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Cauliflower attracts the same set of pests as every other brassica, and the great majority of them arrive by flying in. Cabbage white butterflies, cabbage root fly, aphids, flea beetles and pigeons all land on an unprotected plant. A fine mesh net over hoops, put up on the day the plants go in and sealed at the edges, stops nearly all of them at once.
Netting after the pests are established does the opposite: it shuts the predators out and keeps the caterpillars in. That is why it goes on at planting, not at the first sign of damage.
What is left after netting is mostly soil-borne, and clubroot is the one that matters. It cannot be sprayed away and it lasts in the soil for the best part of two decades, so it is handled by rotation and pH rather than by treatment.
| Stage | Main threats | First sign |
|---|---|---|
| First fortnight after planting | Slugs, cutworms, cabbage root fly | Plants cut off at soil level, or wilting for no reason |
| Leaf building | Cabbage white caterpillars, flea beetles | Ragged holes, or fine shot-holes across the leaf |
| Full canopy | Cabbage aphid, whitefly | Gray-white colonies on leaf undersides |
| Heading | Slugs, caterpillars inside the wrapper leaves | Frass and holes under the folded leaves |
| Winter, on standing plants | Pigeons | Leaves stripped to the midribs overnight |
| Any stage | Clubroot | Wilting in the heat, recovering at night, stunted growth |
Caterpillars and how to keep on top of them
Cabbage white caterpillars are the most visible cauliflower problem. The yellow-and-black gregarious ones feed in groups on the leaf surface and are easy to spot; the plain green ones sit along the midrib and are nearly invisible until the leaf is in shreds. Cabbage moth caterpillars are worse, because they bore into the head itself.
On a garden scale, hand picking genuinely works. The one thing that makes it work is looking on the undersides of leaves at least weekly, because you want to find eggs and newly hatched larvae rather than half-grown ones that eat a leaf a day.
- Net with fine mesh from planting day Use insect mesh of about 0.8 mm over hoops, tall enough that leaves do not press against it. Butterflies lay eggs through netting that touches the foliage.
- Bury or weight the edges completely Trench the edges into the soil or hold them down with boards along their whole length. A gap the width of your finger is an open door.
- Check the undersides weekly Look for clusters of yellow eggs and for lines of tiny larvae. Rubbing off an egg cluster takes a second; picking off forty caterpillars takes twenty minutes.
- Search the head as well Cabbage moth caterpillars tunnel into the curd where nothing can reach them. Part the wrapper leaves and look at the base of the head each time you check.
- Use a bacterial spray if numbers get away A Bacillus thuringiensis product works on young caterpillars and spares bees and predators. Apply in the evening and repeat after rain.
- Clear crop debris at the end Pupae and eggs overwinter in old brassica stems. Pulling and composting spent plants cuts next season's starting population.
Damage to outer leaves late in the season looks alarming but costs very little. If the canopy is built and the head is filling, remove the caterpillars and leave the plant alone.
Aphids, whitefly and flea beetles
Cabbage aphid is the one that does real damage. It forms dense mealy gray colonies deep in the growing point and between the wrapper leaves, distorting new growth and contaminating the curd where it is very hard to wash out. Because it sits where sprays do not reach, early detection matters more than treatment.
Flea beetles pepper young leaves with small round shot-holes. On an established plant it is cosmetic, but on a newly planted seedling a heavy attack can check growth badly, which with cauliflower means a smaller head.
- Inspect the growing point, not the leaf tops Part the inner leaves and look straight down into the center once a week. Aphid colonies are established there long before the plant looks unwell.
- Squash small colonies by hand A colony the size of a coin can be wiped out between finger and thumb, or blasted off with a jet of water. Caught early this is the whole treatment.
- Spray with insecticidal soap for larger colonies Soap needs direct contact, so aim into the crown and under the leaves rather than over the top. Repeat after five days to catch what hatched since.
- Keep the net on for flea beetles too The same mesh that stops butterflies stops flea beetles reaching newly planted seedlings, which is when they do the damage that matters.
- Water and feed young plants through an attack Flea beetle damage is outgrown by a vigorous plant. Keeping the seedlings moving is more effective than trying to kill the beetles.
- Encourage the predators Ladybirds, hoverflies and parasitic wasps clear aphids effectively. Leave a strip of flowering plants nearby and avoid broad-spectrum sprays that remove them.
Clubroot: the one to design around
Clubroot is a soil-borne organism that swells brassica roots into distorted knobs, so the plant cannot take up water. Above ground it looks like drought: wilting in the heat, recovering overnight, stunted and yellowing, and heads that never fill. Pull a plant and the roots settle it immediately.
There is no cure, and the resting spores survive in soil for fifteen to twenty years. It arrives on boots, tools and bought-in plants, so once a plot has it the aim shifts to keeping it out of the clean beds and making the affected ones tolerable.
It thrives on acid, wet soil. Liming to pH 7.0 and improving drainage will not eliminate it, but they reduce the damage enough that a crop is still worth growing.
| Measure | What it does | How much it helps |
|---|---|---|
| Rotate brassicas on a four year gap | Denies the organism a host | Slows build-up; does not clear an infested bed |
| Lime to pH 7.0 | Suppresses spore germination | Substantially reduces damage on acid soil |
| Improve drainage or build raised beds | Removes the wet conditions it needs | Substantial on heavy ground |
| Raise plants in large pots before planting | Gives a big root system before infection | Often the difference between a crop and none |
| Grow resistant varieties | Resists infection directly | Effective, but resistance is partial and can erode |
| Clean boots and tools between beds | Stops spread to clean ground | The most valuable measure if you are still clean |
| Never compost infected roots | Keeps spores out of the compost pile | Essential; bag and remove them |
Cabbage root fly causes similar wilting but leaves the roots eaten and ragged rather than swollen and knobbly. Pull a suspect plant and look before you decide which one you have; the responses are completely different.
Slugs, cutworms, pigeons and rots
The remaining problems are mostly about vulnerable moments rather than constant pressure. Slugs and cutworms take newly planted seedlings in their first fortnight. Pigeons strip standing plants in winter when other food is scarce. Rots take hold in the head when water sits in it in cold, wet weather.
Each has a narrow window, and protection during that window is far cheaper than dealing with the aftermath. A plant lost in its first week is a whole season's space wasted.
- Protect new transplants from slugs Check the soil at the base at dusk for the first fortnight, and use a barrier or wool pellets around each plant. A cauliflower transplant eaten at the growing point does not recover.
- Look for cutworms if plants topple A plant severed cleanly at soil level overnight means a cutworm. Scrape the soil in a circle two inches (5 cm) around the stump and you will usually find it.
- Net or cage against pigeons in winter Overwintering plants need netting held off the leaves on hoops or a cage. Pigeons can strip a row to bare midribs in a single frosty morning.
- Blanch loosely in wet weather Water trapped in a tightly tied head rots the curd from the surface. In autumn and winter, fold the leaves over lightly or leave the head open.
- Remove yellowing lower leaves Dying leaves against damp soil start the soft rots that then move into the stem. Snap them off and take them off the plot.
- Keep the spacing generous At 20 to 24 inches (50 to 60 cm) the canopy dries after rain. Crowded plants stay wet all day, and every fungal problem follows from that.
Record the date you first see each problem and where in the bed it appeared. Pests arrive on much the same schedule each year, and a note of last year's first sighting tells you exactly when to have the net on.