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The white, cream, pink or purple center of an ornamental kale is not a flower and it is not a sign of ripening. It is the plant withdrawing green pigment from the youngest leaves at the middle of the rosette once the nights turn properly cold, which uncovers the color that was underneath all along. The trigger is sustained low night temperature: expect the change to begin after a run of nights below about 50F (10C), and to deepen sharply once nights sit near or below freezing. Two or three cold nights in a warm week will not do it.
This means a plant that is still entirely green in autumn is waiting, not failing. It is the commonest worry with this plant and almost always the wrong one. Warm autumn nights simply postpone the display, and a mild spell in midwinter can even push a little green back into a head that had already colored. Nothing you add to the soil or the water speeds it up. The only lever you have is a cold, open, sunny position, and patience.
| Night temperature | What you see in the center | What to do |
|---|---|---|
| Above 59F (15C) | Solid green throughout | Nothing. It is too warm. Keep it growing wide |
| 50 to 59F (10 to 15C) | A pale wash at the very center | Stop all feeding now if you have not already |
| 41 to 50F (5 to 10C) | Color spreading out leaf by leaf | Water sparingly. Full sun all day |
| Below 41F (5C), including frost | Full, saturated color | Leave it. Frost improves it rather than damaging it |
Feeding in autumn keeps it green
This is the mistake that ruins more plants than any pest. Nitrogen tells the plant to make more green leaf, which is precisely the opposite of what you want once the cold arrives. A well fed autumn plant answers the cold by pushing new green growth from the center, so the color comes late, comes weak, or is buried under fresh green leaves as fast as it appears. The plant looks vigorous and healthy and completely fails at its one job.
Feed early or not at all. A single dose of balanced fertilizer at planting, or compost worked into the bed beforehand, carries the plant through the growing weeks when you actually want leaf. From roughly six weeks before your first frost, stop entirely. If a plant in a container was potted into a compost with slow release fertilizer mixed in, that feed will still be releasing into autumn, which is why container plants are so often the greenest ones in the garden. In that case there is nothing to do but wait for a harder frost to overrule it.
A plant that colors late is still worth keeping. The display holds right through the cold months and often looks best in the deepest part of winter, so a head that only turns after the first hard frost still gives you many weeks. Note the date color first appeared, alongside the sowing date, and you will know next year exactly how much earlier to sow.
Water, light and the position it wants
Full sun, all day, is not optional. In part shade the rosette opens out loosely to catch more light, the leaves get larger and thinner, the stem lengthens, and the color when it finally comes is muted. Six hours minimum, and in winter the more the better because the sun is low and weak. Somewhere open and exposed, where cold air settles rather than drains away, gives better color than a sheltered corner against a warm wall.
Water is straightforward while the plant is growing in late summer heat: keep the soil evenly moist, because a plant that dries out and checks will shed its lower leaves and expose the stem. Once the weather turns cold, cut back hard. The plant is barely growing, evaporation is low, and wet cold soil rots the crown and the roots. In a container, wait until the top inch (2.5 cm) is dry, then water in the morning so the foliage is dry by nightfall.
Water at soil level rather than over the top. Water sitting in the cup of the rosette on a freezing night is one of the few ways to genuinely damage this plant, causing a soft, brown patch at the growing point that never grows out.
Lower leaves that yellow and drop
Old leaves at the outside of the rosette will yellow, go limp and eventually rot off through the winter. This is normal ageing, not disease, and it accelerates after a hard frost. Pull them off cleanly with a downward twist as soon as they go soft. Left in place they hold moisture against the stem, encourage gray mold, and shelter slugs.
Removing them does expose more stem, which is why the sowing date and the planting depth matter so much. There is a limit to how many you can afford to lose. If leaves are dropping faster than the plant is making new ones at the center, the usual causes are drought at some point in autumn, shade from a neighboring plant, or roots sitting in cold wet soil.
It is edible, but it is not a salad
Ornamental kale is the same species as the kale grown for eating and it is not poisonous. It has simply been selected for color and rosette shape rather than for texture, so the leaves are coarse, thick and often noticeably bitter, especially the outer ones. The colored center leaves are the mildest but they are also the entire point of the plant, so cutting them defeats the purpose.
Treat edibility as a fact rather than a plan. It is genuinely useful as a garnish, where a single small colored leaf on a plate does real work, and outer leaves are fine chopped small and cooked long in a soup or stew. Anything raw and in quantity is a disappointment. If plants have been treated with anything not cleared for food crops, do not eat them at all.