Eating quality of Brussels sprouts improves after a light frost, and the plants will survive down to 20º F. Photo courtesy of Becky Sideman

by Sonja Heyck-Merlin

“Brussels sprouts have become more popular in the last 10 years,” said University of New Hampshire (UNH) Extension Fruit and Vegetable Production Field Specialist Heather Bryant. Her recent presentation was the last in the UNH webinar series “North Country Lunch and Learn.”

“When I was a kid, Brussels sprouts were what your parents fed you when they were mad,” she said. Their rise in popularity is in part due to the breeding of varieties lower in glucosinolate – a chemical that creates the bitter flavor many recall from their childhoods. The vegetable is also nutrient-dense and low in carbohydrates, making them attractive to people on ketogenic or low-carb diets.

Named after the capital of Belgium, Brussels sprouts are members of the Brassicaceae family and are grown for the sprouts that form at the intersection of the stem and leaf. The leaves, similar in appearance to collard greens, are also edible. “Like most vegetables, they like a pH of 6.5 to 6.8 and prefer well-drained sandy loam. They are a very long season crop with most varieties needing 90 to 110 days from transplant,” said Bryant.

Typically, Brussels sprouts are grown as transplants, and according to Bryant they should be started about five to six weeks prior to transplanting and kept between 45º and 85º F while in the greenhouse. Then, Bryant recommends, they should be hardened off for a week prior to transplanting. “Eating quality improves after light frost, and the plants will survive down to 20º Fahrenheit,” Bryant said. “If I was trying to decide when to start my seeds, I would ask myself how late I want to be harvesting them. It’s not much fun to brush snow off, so I would aim for an October harvest and count back from there.”

Bryant recommended transplanting at 15 inches apart in rows three feet apart. Many growers choose to plant the crop in plastic mulch. “Also, as the plants grow, consider trimming the leaves from the bottom third of the plants to improve air flow for better disease management,” Bryant said.

An important management decision growers must make is whether or not to top the plants – cutting a few inches off the uppermost vertical stalk. This practice is often recommended to stimulate the growth points higher up on the stalk and to promote uniform sprout size.

Bryant referred to a 2013-14 study conducted by colleague, Becky Sideman, a UNH Extension sustainable horticulture specialist, that studied the impact of topping Brussels sprouts. According to Bryant, the study showed that topping too early – 60 to 85 days pre-harvest – can cause branching and yield reduction. Sideman’s final recommendation is to top the plants 30 to 60 days pre-harvest once the largest sprouts are a half inch to one inch in diameter.

Sideman is currently conducting a Brussels sprouts variety trial at UNH’s Woodman Horticultural Research Farm. Seed from Diablo, a perennial favorite variety of home gardeners and commercial growers, is no longer available, and Sideman is studying the potential of new varieties.

The entire stalk or individual sprouts are harvested in late autumn. Bryant said, “Store them at 32º Fahrenheit with 95% to 100% relative humidity. If you can keep those conditions, you can store them reliably for three to five weeks.”

One common Brussels sprout pest is the cabbage aphid. According to Bryant, the cabbage aphid began showing up in southern New Hampshire only a decade ago. These aphids, which tend to be a problem on autumn brassica crops, thrive in hot, dry conditions, produce multiple generations per year and survive the winter on brassica host plants, including wild brassicas. Curling leaves are the telltale sign that this pest is present, and Bryant recommends weekly scouting starting in late June. If more than 10% of the plants are infested, it’s time to start spraying with an appropriate insecticide. It’s also important to remove or incorporate the Brussels sprouts residue at the end of the season, and there is some research that supports the use of reflective plastic mulch in managing the cabbage aphid.

Other Brussels sprout pests include imported cabbage worm, cabbage looper and diamondback moth. Floating row covers and exclusion netting can work to prevent the moths and butterflies from laying their eggs, but Bryant warned, “Make sure you’re growing in a field that hasn’t had brassicas recently. Diamondback moth and imported cabbage pupae can overwinter. If you put something over them, you’re basically trapping them inside with their preferred food source, away from any beneficials that might go after them.” If more than 20% of the plants are infested as the sprouts are forming, Bryant recommended that growers begin a spray program.

Bryant noted that growers should be on the lookout for swede midge damage. According to the Cornell University College of Agriculture and Life Sciences, the first discovery of swede midge in the U.S. was in 2004 on a broccoli farm in Niagara County, NY. The insect is native to Europe and southwestern Asia and has been known in North America only since 2000 when it was identified in Ontario, Canada. It has the potential to spread to most brassica growing areas in the U.S.

Black rot, a bacterial disease that thrives in warm, wet conditions, can impact Brussels sprouts. Because black rot can be seedborne, growers may want to consider treating the seed with 122º F water for 25 minutes. Though Bryant said that the disease can be difficult to remove from the soil, some cultural controls include a crop rotation of four years and incorporating and/or removing crop residues.

Another disease impacting Brussels sprouts is Alternaria leaf spot, a fungus that thrives in cool, wet conditions. Bryant compared the appearance of this disease to the concentric rings that appear when a rock is thrown in water. “The damage is largely cosmetic,” Bryant said, “but it can decrease storage life. I have seen cases where someone didn’t think they had a lot at harvest, but in storage the disease spread.”

To control Alternaria, Bryant recommends purchasing disease-free seed, incorporating or removing crop residue and avoiding overhead irrigation. “Minimizing leaf wetness will help prevent against fungal and bacterial diseases because the disease organisms use water like a superhighway to transport the disease,” said Bryant. Protectant fungicides are an option but because they work as a barrier, they must be applied before the disease appears.