Question of the Week
Answer: Yellowing or browning of white cauliflower heads can be attributed to frost damage, sun damage, or boron-deficient soil.
Cauliflower is a cool-season plant belonging to the Brassica family. However, it can be damaged if the temperatures are too low or if exposed to a hard freeze. If you have experienced extremely cold night temperatures, this may be the reason for discoloration.
If you have ruled out frost, are you blanching your cauliflower heads? Blanching involves pulling the leaves up over the head of the cauliflower to shade it which ensures the cauliflower head remains a crisp, white color. The leaves can be tied together but this makes it easy for water to collect, which could rot the head of the cauliflower. A safer bet is to bend the larger outside leaves over the crown and loosely tuck the edges to hold it in place, which allows for proper air circulation. Some varieties of cauliflower are even “self-blanching.”
If you have not experienced a hard freeze and you have been blanching the cauliflower heads, it is likely your problem is boron-deficient soil. This will cause the heads to turn brown and leaf tips to curl or become distorted. A soil sample could help determine if your soil is deficient. However, since the uptake of boron by the plant depends on different factors, such as the percent of organic matter in the soil, moisture levels, and the pH, a soil test that indicated sufficient boron levels would not eliminate the possibility of a boron deficiency.
Boron is one of seven essential trace minerals that plants need in very small amounts to grow well. Cole crops need higher amounts than other crops. It's easy to over-apply boron in trying to rectify a deficiency. Consider looking at other factors like soil moisture levels, good pH levels, and organic matter content of the soil before applying boron. If it has been dry, keeping the soil moist with irrigation may help the uptake of boron. Fixing the pH and building organic matter are both longer-term solutions that will help with future plantings. For subsequent crops, provide boron by adding compost to the soil, or plant fall cover crops of vetch or clover. A foliar feeding with kelp or fish emulsion could help, as these both are high in boron. If you decide to apply boron it's recommended at a rate of only .1 to .25 lb/acre as a foliar spray. The amount used depends on the percentage of boron in the product you are using.
For more information about growing cauliflower, refer to the ATTRA publication Cole Crops and Other Brassicas: Organic Production, available at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=27.
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