This is seen fairly often in the garden, where a plant’s foliage develops a pink-purple tint at the edge of the leaves, and if not addressed it can turn the entire leaf purple. It often affects peas and brassicas, and is usually down to one of two things… cold weather or a nutrient deficiency.
Nutrient Deficiency:
The purple is usually a sign of the plant being deficient in phosphorous, and this can happen even if the plants are growing in fresh compost if the plants are underwatered and thus unable to draw up nutrients from the soil.
Cold Weather:
Often at the start of the growing season, in April and May, you’ll find that the wind temperature is quite cold, blowing in from the north/east. Night time temperatures can be low still as well. It can often cause this pink-purple discolouration, and you’ll most likely find that plants start producing lots of lovely green leaves as soon as the temperatures settle as we get closer to summer.
Or Both:
Interestingly, the two reasons can be connected – if a plant is cold and the soil temperature is cold, the root system and aeration of the soil is affected and the plant is unable to draw up enough phosphorous.
What to do?
If you’ve planted into fertile soil, then the best thing to do is to cover affected plants with fleece or a cloche until the weather warms properly. Keep the plants well watered, and if you suspect poor soil quality, add a fertiliser or mulch with fresh compost.