A few simple techniques keep brassica heads and curds clean, white and pest-free right through to harvest — here's what actually needs doing, and when.
Last updated 13 July 2026
If you're growing calabrese or purple sprouting broccoli this season, our full guide to growing broccoli in the UK covers sowing through to harvest in detail.
"Blanching" gets mentioned a lot around brassica heads, but it's worth being precise about what it actually is and which crops need it. True blanching — excluding light from a developing curd to keep it pale and tender — is really a cauliflower technique, not a broccoli one. Broccoli and calabrese heads are meant to be green, so they don't need blanching at all; what they do need, just as much as cauliflower, is protecting from pests once the head starts to form.
Blanching Cauliflower Curds
As a cauliflower curd starts to swell, exposure to sunlight can turn it yellow, grey or even slightly bitter. Traditional blanching solves this by loosely tying the plant's own outer leaves up and over the curd once it reaches about golf-ball size, using soft garden twine or a rubber band. Check every few days and retie as the curd grows, since a leaf pulled too tight can trap moisture and encourage rot. Many modern varieties are bred as "self-blanching," curling their leaves naturally over the curd without help — worth choosing if you'd rather skip the extra step.
Protecting Heads and Curds from Pests
Whether you're growing cauliflower, calabrese or purple sprouting broccoli, the bigger threat to a developing head usually isn't light — it's pests. Cabbage white butterfly caterpillars can shred a curd from the inside out within days, and pigeons will strip exposed heads and leaves bare, especially over a hard winter when little else is available to them. Fine insect mesh netting, pegged down securely at the edges so nothing can crawl underneath, is the most reliable protection against caterpillars; a sturdier bird net does the job against pigeons. Whichever you use, it needs to go on before the head starts forming and stay in place right through to harvest — for a full breakdown of mesh sizes and timing, see our guide to protecting brassicas from cabbage white butterfly.
Grower's note: check under netting weekly even once it's in place — slugs and small caterpillars that get in before the net goes on can do just as much damage trapped inside as pests would from outside.
Other Common Issues with Brassica Heads
Beyond blanching and pest netting, a handful of other problems commonly affect brassica heads: club root causing stunted, wilting plants in soil that hasn't been rotated, flea beetle riddling young leaves with small holes, and whitefly clustering on the undersides of older leaves. None of these are specific to the head-forming stage, but a badly affected plant will rarely put on a good curd or head regardless of how well it's netted or blanched. Our garden problems, pests and diseases hub covers all three in more detail, along with the wider range of issues that can affect a UK vegetable bed.
Getting the Timing Right
Start checking cauliflower curds for blanching as soon as they're visible between the leaves — usually a small, pale patch the size of a coin. Netting, by contrast, should already be in place well before that point, ideally from the day plants are transplanted, since caterpillars and pigeons will happily target young leafy growth as well as a forming head. Getting netting on early and blanching curds as they appear, rather than waiting until damage or discolouration is obvious, makes the difference between a clean harvest and a disappointing one.
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Get the UK Garden Planner →Frequently Asked Questions
Do broccoli heads need blanching?
No. Broccoli and calabrese heads are meant to stay green, so they don't need blanching. Blanching (excluding light to keep the curd pale) is specifically a cauliflower technique.
How do I blanch a cauliflower curd?
Once the curd reaches about golf-ball size, loosely tie the plant's outer leaves up and over it with soft twine, checking and retying every few days as it grows. Self-blanching varieties do this naturally without help.
What's the biggest threat to a developing brassica head?
Usually pests rather than light exposure — cabbage white butterfly caterpillars and, over winter, pigeons stripping heads and leaves. Fine mesh netting from transplanting through to harvest is the most reliable protection.
When should I net brassicas against pests?
From the day of transplanting, well before a head or curd starts forming. Waiting until a head is visible gives caterpillars and pigeons time to damage the plant first.
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