Kale in September: This Month Decides Your Winter Harvest
You planted it in summer and you'll harvest from October — but the leaves that carry you through winter are forming right now. Here's what matters in these weeks.
Kale is the plant still standing in your raised bed long after everything else has been cleared away. Nothing spectacular happens in September — and yet this is the month that decides whether you’ll be picking thick, firm leaves in January or just a few nibbled leftovers. Here are the points that matter now.
September Is for Care, Not for Planting
Kale goes into the ground as a young plant in June or July. Planting it now leaves far too little time — you simply won’t get a usable plant before winter.
What’s already standing in your bed, on the other hand, is only just hitting its stride: September and October are the weeks when most of the leaf mass you’ll later pick is formed. Harvest then runs from October right through to March. Which is exactly why what you give the plant now — calm, protection and stability — makes the difference.
Cabbage White Butterflies: The Last Round
Kale is a brassica, and cabbage white butterflies know it. They lay their final eggs of the year in September, and the caterpillars chew through exactly the leaves meant to carry you through winter. Check the undersides of the leaves and the heart of the plant twice a week and pick off eggs and caterpillars by hand. Children usually take to this job with far more enthusiasm than adults do.
If things get out of hand, a Bacillus thuringiensis product works specifically on caterpillars and nothing else. The calmer route is fine mesh insect netting: lay it over the plants properly once and you’re done until the first frost. Mealy cabbage aphids like to hide in the heart leaves — a soft soap solution sprayed into the leaf axils usually sorts them out.
Less Nitrogen, More Stability
Kale is a heavy feeder — but that applies to spring and summer, not to September. Piling on nitrogen-rich fertiliser now gives you soft, water-filled tissue, and that is exactly what frost damages first. A thin layer of mature compost around the plant is plenty.
What matters more right now is stability. Kale grows a good 80 centimetres tall on a single stem, and autumn storms love to flatten it. Mound a few handfuls of soil around the base of the stem and press it down firmly. For particularly tall plants, a stake beside them does the same job.
The Frost Myth, Sorted Out
“Kale needs frost or it won’t taste good” — you hear it everywhere, and it’s only half true. What actually happens is that during sustained cold the plant converts starch into sugar over several weeks, a perfectly ordinary antifreeze mechanism. So it’s the cool spell that shapes the flavour, not one single frosty night.
On top of that, many newer varieties already taste mild in October without any need to wait for sub-zero temperatures. Harvest from the bottom up, leaf by leaf, and leave the heart of the plant standing. It will keep pushing out new growth all winter, so you get fresh leaves again and again instead of everything at once.
Who Belongs Next to It — and Who Doesn’t
Good neighbours for kale are peas and beans, because they bring nitrogen into the soil, plus lettuce, potatoes and tomatoes. It does not get along with strawberries — or with mustard, and mustard is a special case: it’s a brassica itself and therefore a host plant for clubroot. Sowing it as a green manure right next to or after cabbage crops risks bringing in the very disease you were trying to avoid.
The better green manures after brassicas are phacelia or buckwheat — but both need to go into the ground by August at the latest and neither tolerates frost. For a sowing in early September, winter rye is usually the only option left in most regions; note the other two down for next season instead.
For your crop rotation, that means: wherever kale is growing this year, no other brassica should follow for the next three to four years. Make a note of its exact position now — come spring you definitely won’t remember.
Your Bed Planner Shows It Instantly
In Rootivo, the bed planner draws companion-planting lines straight from your kale — green to the neighbours that work, red to the ones that don’t. And because every planting is saved with its year, the crop rotation check speaks up on its own next spring if you try to put brassicas back in the same spot.
Im Lexikon: Grünkohl