Radicchio

Sow in summer for a fall crop, not in spring

Sow seed 1/4 to 1/2 inch deep in midsummer, either directly in the bed or indoors in flats about four weeks ahead of setting plants out. Aim to have plants sized up as the nights start cooling, and keep the seedbed shaded and damp if soil is running hot. Radicchio heads properly only as temperatures fall, and sustained heat above 75F pushes it to bolt instead of forming a head. A crop that matures into cooling fall weather colors up red and stays sweet, while a spring sowing runs to seed as days lengthen.

Set plants out at head spacing, about four weeks after sowing

Move transplants to the bed about four weeks after sowing, in soil held between pH 6.0 and 6.8. Space them 10 to 12 inches apart for full-size heads, or 6 to 8 inches apart if you want mini heads about 4 to 5 inches across. Radicchio wraps a head rather than spreading, so spacing sets head size directly. Crowded plants trade head density for leaf, and heads that touch hold water between them and rot at the base.

Keep it at an inch of water a week, and do not let it check

Give the bed about one inch of water per week counting rainfall, and keep it steady rather than letting the bed swing between bone dry and soaked. Expect to water this crop a little longer into the season than you would lettuce on the same ground. Radicchio needs a longer run to maturity than lettuce, so it sits on the same shallow root zone for more weeks and needs the extra irrigation to finish. Steady moisture matters more than volume: when watering or rain follows a dry spell the plants resume growth suddenly, and that check-then-surge is what shows up as tipburn, the browning of leaf edges inside the head.

Work in a complete fertilizer, then side-dress at four inches tall

Work a complete fertilizer into the bed at about two pounds per 100 square feet before planting. Once the plants stand about four inches tall, side-dress with roughly one pound for every 25 feet of row and water it in. The whole crop is leaf, so head weight tracks nitrogen supply during the frame-building weeks. Feeding the plant while it is still open and growing fills the head out before cold shuts growth down.

Forcing types: cut back to the crown, then lift the roots to force

If you are growing a forcing type such as Red Verona or Treviso, cut the leaves off to within 1 inch of the crown about two to three weeks before your first frost. Then dig the roots and store them in a burlap bag in a cool, dark place - the head forms in storage, not in the bed. Do not leave the cut-back plants in the ground and wait for them to re-head. Forcing types do not wrap a tight head on their own in the field. Cutting the top away and lifting the root sends the root's stored reserves into a fresh, blanched, tightly folded head, and the cool dark storage is what blanches it. Leaving a cut-back crown in the bed through frost gives you neither the old leaves nor a forced head.

Scout for aster yellows, cutworms, slugs and flea beetles - pull, do not spray

Walk the bed weekly. Pull and destroy any plant that is stunted with yellowed leaves and stems, and keep perennial broadleaf weeds knocked back around the planting. Watch also for cutworms, slugs and flea beetles chewing young plants. Aster yellows is a phytoplasma carried by aster leafhoppers, and once a leafhopper picks it up it transmits the pathogen every time it feeds for the rest of its life. An infected plant never recovers, so it is only a reservoir the next leafhopper feeds from. The pathogen also survives in perennial weeds from one season to the next, which is why clearing them out of the planting matters.

⚠ Do not reach for an insecticide here - leafhoppers are quick-moving and sturdy and are hard to manage effectively with insecticides, with few effective products available to home gardeners. Roguing infected plants and clearing perennial weed hosts is the control that works.

Cut when the head is full and solid, and let frost hit it first

Squeeze the heads and cut at soil level once they feel full and firm rather than spongy. Let a few frosts pass over the crop before you cut if you can, and trim off outer leaves the cold has burned. Store the cut heads cold and damp, at 32 to 34F and near-saturated humidity, where they will hold two to three weeks. Firmness, not the calendar, is what says the head has finished wrapping - days to maturity shift by variety and by how warm the run-up was. Cold weather deepens the red and takes the hard edge off the bitterness, and the tops shrug off short spells down to about 20F. If frost kills the outer green leaves, the red head underneath is usually fine.