Napa Cabbage (Chinese Cabbage)

If you want a spring crop, start seedlings indoors and set them out only once you are past any stretch of 10 or more days between 35 and 50 F. Do not direct-sow early into cold soil. Choose a variety with real bolt resistance - the Connecticut Agricultural Experiment Station's Bulletin 1061 Brassica Trials found Bilko, Nikko, and Rubicon still made good spring heads while Yuki, Taranko, Jazz, Apollo, and Monument bolted. A prolonged cold period after the plant is already growing vernalizes it, and a vernalized plant switches from making a head to making a seed stalk. The 10-days-between-35-and-50-F trigger is Clemson HGIC. Spring yields in the CAES Bulletin 1061 trial ran 11.6 T/A against 14.3 T/A in fall almost entirely because of bolting.

Direct-sow a half inch deep in midsummer, timed so the heads fill during the cooling, shortening days of autumn. Chinese cabbage takes about two months to mature, so back up roughly 60 days from your average first hard freeze and add a week of margin. Sow three seeds every 18 inches for full-size napa heads, then thin to one plant per spot after emergence. This is a fall crop everywhere it is grown well. Heat and long days push it to bolt instead of head, so the goal is to make the plant do its heading in weather that is getting cooler, not hotter.

Give the crop about one inch of water per week from rain or irrigation, more on sandy ground. Dig down with a trowel to confirm the water actually reached the root zone instead of assuming it did. Water in the morning so the foliage dries. Organic mulch over the bed helps hold the moisture and keeps the soil temperature down. This plant grows fast and shallow, and an interrupted growth curve is what produces a loose head or a bolt. Steady moisture also keeps soil temperature from spiking, which is one of the triggers that makes a susceptible plant abandon leaf growth for flowering.

Work a complete fertilizer such as 5-10-10 into the bed at about 3 pounds per 100 square feet before planting. Then side-dress once the plants are roughly half grown and before the head starts to close, using 34-0-0 at 1 pound per 100 feet of row or calcium nitrate at 2 pounds per 100 feet of row. Band it along the row 4 to 6 inches out from the stems and keep it off the leaves. Side-dress again if you are on sandy soil or heavy rain has leached the bed. Napa cabbage is a heavy nitrogen feeder building a lot of leaf in a short window, and any check to growth in that stretch gives you a loose, small head. The nitrogen has to land while leaves are still expanding, because once the head closes the plant cannot use it.

Starting right after the plants come up or go in, check both the upper and lower leaf surfaces at least once a week, and more often as the season warms. You are looking for imported cabbageworm, cabbage looper, and diamondback moth larvae plus their frass. Treat while the larvae are still small. Using the UMN cole-crop thresholds: from transplant until the head starts to cup, act when 30 percent of plants carry one or more medium-to-large imported cabbageworm larvae, or 50 percent carry five or more diamondback moth larvae. Once heads are cupping, tighten to 10 percent of plants infested. These thresholds were set for cabbage, not napa specifically, so treat them as a guide. Caterpillars bore into the folding head and tunnel where no spray can reach, so the whole decision has to be made before the head closes. Small larvae are also far easier to kill than large ones, which is why the threshold tightens once cupping starts.

When scouting puts you over threshold, spray Bacillus thuringiensis and cover the leaves thoroughly, including the undersides. Bt only works if the caterpillar eats it, so coverage and small larvae are the whole game - repeat as new hatches appear rather than pushing the rate. If Bt is not holding, spinosad is the next step. Skip the broad-spectrum pyrethroids unless you have no alternative; they wipe out the parasitic wasps that are doing much of this work for you. Bt is a stomach poison specific to caterpillars, so it takes out the cabbageworms and loopers without touching the predators and parasitoids that suppress the next generation. Timing it before the head closes matters because larvae that get inside are untouchable.

Lay a lightweight floating row cover over the bed the day you sow or transplant, and keep it on through the first few weeks of growth. Anchor the edges with soil or pins and hold the fabric up on wire hoops so wind cannot whip it against the seedlings. Napa cabbage needs no insect pollination, so the cover can stay on as long as you like. Flea beetles, cabbage maggot flies, and the cabbageworm and looper moths all arrive to lay on young plants, and a physical barrier at that stage prevents the damage rather than treating it. The cover also buffers wind and cold during establishment.

Squeeze the head: when it feels firm and dense, cut it off just above the outer leaves. Do not leave a mature head standing in the garden hoping it will size up more. Harvest the whole planting as it comes ready rather than picking at it. Napa cabbage has no holding ability once it matures. A finished head left in the ground splits, or the plant reads the mature head as the end of its job and throws a seed stalk straight up through the middle of it.