University of Vermont Extension 
Department of Plant and Soil Science

Summer News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
Cabbage and kale were named as vegetables of the year in 2007 by the National Garden Bureau to give them wider recognition.  Their many good qualities often may be overlooked, such as their excellent growth in cool, northern climates; their hardiness even after frosts; their ornamental value, even mixed in flower gardens; and their high nutritional value.  If you are growing, or decide to grow, these and related cool crucifer crops (those in the crucifer family such as broccoli and cauliflower), there are several problems that might make your plants not look or grow like they should.  Several solutions relate to proper culture.

If cabbages don’t form the traditional “heads” or tight group of leaves in the center, perhaps they are a loose-leaf variety.  Otherwise, failure to produce heads may be from improper growing conditions.  Cabbages need to be planted early in the season, even before last frost, in order to form heads before daytime temperatures routinely get above 80 degrees F.   This timing of heat of course varies from year to year.  Loose, puffy heads also can be caused by too much fertility, and improper watering—too much or too little.

On the other hand, cabbages may form nice heads only to split at harvest time.  This is more often a problem on varieties that mature during warm weather.  Keep plants uniformly moist, particularly at this stage of growth.  Splitting also might be prevented by root pruning, or cultivating near stems, when heads are about mature.
If heads don’t form, but instead send up a flower stalk, this is referred to as “bolting.” This is caused by too high temperatures, or when growth resumes after a prolonged cool period.  If you see this in mid-summer, consider planting another crop in late summer to mature during the cooler temperatures of fall.

Dark or blackened areas on the inside leaves of cabbage heads is termed “internal tip burn.”  It could have many causes including insufficient fertility when heads are forming or too much when heads are mature, low soil moisture, or a deficiency of boron or calcium.  A soil test from your local Extension agency can determine if nutrition is incorrect.

If you harvest cabbage, and it has black streaks from the core stem into the head, this is most likely black rot.  You can tell it easily from the foul-smelling decay.  This disease is caused by a bacteria that comes in on the seeds, so the only control is to plant resistant varieties.  Watch for this disease on non-resistant varieties, and if the plants are flooded in heavy rains.

Black, circular spots on the outer leaves, as the heads mature, usually are caused by Alternaria leaf spot disease.  Yellow lesions on the outer leaves, with downy growth beneath, is caused by downy mildew disease.  Check with your local complete garden store for proper and least toxic sprays for these diseases, and for possible insects, and follow all label precautions.

There are several insects to watch for on cabbages and related crucifers.  The most common are the cabbage looper and imported cabbage worm, which can destroy plants.  Slow and stunted growth may be caused by soil aphids on roots, or green aphids on leaves. 

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