Managing nitrogen timing in summer-harvested broccoli

Picture of tight and loose heads, with cross-section of the heads
Late nitrogen application can cause early side-branch elongation, making the heads less desirable on the market. The top image shows a tight head with solid side branches. The lower image has a head that has begun to loosen at the sides. This effect can be reduced by having the last nitrogen application at least four weeks before harvest so that the growth rate is slowing down a bit just as harvest begins.

One of the challenges with raising broccoli in the East is getting heads to stay dense. In the warmth of summer, the outer branches of broccoli tend to start elongating a little before harvest maturity. They “blow up” in the words of many producers. The result is a head that doesn’t pack tightly in the box and has soft edges that are prone to damage in handling.

The solution is to let growth slow a little during the week before harvest. Growth is promoted by the combination of warmth, water, nitrogen, and sunlight. Warmth is a given for harvests in July and early August, sunlight we have no control over, and abundant water sometimes comes whether we want it or not. The main management tool is nitrogen.

Slowing growth by reducing nitrogen is a considerable challenge because abundant nitrogen is needed during the vegetative growth to get strong, healthy, fast-growing plants. The best approach is to supply nitrogen relatively early in the growing period, and not add nitrogen in the last four weeks.

Many popular broccoli varieties are harvested starting only eight weeks after transplanting. Therefore, the last nitrogen application should be only four weeks after transplanting. At that time, the foliage is near full cover, which a good time for a traditional side-dress application as well as cultivation to get escaped or newly germinated weeds. Fertigation through a trickle-irrigation system would be during the fourth week. At that time, the plants are large enough to take up the nitrogen but not so far along that excess growth at harvest will cause loose heads.

Applying all of the nitrogen before planting is a possibility. Ordinarily, applying 120 to 150 pounds per acre of nitrogen preplant is ill-advised because of the high likelihood of leaching before the crop takes it all up. However, because broccoli is only in the ground for about nine weeks through the end of harvest, and reaches its maximum uptake five weeks after transplanting, the risk of leaching loss is relatively low compared to the typical situation. Pre-plant application of the fertilizer opens up production options that don’t allow side-dressing or liquid fertilization.

This early-nitrogen approach is also helpful in reducing hollow stem. Hollow stem is likewise a symptom of excessive late vegetative growth. The main tool for managing hollow stem is adjusting the plant population. If hollow stem is a problem, it’s likely that both yield and quality will be improved by spacing the plants closer together. In New York we have found an in-row spacing of 8 inches to work quite well. But limiting late nitrogen also tempers the growth rate at the right time.

This article was published in VegEdge on June 5, 2019. A publication of the CCE Cornell Vegetable Program.