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Rescue problem brassicas

27 June 2023
plant rescue 2
White caterpillars can quickly demolish young cabbage seedlings.

Sarah O'Neil provides advice on dealing with brassica issues.

Brassicas are a great cool-season crop to fill the garden over the winter months when there isn’t much else to grow. Cabbage, broccoli, cauliflower and other members of the brassica family are a staple in our Kiwi diets and are so versatile to use in the kitchen. Although don’t be too hard on kids who don’t like Brussels sprouts as it has been proven their taste buds haven’t matured enough to appreciate the flavour. If you haven’t had them since you were a kid, give them another try, you might enjoy them.

Botanical name: Brassica oleracea
Common names: Cabbage, broccoli, cauliflower, kale, Brussels sprouts, kohlrabi, and gai lan to name a few.
Planting time: June–April depending on the variety, always check the label.
Habit: Large leafy plants with thick waxy leaves. Allow 50–75cm per plant.
Hardiness: Cold hardy
Recommended growing medium: A firm but free-draining rich soil in full sun. Also suitable for growing in containers.

Loose heads & hearts

Issue Things have gone wrong when brassica heads are loose, without the expected solid, firm hearts.
Cause The most common cause is the soil is too soft. Brassica don’t like loose soil. It can also be caused by too much nitrogen or not enough organic matter in the soil.
Solution Before planting, firm the soil down. This can be done by carefully walking over the soil. Prepare the garden bed well to meet the specific needs of the crop you are growing.


Issue When your plants completely bypass the edible stage and begin to bloom and set seed.
Cause This is caused by a pause in the growth and can either be out of our control with erratic temperatures or in our control with inconsistent watering, root exposure or other stressors such as high pest or disease pressure. A variety grown in the wrong season may easily bolt.
Solutions Unfortunately, we can’t control the weather, however a consistent watering schedule throughout the life of the plant can help avoid bolting. Make sure the plant is planted at the correct depth, and a good thick mulch can help protect the roots, retain moisture and soften extreme temperatures in the soil. Ensure you have the right variety for the time of year – check the seed packet or label for suggested planting dates.

Holes in the leaves

Issue Finding holes in the leaves is a telltale sign something is eating your plants.
Causes There are a couple of likely suspects – slugs and snails and cabbage white caterpillars. There are other creatures that also enjoy eating brassica plants, too.
Solutions For the slugs and snails you can take a hands-on approach and collect them and remove or despatch them. A beer trap sunken into the garden will lure them to a boozy death, or use snail bait sparingly to keep them away.
For the cabbage whites, there is less risk in the winter months as the butterflies are less likely to be about. Monitor plants often and rub off eggs under the leaves or pick off caterpillars for a small problem. Exclude them with a net over the plant or garden bed. For a more serious problem there are organic and non-organic sprays that will take care of any caterpillars feasting on your leaves.

Stunting & wilting

Issue Failure to thrive and small plants, often with purple colouration. Wilts readily on hot days and even death. The roots will be swollen and distorted.
Cause Club root is a disease caused by a soil-borne micro-organism from the slime mould family. The population can build up in the soil over the years to reach problematic levels. Spores can remain in the soil for more than 20 years meaning you can’t grow brassica there again.
Solutions Practise crop rotation and don’t plant brassica in the same spot for at least three years to prevent build up of disease. Adding lime to the soil raises the pH and makes the soil less attractive.

Read more from Sarah, aka Sarah the Gardener, on topics ranging from under-cover growing to kids, in every issue.

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