Dr. Ajay Nair
Associate Professor and Extension Vegetable Specialist
Department of Horticulture
Iowa State University
Most vegetable growers in Iowa are selling their produce through direct-to-consumer markets such as CSAs and Farmer’s Markets. Broccoli (Brassica oleracea var botrytis) is one of the staple crops widely sold in these markets. It is a member of the mustard family of plants and is closely related to Brussels sprouts, cabbage, cauliflower, kale, and kohlrabi. Broccoli is well-adapted to all areas of the state. The edible part of broccoli is the immature flower head. There are two distinct forms of broccoli: heading and sprouting broccoli. Heading broccoli is the form most commonly grown in the United States and is characterized by its branching cluster of green flower buds atop a thick, green flower stalk, with smaller clusters that arise like sprouts from the stem. The sprouting broccoli makes a dense, white curd like cauliflower.
In Iowa broccoli is typically planted early to mid-April and harvested mid-to-late June. Given the time of harvest it is often challenging to preserve the quality of broccoli in open fields. Broccoli head is susceptible to broccoli brown head (associated with warm temperature), head rot (favored by cool temperatures and prolonged periods of moisture), and other physiological disorders. One of the major challenges growers face with broccoli is the ‘hollow stem’. It is initiated by mechanical separation of stem tissue during rapid expansion and growth which leads to hollow stem (Fig.1 A and B). The precise answer to occurrence of hollow stem is still not known, however, Boron (B) deficiency, high nitrogen (N) application, and plant spacing are often considered to be the cause. If hollow stem is accompanied with browning of heads, boron deficiency may be a causal factor. Wider plant spacing can also lead to hollow stem. Broccoli is often planted on raised or flat beds with or without plastic mulch. The ideal plant spacing should be 10-12 inches within row and 12-15 inches between.
Given the role of B, it is recommended to apply required B based on soil test results. Boron plays a major role in the cell wall biosynthesis that primarily influences many growth factors including tissue differentiation, transport of carbohydrates and proteins, and cell membrane functions. Soil organic matter (OM) is the primary source of B, which can be readily leached from soils with excess rainfall and irrigation. Leaching of B is greatest in coarse-textured soils with low organic matter. In vegetable crop production, B is enough if soil test shows 0.7 ppm (Mehlich III extraction method). If lower than that, it is recommended to apply B. Soil application can be made using Borax or Solubor. Suggested rate of 0.5 to 3.0 lb. B/acre depending on crop sensitivity and soil type. It is advisable to check with vegetable specialists before applying B as it is a micronutrient and over application could lead to imbalances and toxicity issues. Broccoli is a heavy feeder so providing optimum N is critical for high yield and quality. N application rates could range from 120 to 180 lb. N/acre. Higher than required application can significantly increase the chances of hollow stem.