Why Are My Carrot Leaves Turning Red or Purple?

A red or purple flush spreading through carrot foliage can be harmless or a warning, depending on the cause — so this is one to look at carefully rather than ignore. The reddening comes from stress pigments, but what is causing the stress ranges from a cold snap (no problem) to carrot fly larvae chewing the roots (a real problem). Let me help you tell them apart.

Cold weather: usually harmless

Carrot foliage often takes on a reddish or purple tint in cold weather, especially after cold nights, through the stress pigment anthocyanin — the same response many plants show. This kind of cold-induced reddening is harmless; the plant is simply reacting to the chill, and it usually greens up again as conditions warm, while the roots keep developing fine (and cold actually sweetens carrots). If your carrots reddened during a cold spell and are otherwise healthy and growing, this is the benign explanation.

Phosphorus shortage

Reddening or purpling can also signal a phosphorus deficiency, which carrots show as a purple-red tint to the leaves along with slow growth. As with other crops, this is often not a true lack of phosphorus in the soil but cold soil preventing the roots from taking it up — so it frequently overlaps with the cold-weather cause and eases as the soil warms. Genuine phosphorus deficiency in poor soil is possible but less common. If reddening comes with stunting and persists in warm conditions, consider the soil.

The warning sign: carrot fly

Here is the cause you must not miss. One of the classic early symptoms of carrot fly attack is the foliage turning red, bronze or purple, as the larvae damage the roots below and the stressed plant discolours above. So reddening carrot tops, especially with stunted or wilting plants, is a recognised warning to check for carrot fly. Dig up a plant and inspect the root for rusty-brown tunnels. If you find them, act on carrot fly control (barriers and the measures in our carrot fly guide) to protect the rest of the crop.

How to respond

Read the context: reddening during or after cold weather, on otherwise healthy plants, is harmless — wait for warmth. Reddening with stunting that persists in warm soil suggests phosphorus or soil issues. Reddening with stunted, struggling plants, especially in early-to-mid summer, is a red flag for carrot fly — check the roots. Because the carrot fly possibility is serious, it is always worth lifting one plant to inspect the root when the tops redden unexpectedly. Rule that out, and you can relax about a simple cold or phosphorus flush.