There is no need for alarm when cauliflower heads in your garden turn purple around the outside edge. Assuming you aren't growing one of the purple varieties, your cauliflower is just being cauliflower, and the coloration is not caused by a disease, fungus or pest.
Why Cauliflower Turns Purple
Genetically, many cauliflowers tend toward purple, red or blue pigments. Cauliflower is a cool-weather vegetable, and when a lot of sunlight and heat hits the developing heads of these cauliflowers, they can display a purplish tinge along the edges of the heads.
The edible portion of the plant is the head of flower buds, called the curd. The purple color in cauliflower is caused by the presence of anthocyanin, a harmless water-soluble pigment in the curd. Sun exposure exacerbates its effect on the developing cauliflower head.
Ways to Prevent a Purple Tinge in Cauliflower
- Purchase a self-blanching variety developed to reduce tinting
- Schedule the development of the cauliflower for the cool months in fall
- Cover the head as it develops with surrounding leaves, a process known as blanching.
To get nice, perfectly white cauliflower in the garden, you need to blanch the cauliflower by covering the developing head with leaves. Even if the cauliflower is already showing a tint around the edges, you should get that head of cauliflower covered up and out of the sunlight. To blanch the cauliflower, draw the surrounding leaves up and around the head of cauliflower.
Tie the leaves with garden twine or use a rubber band to hold the leaves closed. That's all there is to it. You won't have to worry about any further coloring of the cauliflower curds.
When to Blanch
Start checking your plants about 30 days after you transplant them into the garden. When you see a cauliflower curd that is about the size of an egg, it is time to blanch.
The curd continues to grow rapidly, and you'll probably need to adjust the positioning of the leaves covering the curd as this happens.
Is Purple Cauliflower Safe to Eat?
If your cauliflower has already turned purple on the edges, don't despair. It is perfectly safe to eat it either raw or cooked, although the flavor may be subtly different from the familiar white variety. When you cook the cauliflower, the purple tinge turns gray—not the most appetizing hue—particularly when the cooking water is hard or has an alkaline pH. A bit of vinegar added to the water minimizes the color change.
Purple Cauliflower Varieties
In the past decade, colored cauliflower started showing up at produce stands. The colors are not artificial. Brightly colored cauliflower has become increasingly popular at gourmet markets, and seeds are available for lime green, purple and orange varieties from most of the popular garden seed companies.
In the case of these brightly colored cauliflower varieties, there is no need to blanch. Bright sunlight intensifies the color of the curds.