Gadenin日本語Get the appCount backwards from the cold, not forwards from the seed packet
Radicchio does not respond to a calendar, it responds to falling night temperatures. The red pigment and the tight heart both appear when nights settle in the 40s F (4 to 10C) and days stay mild. Everything about the sowing date is really a way of arranging for the plant to be about two thirds grown when that weather arrives.
The practical rule is to sow 90 to 110 days before the first hard frost you normally get. That usually puts the sowing somewhere between early summer and the end of summer. Sown earlier, the plant reaches full size in heat, sprawls open, and often runs to flower. Sown later, it is still a small rosette when the ground goes cold and it simply stops, sitting there green and flat until spring.
A second, smaller sowing two to three weeks after the first is worth making every year. Autumn arrives at a different pace each season, and having two ages of plant in the ground means one of them will meet the cold at the right size. The one that misses is not wasted, it is a spring plant instead.
| Sowing window | What you are aiming at | Typical result | Best use |
|---|---|---|---|
| Late spring | An early autumn cut | Bolts or stays open and bitter in heat | Cutting leaves only, not heads |
| Early summer | Heads before the first hard frost | Reliable heading in a normal autumn | Main sowing |
| Late summer | Heads through late autumn | Heads late, or overwinters and hearts in early spring | Insurance sowing |
| Early autumn under cover | Overwintered plants | Tiny rosettes all winter, tight hearts in early spring | Cold-frame or tunnel growing |
Depth, spacing and the first three weeks
The seed is small, flat and pale, and it wants a quarter of an inch (5 mm) of cover and no more. Anything deeper than half an inch (12 mm) gives ragged, half-empty rows. Firm the bottom of the drill with the edge of a board before you sow so every seed sits at one depth and the row comes up as a single flush, then sift fine soil over the top and press it down with your palm.
Germination takes four to eight days at soil temperatures of 60 to 75F (15 to 24C). Above 85F (30C) it goes slow and patchy, which matters because most useful sowings go in when the ground is at its hottest. Sow in the evening, water the open drill before the seed rather than after, and cover the row with a board or a piece of shade fabric until the first shoots break the surface. Lift it the same day they appear or the seedlings stretch and never recover their stockiness.
Sow three or four seeds at each station and thin ruthlessly to one. Radicchio at half its proper spacing does not make two small heads, it makes two open rosettes that trap water in their centers and rot in the first wet week of autumn. The spacing is what makes the head.
- Water the drill first Run water along the open drill and let it sink in before the seed goes down. Watering after sowing floats the fine covering away and drags the seed into clumps at the low points of the row.
- Sow in stations Three or four seeds every 10 to 12 inches (25 to 30 cm), with rows 14 to 18 inches (35 to 45 cm) apart. Wide-leaved types that spread before they close want the wider end of both ranges.
- Thin at the second true leaf Leave the single sturdiest seedling at each station and snip the rest off at ground level rather than pulling them, so the survivor's roots are not lifted.
- Check the spacing again a fortnight later Once each plant has five or six leaves, walk the row and remove anything that has crowded back in. This is the last moment when spacing can still be fixed without losing the head.
Thinnings are perfectly good eaten small and green. What they are not is a second crop left in place, so do not leave two plants at a station in the hope that both will heart.
Sowing in modules and moving plants out
Radicchio transplants more willingly than most chicories, which is useful when the ground is still full of a summer crop at the right sowing moment. Sow two seeds into each cell of a module tray, thin to one, and grow the plants on in the shade of a wall or under fabric where the compost will not dry to dust between waterings.
Plant out at four true leaves, no later. A module plant held too long spirals its roots and then sulks for two or three weeks after planting, and that check in growth is one of the most reliable ways to trigger bolting. Set the plants at their final spacing, water them in properly the same evening, and shade them for two or three days if the weather is bright.
Set the crown at exactly the depth it grew in the tray. Buried too deep, the lowest leaves sit on wet soil and the base of the plant goes soft. Planted proud with roots showing, the plant rocks in wind and stalls.
Bolting, and the difference between bolting and waiting
Radicchio is biennial. Its natural plan is a rosette and a root in the first season, a winter, and a flower stem in the second. A young plant that is checked hard, by heat, by drought, or by a cold snap followed by warmth, can be persuaded that its winter has already happened and will send up a flower stem in its first season. When that happens the head never forms, the leaves turn woody, and the bitterness goes from pleasant to inedible. Pull the plant; there is no way back.
What looks like failure far more often is not bolting at all. A summer-sown plant that sits green, loose and open through warm weather is behaving exactly as it should. It is holding, waiting for the nights to drop before it closes and colors. Every year gardeners pull perfectly good plants in early autumn because they are still green. Look at the growing point: a bolting plant has a thickening stem rising out of the center, while a waiting plant has a flat, tight cluster of small leaves at the crown.
Record the sowing date and which end of each row it went into, along with the date of your first real frost. After two seasons those two numbers together will tell you your own correct sowing window better than any general advice can.