Crop planning deep dive: Brassica bonanza

Authors: Natalie Hoidal and Marissa Schuh, Extension Educators, University of Minnesota Extension. Originally published 2021, updated 2025.


We are reviving and revising a series of crop-specific deep-dive articles we published in 2021. This week we are focusing on crops in the Brassica family, and tips for growing them successfully. By planning ahead and anticipating challenges, you can get ahead of common problems that limit yields.

Bolting

Brassicas originated in the Mediterranean, where they were originally biennial plants that required a 2-3 month cool period (winter vernalization) to induce flowering. Since winter in Greece is a bit like spring in Minnesota, growers in cool climates have been able to grow Brassicas in the spring. Because our spring is followed by a very warm summer, growing spring Brassicas in Minnesota can be a bit of a dance with the weather. While each plant is a bit different in its requirements, the basic idea is that they prefer conditions in the 60s early in development where they can put on leafy biomass, and eventually with enough cool nighttime temperatures, they will experience enough cold weather to trigger vernalization, which allows for head formation (or enlarged storage roots in the case of kohlrabi, turnips, etc.). If plants experience too much cold early on, they can experience “buttoning up”, which happens when the plant produces a head before it’s really ready. Once plants enter their head formation stage, they are far more susceptible to bolting with high temperatures. For a very in-depth look at the conditions that cause bolting in a range of crops, see this article from our colleagues in Michigan.

Bolting broccoli, photo NH

Based on this information, some tips for preventing bolting include:

• Choose varieties based on season: plant varieties recommended for spring in the spring, fall varieties in the fall, and heat tolerant varieties in summer. Certain varieties have been specifically bred to better withstand the temperature fluctuations of each season.
• Consider using row cover early in the spring if temperatures are consistently dipping below 50 degrees F to hold off vernalization.
• Transplanting can help, especially for spring broccoli. Plants will already have a few leaves, and may be able to more quickly reach maturity before hot summer weather arrives.
• Hedge your bets with succession planting. Since spring weather is so unpredictable, planting a new succession every couple of weeks will help to provide some resilience in the case of very high or very low temperatures.
• For broccoli and cauliflower, harvest based on bud size rather than head size.
• You can always stick to growing your head-forming Brassicas in the fall if your markets allow for it. Fall grown Brassicas get to experience conditions much more similar to the Mediterranean conditions they were bred for: a warm summer to put on leafy green biomass, followed by a cool fall with ideal head formation temperatures.

The Very Hungry Caterpillar Complex

Size alone isn’t enough for caterpillar ID. Diamondback moth is smooth, and tapered at each end. Imported cabbageworm is velvety. Cabbage looper and smooth and moves in an inchworm fashion. Photo: Marissa Schuh, University of Minnesota Extension.

The three pests in the cole crop caterpillar complex make sure there is some level of pest pressure no matter where we are at in the season. Diamondback moth is the first arrival most years, with larvae chewing many small holes in leaves. In the spring the adult imported cabbageworm is one the first butterflies flying, though it sometimes takes a while to start seeing the velvety larvae feeding on brassicas. Cabbage looper is the last arrival. Unlike the other two, cabbage loopers cannot survive the Minnesota winters, so when they show up in the season is dependent on weather fronts bringing them up from the southern US.

While these three can sometimes be found feeding on one unfortunate plant, they vary in their timing and capacity for damage, so being able to tell them apart aids in scouting and spray decisions. 

There are many thresholds developed by research in different states. The most straightforward, conservative threshold is from University of Connecticut. These are based on weekly scouting of 25 plants and counting plants that have at one or more caterpillars.

• For young plants, caterpillar control is only needed if plants are growing slowly and 35% of plant are infested.
• For kale, collards, and other leafy brassicas, use a 10-15% threshold throughout the season.
• For broccoli, cabbage, and other heading brassicas, use a 20% threshold. Make sure to scout and spray crops as heads form.

Bt is effective against all caterpillars in the complex. Understanding the nuances of using Bt is key in its successful control. Caterpillars must ingest the product while it is active. This means it needs to reach the caterpillars where they are feeding. Bt also breaks down in direct sunlight and washes away with water, so reapplication will be needed. Finally, Bt works best against these caterpillars when they are young and small (under a quarter inch in size). This highlights how scouting and early detection will help catch these pests when they are most easily controlled. 

For other management options, see the Midwest Vegetable Production Guide.

Cabbage Maggot

The biology of cabbage maggot makes it an incredibly hard pest to control. The adult flies look similar to a common house fly, making scout or trapping untenable. The eggs are too small to scout for, and once they hatch, the maggots enter the plant, where they are protected from sprays. This means the place in the pest’s biology where we can intervene is at egg laying and egg hatching. 

Cabbage maggot larvae and pupa in the root zone of a wilted cabbage plant. Photo: Marissa Schuh, University of Minnesota Extension.

Predicting this is difficult, though we do have some tools. Models based on how temperatures impact cabbage maggot development can be found on the University of Minnesota VegEdge page. As temperatures are warm enough for cabbage maggot to develop (over 43° F), the map will start to change colors. In the spring, green is the color to look for. If the area you are in is green, it means we’ve had enough warm weather to allow cabbage maggot adults to start flying and laying eggs. Our window for cabbage maggot control is at egg-laying. This means that when the model says cabbage maggot adults are flying in our area, we should do what we can to prevent them from laying eggs. This could be by deploying a row cover, transplanting large plants with more robust root systems, or by holding off on planting our most susceptible crops.

While we've had some development of cabbage maggot, but we aren't to the point where adults are likely emerging anywhere in the state. 

Flea Beetles

Flea beetles are typically an early spring problem in Minnesota, with adult beetles peppering seedlings and leaves with many small holes. In recent years, we have observed flea beetles causing problems as late as August. 

Flea beetles are small, black, and shiny. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org.

Options for control include...

• Waiting to plant until June when the first wave of flea beetles has passed
• Planting larger transplants that will be able to power through flea beetle feeding
• Using a row cover when plants are small (though this makes weeding hard)
• Use a trap crop like mustard, and plant the trap crop early so it will 7 to 14 days ahead of your main crop
• Kaolin clay can provide a physical barrier to feeding (but doesn’t always work)
• If 10-20% of the leaf area of plants are damaged, consider spraying

If you want to learn more about flea beetles, visit the main UMN Extension website.

For more information on chemical controls for the pests mentioned above, visit the Midwest Vegetable Production Guide.

Brassica Diseases

While bugs are often more noticeable, diseases like black rot and alternaria are increasingly worrisome. For more information on brassica diseases and their management, see our listing of common diseases in vegetable crops. 

Variety selection

In 2021 and 2022 we completed extensive broccoli variety trials. We had a research site in Waseca that we infected with black rot (and where we allowed Alternaria to proliferate), and 87 growers participated in the trial, doing mini variety comparisons on their farms and in their gardens. Based on these trials, the favorite varieties were:

• Favorite spring varieties: Wolfman, Eastern Crown, and Green Magic.
  
• Favorite fall varieties: Belstar & Covina. Eastern Crown, Dimplomat, Imperial, and Abrams were runners-up, but had less disease tolerance than Belstar and Covina.
  

You can read the full report here. 

In addition to standard head broccoli, many growers have started to grow broccolini. You can read the results from a preliminary broccolini variety trial by clicking here.