Gadenin日本語Get the appCold nights are what make the head, not time
Radicchio has two separate things it needs to do, and both are triggered by cold rather than by age. The first is closing: the inner leaves stop expanding outwards and start wrapping, pulling the rosette into a firm heart. The second is coloring: the green pigment fades from the leaf blade while red pigments build, until the leaf is wine-colored with a white midrib. Neither happens reliably while nights stay above 55F (13C).
The weather that does it is a run of nights in the 40s F (4 to 10C) with days that stay mild and bright. Two weeks of that changes a plant completely. A single cold night does nothing, and a cold snap followed by a warm week can even stall the process, which is why the head often seems to appear all at once after a slow month of apparently nothing.
This is also why a plant that heads beautifully in a cold autumn will make a loose, half-green head in a mild one. The plant is not inconsistent. It is reading a different autumn.
| Night temperature | What the plant does | Leaf color | What to do |
|---|---|---|---|
| Above 60F (16C) | Grows outward, stays a flat open rosette | Fully green | Keep it watered and wait |
| 50 to 60F (10 to 16C) | Slows outward growth, inner leaves begin to cup | Green with faint red veining | Stop feeding with nitrogen |
| 40 to 50F (4 to 10C) | Wraps and firms the heart | Reddening from the veins outward | Keep the crown dry and open |
| Below 32F (0C), light frost | Heart firms further, sugars rise | Deep red, outer leaves scorched | Strip damaged outer leaves and check firmness |
A green, loose summer plant is not a failure yet
This is the single most common mistake with radicchio. A plant sown in early summer will spend the whole of late summer looking wrong: flat, sprawling, green as a lettuce, with no sign of a center. Gardeners conclude they have grown the wrong thing or done something wrong, and they pull it or let it go to weeds. Three weeks later, when they would have had a head, the row is empty.
Hold your nerve. The plant is doing the correct thing in the wrong weather, which is to build the leaf area and the root that will pay for the head later. As long as the crown is flat, the leaves are healthy and there is no stem rising from the center, the plant is on schedule. Judge radicchio in the middle of autumn, not at the end of summer.
The one thing worth doing during that green phase is to stop feeding it. Nitrogen given late keeps the plant in the outward-growing mode you are trying to leave behind. Once the days start shortening noticeably, the plant should be living on what is already in the soil.
Mark on your notes the date the first leaves show red veining. That date, compared with your sowing date, is the most useful single number you can carry into next season for setting your sowing window.
Cutting a stubborn plant back to a stump
If a plant is still open and green well into autumn and the nights are already cold, there is a reliable trick that seems brutal and works. Cut the whole plant off horizontally about an inch (2 to 3 cm) above the crown, taking every leaf and leaving a bare stump with a live growing point in it. Clear the cut leaves away from the row rather than leaving them to rot around the stumps.
What regrows from that crown is different from what you cut off. Because the plant is now regenerating in cold weather with a large root behind it and no leaf canopy, the new growth comes tight, small and strongly colored, and it wraps into a dense red heart instead of an open rosette. It is often better looking and firmer than a head that formed the ordinary way.
Allow four to six weeks from cutting to a usable heart, more in cold, dull weather. The stump looks dead for the first ten days, which is normal. Only give up on it if the crown itself goes soft and brown, which means rot rather than dormancy.
- Choose the plants worth cutting Cut only plants with a thick, healthy crown and no flower stem. A thin or bolting plant has no root reserves to rebuild from and will just sit there.
- Cut clean and high Take the leaves off in one horizontal cut about an inch (2 to 3 cm) above the crown with a sharp knife. Cutting into the crown itself removes the growing point and kills the plant.
- Clear and open up the row Take all the cut leaves away. Wet leaf litter sitting against a fresh cut is the one thing that turns this into rot instead of regrowth.
- Protect from heavy rain, not from cold A cloche or a low cover keeps water out of the open crown. Do not warm the plants; the cold is what makes the regrowth tight and red.
- Check firmness weekly Squeeze the new head gently after a month. Cut it when it resists like a firm apple rather than giving like a cabbage leaf.
Things that stop a head forming at all
Crowding is the commonest cause. Plants closer than about 10 inches (25 cm) shade each other's crowns, and a shaded crown stays flat. Too much nitrogen is the second: a well-fed plant keeps making big soft outer leaves right through the cold and never switches modes. Shade is the third, since a plant under a hedge or on the north side of taller crops gets fewer of the bright mild days that go with the cold nights.
A late check in growth will also do it. A plant that dries out hard in early autumn and is then flooded often responds by making a mass of new outer leaf rather than closing. Water evenly, and if you have to choose, err on the dry side once the nights turn, because a slightly hungry, slightly dry plant hearts more readily than a lush one.