There is no need for alarm when cauliflower heads in your garden turn purple around the outside edge. Assuming that you aren't growing one of the deliberately 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 heat and sunlight hits the developing cauliflowers, they can display a purplish tinge along the edges of the heads.
The purple color in cauliflower is caused by the presence of anthocyanin, which is a harmless, water-soluble pigment in the curd. Anthocyanin is the same pigment found in red cabbage. Sun exposure exaggerates its effect on the developing cauliflower head.
Ways to Prevent a Purple Tinge
Cauliflower is usually ready to harvest within 60 days. It needs daytime temps of between 70 to 85 F to avoid turning purple. Starting cauliflower earlier can avoid sweltering temperatures; however, you have to be careful though and strike a delicate balance because if you plant too early, a late season frost can kill young cauliflower.
Also, low soil fertility can cause white cauliflower to turn purplish. Cauliflower grows best in well-drained yet moisture-retentive, fertile soil with a pH of 6 to 7. Use a pH probe to test the pH level. To ensure you have a fertile soil, enrich the soil with compost when planting. If you use a compost rich in organic matter you will not need to fertilize. If you do not amend the soil with compost when planting, you will need to fertilize monthly with a 5-10-15 NPK fertilizer.
There are several ways you can go about preventing the purple hue from occurring:
- Schedule the development of the cauliflower for the cooler fall months.
- Cover the head as it develops with surrounding leaves. This process is known as blanching.
- Purchase a self-blanching variety developed to reduce tinting.
- Use floating row covers to inhibit temperature fluctuations and direct harsh sunlight.
To get 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 the cauliflower. Tie the leaves with garden twine or use a rubber band to hold the leaves closed, and 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 transplanting them into the garden. When you see a cauliflower curd 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. Purple cauliflower has some added natural sweetness. Purple cauliflower is usually described as having a mild, sweet, nutty flavor. Some say purple cauliflower has a slightly "off" flavor best eaten raw since cooking deepens the "off" taste.
When you cook the cauliflower, the purple tinge turns gray, mainly when the cooking water is hard or has an alkaline pH. Its appearance may look unappetizing, but a bit of vinegar added to the water minimizes the color change.
Some cooks prefer to use purple cauliflower when composing dishes because it gives plates more visual interest. Purple cauliflower used in a green salad makes the colors pop—green and purple are complementary colors—allowing the chef to create a feast for the eyes. Another fun way to go is pairing purple foods together for a monochromatic smorgasbord; think: radicchio, purple cabbage, Japanese eggplant, purple potatoes, the list goes on.
Purple Cauliflower Varieties
Brightly colored cauliflower has become increasingly popular at gourmet markets, and seeds are available for purple Graffiti cauliflower, orange Cheddar cauliflower, and lime-green Broccoflower or Romanesco varieties from most popular garden seed companies. Other popular purple colored varieties are Sicilian Violet, Violet Queen, Rambo Purple, and Purple Cape.
In the case of these brightly colored cauliflower varieties, there is no need to blanch, as they're meant to look that way. Bright sunlight intensifies the color of the curds.
Kapusta-Duch J, Szeląg-Sikora A, Sikora J, et al. Health-promoting properties of fresh and processed purple cauliflower. Sustainability. 2019;11(15):4008.