Cole crops harmed by hot weather

Published: August 22, 2018

For lovers of cole crops or those relying on their production for income, 2018 has been a disappointing year thus far. The combination of a late spring and early summer has resulted in reduced yield and quality or, in certain cases, no yield at all. Commercial growers and home gardeners alike have been left wondering what went wrong this spring.

The term "cole crop" is derived from kohl, the German word for cabbage, and given to a number of cultivar groups of Brassica oleracea. In its uncultivated form, the latter is commonly known as wild cabbage. Significant cultivar groups of B. oleracea include Acephala (kale and collard greens), Botrytis (cauliflower and Romanesco broccoli), Capitata (cabbage), Gemmifera (Brussel sprouts). Gongylodes (kohlrabi) and Italica (broccoli). All likely were derived by artificial (human) selection from phenotypic variations of B. oleracea that appeared throughout antiquity. Today, cabbage, broccoli and cauliflower represent the three most economically important cole crops in the United States.

The cole crops are classified as "cool season" vegetables which grow best under cool, moist conditions. For example, cabbage carries on the greatest amount of photosynthetic activity (growth) at temperatures in the 59-68 degree F range. Properly hardened, many of cole crops can withstand temperatures well below freezing for short periods of time. Young plants tend to be more cold tolerant than older plants.

Conversely, cole crops do not respond well to hot weather. For example, cabbage totally stops growth at temperatures above 77 degrees F. Broccoli and cauliflower are even more sensitive to high temperatures. Any condition that causes stress on cole crops during their period of active vegetative growth can lead to crop failure. At our latitude, temperature stress is a frequent cause.

Because of their ability to withstand frost but their aversion to hot weather, cole crops typically are planted (outdoors) in Missouri in March or early April. Exact planting date depends largely upon latitude and local conditions. The hope is for plants to have sufficient cool weather to make significant growth before warm weather ensues. Cool temperatures provide the plant with an environment conducive for growth that results in the formation of heads, spears or curds in cabbage, broccoli and cauliflower, respectively.

In most years, this production regime results in an adequate crop spring crop for Missouri growers. In 2018, it did not. Most of Missouri was subject to a combination of climatic events this past spring that rarely, if ever, were experienced before. Borrowing from a movie title, it was the "perfect storm" for cole crop failure.

Weather data collected at Sanborn Field on the University of Missouri campus revealed that April 2018 was the second coldest on record. The abnormally cold temperatures resulted in below average growth during a period when cole crops usually make significant growth.

April was followed by much warmer weather in May and June. Again at Sanborn Field, weather data revealed May to be the warmest on record with an average high of 84 degrees F. The latter is well beyond the point cole crops stop growth. Additionally, there was no respite to high temperature stress in May. Every day the recorded high daily temperature was above the historical average.

The abnormally warm temperatures continued into June when the average high temperature at Sanborn Field was 89 degrees F and the average low 69. The latter is significant, since warm night temperatures result in elevated rates of respiration which depletes the meager amount of food manufactured by plants during the day.

This transition from being somewhat "behind schedule" (growth-wise) at the end of April to the stifling temperatures of May and June resulted in widespread cole crop failure during spring 2018. At best, yields were greatly reduced and quality was poor.

Additional temperature-related disorders that can reduce cole crop value include "riciness" and curd bracts in cauliflower and knuckling in broccoli, Ricing occurs when cauliflower flower buds develop, elongate and separate. Curd bracts are leaves that develop between curd segments. Both are caused by high temperatures. Knuckling, caused by fluctuating temperatures, occurs when broccoli makes uneven growth leading to a bumpy appearance.

The question arises then, "What can be done to improve the likelihood of a successful crop"? Cultural practices that allow for earlier planting in the spring so that cole crops can make significant growth before the advent of hot weather should be explored. These include the use of floating row covers, plastic mulch or high tunnels.

Depending upon thickness, row covers can provide from 4 to 8 degrees F of frost protection. At our latitude, this would allow cole crops to be planted three to four weeks earlier in the spring. The result would be longer exposure to cooler temperatures favored by cole crops before high temperatures slow growth.

Plastic mulch tends to warm the soil earlier in the spring. Research has demonstrated black plastic mulch increases soil temperature about 5 degrees F at a depth of two inches, when compared to bare soil. The combination of warmer roots along with favorable cool spring weather has been shown to hasten the maturity of cole crops from between one and three weeks. Thus, the crop has been harvested before temperatures get too warm. Clear plastic does an even better job of warming soil than black plastic, however, it does not suppress weed growth.

Finally, high tunnels are rudimentary, unheated greenhouses covered with a single layer of polyethylene plastic. Although they do represent an added production expense, their use has been demonstrated to increase day temperatures by 10 to 30 degrees F and offer up to four degrees of frost protection. The latter allows for the planting of most crops two to four weeks earlier than normal. Passively cooled, high tunnels provide ideal growing conditions for cole crops very early in the season. Once the crops have been harvested, warm weather vegetables can be planted to make additional use of the valuable space. For an extra early start, a combination of floating row covers placed over plants in a high tunnel has proven to be workable. Additional information about high tunnels can be found at http://hightunnels.org/.