Not soil sickness — more likely a nutrient or pH problem, or simply insufficient feeding. — Fix: Test the pH, feed appropriately, and top up with compost. Do not excavate a border for a problem that is a feeding one.
Questions people ask
How long before greenhouse border soil goes sour?
With the same crop every year and no intervention, decline typically becomes obvious somewhere between the third and fifth season. With annual compost, rotation and winter flooding, a border can crop well for a decade or more.
Can I just add more compost and feed?
It helps with the nutrient side and is worth doing every year, but it does not remove root pathogens or eelworm. If the roots are visibly damaged, the soil has to be replaced or bypassed.