What does it mean when the tops of the Brussels Sprouts start to turn purple?
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Purpling on brassicas can be caused by several common occurrences, and thus this is a common problem. It can be caused by soil acidity, so liming might be a good idea, but I think it is probably one of the following:
Potassium Deficiency. This is a common problem in light or sandy soils, and in poor soil. The plant cannot gather enough potassium in the soil, and it causes a purple color on the leaves, buds, stems, or a combination of these. Also, some plants, including some brassicas, are inefficient at taking up potassium during cold weather. Thus, many plants become purplish in cold weather.
Variety. Different varieties of Brussels Sprouts were developed to have purple coloring, and it is not uncommon for some sprouts that were meant to be green to develop some purple coloring.
None of these things really affect the flavor of the sprouts, so you do not have to worry about that. It is a good idea to keep your soil in good health by maintaining a good amount of organic matter in the soil, which is best accomplished by adding plenty of compost each year, and keeping your plants mulched. Also, if your soil becomes too sour, you can add lime.