Why have my plant's leaves gone purple or red?

Purpling looks like a deficiency and is very rarely one in British gardens. Phosphorus is abundant in most UK soils and is not readily leached out, so genuine shortage is uncommon. What is common is a cold, wet April in which every tomato, sweetcorn and brassica seedling on the country's windowsills goes purple at once.

Step by step

1. Check the calendar and the temperature first.Purple undersides and stems on young plants in spring, particularly after a cold snap or after moving plants outside, is cold-induced phosphorus lockout. The nutrient is there; the root cannot take it up in cold soil. Nothing is wrong with the compost.
2. Do nothing but wait, and keep them warmer.Bring plants back under cover if a cold spell is forecast, avoid planting out before the soil has warmed, use a cloche or fleece, and do not water with cold water straight from an outside tap onto small plants in cold weather. Colour returns as the soil warms.
3. Rule out a genuinely restricted root system.A plant that has outgrown its pot, or whose roots have been damaged, cannot take up phosphorus either — same symptom, different cause. Check whether the plant is pot-bound and whether the roots are healthy before concluding anything about nutrients.
4. Check whether the plant is short of water.Drought stress produces purple and red tones in many plants, along with dull colour and a hard, tight look to the growth. Water thoroughly and see whether the next growth is normal.
5. Consider the variety.A great many plants are simply purple-leaved or purple-stemmed, and many more colour up in strong light — the red on the outside of a lettuce, the bronze on a young rose shoot, the purple on the underside of a tomato leaf in sun. If it is on the sun side only and the plant is growing well, it is pigment, not a problem.
6. Note the autumn case.Purple and red in autumn is normal senescence. Deciduous plants withdraw nutrients and the remaining pigments show. So do evergreens under cold stress in winter — nandina, some grasses and many conifers go bronze deliberately and green up in spring.
7. Only consider feeding as a last step.If a plant is purple in a warm season, is not pot-bound, is watered properly and has healthy roots, then a balanced feed is reasonable. On acid-loving plants, and on soil that is very alkaline or very acid, correcting the pH matters more than adding phosphorus.
8. Look at the pattern if the purple is not uniform.Purple confined to the leaf margins with a scorched edge is more likely potassium or salt damage. Purple only between the veins with green veins is a different pattern entirely and belongs with the interveinal chlorosis diagnosis.

Tips

• Sowing later into warm soil beats sowing early into cold soil. A crop sown three weeks later usually overtakes one that sat purple and stalled.
• Do not add superphosphate or bonemeal in response to spring purpling. It does nothing at low soil temperature and adds phosphate the soil did not need.
• Black plastic or a cloche laid over a bed for a fortnight before planting raises soil temperature meaningfully and prevents the whole problem.
• Sweetcorn, tomatoes, aubergines, peppers and brassicas are the plants that show cold purpling most obviously. Take it as a thermometer.

Common mistakes

• Feeding phosphate to purple spring seedlings — The soil is not short of it. The root cannot absorb at that temperature, so more in the soil changes nothing and adds to nutrient run-off.
• Planting out as soon as the air feels warm — Soil temperature lags air temperature by weeks. A warm afternoon in April sits over cold soil, and that is exactly what causes the purpling.
• Assuming purple always means a problem — Enormous numbers of plants are purple by design or colour up in sun. Judge by growth rate, not by colour.

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