There’s no better way to avoid potential plant problems than to know when and how they might occur, and to be ready for them so that they do not cause severe problems.
One group of vegetables that require special knowledge are the cole crops. The word “cole” comes from the German word “kohl,” which means cabbage. So cole crops are members of the cabbage family, along with Brussels sprouts, Chinese cabbage and kohlrabi. (Kohlrabi, translated from German, means “cabbage turnip.”)
The ideal temperature for cole crop growth is between 60 and 68 degrees F, making them cool season vegetables. In the northeastern U.S., cole crops can successfully be grown in spring and autumn, when temperatures occur easily in that range. Cabbage can withstand short durations of temperatures as low as 25 degrees, while broccoli and cauliflower plants tolerate light frosts. The upper limit for cole crop growth is about 80 to 85 degrees.
All these crops lose quality when temperatures exceed 80 degrees. Cabbage is the most heat tolerant, but prolonged high temperatures cause puffy heads with long cores and increased tip burn. High temperatures cause broccoli and cauliflower heads to become loose and branchy and may favor the development of bracts (leaf-like structures) in the heads. Broccoli buds turn yellow and flower rapidly in hot weather, while cauliflower buds develop a fuzzy, “ricy” appearance.
In young broccoli and cauliflower plants, there is a fine balance between vegetative and reproductive growth. If the balance tips toward reproduction while the plant is young, root and leaf growth are restricted and the developing head becomes exposed on a small plant. The head never reaches marketable size and soon breaks up as the flower stalks elongate.
This “premature” head development is called buttoning. Buttoning is most likely to occur when well-developed young plants experience stress, especially cold temperatures. Follow these guidelines to help prevent buttoning:
- Keep soil temperatures above 60 degrees and daytime air temperatures above 70 degrees while growing transplants in the greenhouse. Avoid temperatures below 50 degrees. Harden by gradually reducing water or withholding nitrogen for the final week before field planting, rather than by lowering temperatures, but do not stress the plants.
- Set plants in the field when they are about four weeks old. Plants more than five weeks old are especially likely to button. Avoid transplant shock. When the largest leaves are three to four inches wide or wider, exposure to field temperatures below 50 degrees for more than four days promotes buttoning.
- Prevent nutrient stress. Apply recommended pre-plant fertilizers. Use starter solutions when transplanting. Side-dress young plants with nitrogen as needed.
- Prevent moisture stress. Plants grown in cold, wet soils experience slow growth. As plants develop, irrigation may be required.
- Prevent stress from weeds, pathogens and insects.
- Buttoning is more common in cauliflower than in broccoli. Also, varieties differ in their tendency to button. The earliest maturing varieties are most likely to button.
Young plants of biennial vegetables, including cabbage and cauliflower, which are exposed to prolonged temperatures below 50 to 55 degrees are likely to show bolting (premature flowering) later in the season. Bolted plants are worthless for market. Set plants in the field when they are about four weeks old. Following the same temperature guidelines shown above for the prevention of buttoning also will minimize bolting.
Broccoli should be cut when heads are three to six inches in diameter and before the flower buds open to show any yellow. Leave about eight inches of stem when cutting heads. On some varieties, side shoots will develop off of the main stem after the central head is harvested. These also can be harvested.
Harvest cabbage when the head feels firm and solid when pressed with the thumb and fingers. Split heads are a sign of over-maturity. Allow some wrapper leaves to remain on the head to protect it in transit to market.
Most cauliflower varieties have to be tied to keep sunlight off the curds (heads) in order to maintain a white color. This process of excluding light is called blanching. Blanching is accomplished by using a rubber band to tightly cover the curds with the outer wrapper leaves. Do not choose “self-blanching” varieties, as these varieties sometimes do not fully cover the developing curds, especially under warm temperatures.
When tying cauliflower, check the field every three to four days as maturity approaches. Any plant with a visible white curd of at least one inch in diameter should be tied by gathering the leaves together and placing a rubber band or piece of twine around them. The same color band should be used for all plants tied on the same day to enable harvesting of all plants with the same color band at once. Three or four days later, repeat the process with a different color band. Avoid cracks or spaces between leaves that will allow sunlight to penetrate and discolor the head.
After the heads form, they grow rapidly. Starting four days after tying, check several heads to see if they have attained market size. Time from tying to harvest varies from about four days to two weeks, depending on the temperature and available moisture. Each head should be six to eight inches in diameter and weigh 1.5 to two pounds. Good quality cauliflower is pure white, well domed, tight, solid and clean. There should be no hollow stem and no insect or disease damage on the head. If 75 percent of the heads are ready for harvest, cut all heads tied the same day as those checked. Although some small heads may be harvested, this process eliminates the need to check every head individually. Before packing, wrapper leaves must be trimmed to about one to two inches above the head. If heads are film-wrapped, the wrapper leaves are trimmed flush with the top of the head.
During damp, humid weather, the curd is very susceptible to rot-causing pathogens. Tying leaves tightly around the curd to exclude light will increase the chances of disease. In such cases, tie the leaves together as high above the curd as possible to allow some air movement.
Kohlrabi is easily grown from seed, taking only about 45 days from seeding to harvest. The plant part eaten is a swollen above-ground stem which forms a bulb, measuring about three inches in diameter. It has a turnip-like flavor, but much milder. Kohlrabi is desired by people of European descent, and small quantities are easily sold at retail farmers markets.
Brussels sprouts take 60 to 90 days to mature. They do best as a transplanted autumn crop in areas where temperatures are cool. To hasten the maturity of the individual sprouts, pinch out the growing point (top) of the plant. This breaks the plant’s apical dominance, causing the sprouts to quickly develop.