Understanding Cover Crop Classifications, Seed Selection, and Mixes

This article is part of a series on cover crop species selection by ISAP’s Soil Health Specialist, Jim Isermann. The first article provided a general introduction to cover crops and highlighted important considerations for determining which cover crops will be the best addition to a cropping system and a specific farmer’s operation, and future articles will focus on individual cover crop species. In the following article, Jim examines three common and useful ways to categorize cover crop species, shares some considerations on cover crop seeds, and provides a quick primer discussion on mixes. 

Cover Crop Classification

There are several ways to classify cover crop species, but there are 3 that are the most practical to help in cover crop species selection for farmers. Those are:

  • By season: cool season vs. warm season.
  • By overwintering vs. winter kill (terminal).
  • By family: grasses, brassicas, legumes, and other broadleaves.

There are additional ways to classify cover crops – C3 vs. C4 species, whether they are associated with mycorrhizal fungi, etc. – but the three categories mentioned are probably the most critical and helpful for a farmer when planning a cover crop strategy.

Cool Season vs. Warm Season

Whether a cover crop is cool season or warm season plays an important role in how effective the cover crop will be within a given planting and growing season. This classification relates primarily to what conditions the plant thrives under. Generally, few annual species classified as warm season will overwinter, but cover crops classified as cool season species are not guaranteed to overwinter either. When considering annual species that are commonly used as cover crops, warm season plants such as sorghum-Sudan and millets will not withstand any frost, while cool season species such as turnips and oats will withstand temperatures down to the lower 20’s (or lower) but generally will not overwinter in central Illinois.

Cover crops can be planted together, or in “mixes,” to increase diversity and maximize soil health benefits.

Within corn and soybean systems, warm season annuals can be used as a tool on prevent plant acres or following small grains in rotation, and occasionally following a corn silage crop. When planted in hot summer months and provided with enough water, warm season species can provide impressive growth and soil health benefits. However, given the usual timing of cover crop seeding in corn and soybeans systems in Illinois, warm season annuals are not generally utilized as cover crops on a large number of acres. We do, however, see some species classified as warm season show up in mixes on occasion. In Illinois, we rely more on cool season species that thrive under the cooler conditions of fall and spring to fill in the gaps around our warm season (corn and soybean) cash crops.

Over Wintering vs. Winterkill

How many species do you see? Working with trials across the state we’ve seen combinations work and not work. It all starts with the planning process.

Knowing if a cover crop is an overwintering or a winterkill species is helpful during species selection. When growers are first starting out, they often are not ready to handle an overwintering species that is still growing and building biomass in the spring and needs to be terminated. Finding a winter kill species can be a good option. This grouping is somewhat vague given that whether a species will overwinter is largely dependent on the winter weather and establishment date of the cover crop. As noted above, the warm season species such as sorghums and millets will die off at the first frost, but species such as turnip or oats require temperatures below 20 degrees or less to winter kill.

In situations with early snow cover, these species can be insulated from cold temperatures, and even in a relatively harsh winter can green up in the spring unexpectedly. If these cool season species do overwinter, they are generally easy to kill with either an herbicide application or mechanical tillage if necessary. Even within species, certain varieties may be prone to winter kill while others are more winter hardy. Spring vs. winter varieties of a species will be covered in a future article. The winter hardiness of species such as annual ryegrass and barley will rely heavily on when they are seeded – an early establishment can be crucial to ensure overwintering. And of course, the winter hardiness of individual varieties will play a huge role as well.

Cover Crop Families

Classifying by species family is one of the most helpful ways to build a successful cover cropping program. This, in conjunction with knowing the above distinctions of overwintering and warm vs. cool seasons, will help to ensure you are using cover crop species in conditions where they can express their beneficial characteristics and provide value for your system. Within each family, a species will be classified as cool season or warm, and if they overwinter or not. Note that the family does not put them into these categories. For example, cereal rye is a cool season and sorghum-Sudan a warm season, but both are grasses.

The primary cover crop families we will review are grasses, legumes, brassicas, and broadleaves. The distinct characteristics of each family will require additional details, so we will address them further down the line. The below chart from NRCS is not an exhaustive list of species and includes some we do not see much in the Illinois, but the chart is a great illustration of the relationships between families and seasons of many cover crops.

The Agricultural Research Service’s Cover Crop Chart is designed to assist producers with decisions on the use of cover crops in crop and forage production systems.

Varieties, Brands, and VNS

When ordering cover crop seeds, you will come across a few distinctions that are important to consider. Sometimes, you will see a specific variety listed, such as Aroostook rye or Elbon rye. Often, you will have an option of VNS (Variety Not Stated), and occasionally you will see what is considered a “brand”. When looking at the VNS option, it is important to understand that Variety Not Stated simply means just that, that the variety is “not stated” on the bag. It is not necessarily a reference to the quality of the seed or even if the variety is necessarily unknown. It does mean you probably will not know specifics about the variety, so depending on your application you may want to search out a specific variety. In some circumstances you may not want to use VNS.

Field days have been a great way to be able to see cover crop species or mixes that you might not normally see.

Whether or not it is a concern that a cover crop seed is labeled VNS will usually depend on the specific species and circumstances. Cereal rye is the easiest example here. VNS is pretty much the industry standard in Illinois. If you are purchasing from a “reputable” company, you will be receiving a clean winter hardy species that is one of a few varieties grown for grain yield by their suppliers. Most applicators in Illinois are going to seed cereal rye in late fall and chemically terminate early in the spring. The cereal rye will probably not go beyond 24 inches in height and will not set seeds. Frankly, for these applications, using a VNS cereal rye variety is usually sufficient.


However, a farmer looking to roller crimp for termination or looking for forage or grain value will want to look for specific varieties with characteristics that will better suit their system. An alternate example is annual ryegrass. Available annual ryegrass varieties vary widely in characteristics including winterhardiness, dormancy, and whether the seed is clean of multiple varieties. For cover crop use in Illinois, you should never purchase a VNS annual ryegrass without more information.

Brands are another option that some cover crop companies utilize to distinguish a bag of cover crops. These are often held to a higher level of quality over VNS, in terms of cleaning and perhaps seed germination but are generally a similar variety to VNS. Again, there is nothing wrong this, but make sure you understand you are not purchasing a specific variety here. We will give some examples of this when we discuss individual species.


The last thing to cover before moving on to individual species is a quick discussion on mixes. Mixes of species can be used for multiple reasons, most notably their positive effect on the soil and their potential synergistic effect on each other (more on that later). Each different species has a different effect on the soil and the biology below and above ground. Plant roots play many roles in supporting the biology below the soil surface by providing root exudates and the food and environment for many soil dwelling species of organisms to survive and thrive. For this reason, many will look for multiway species blends in an effort to support these populations. Simple blends of the three primary families of grasses, brassica and legumes are very popular in Illinois corn and soybean systems.

Don’t be intimidated by advanced systems of cover cropping combinations. Learning why these covers are placed in a certain combination can inspire you to use a 2-way mix, simplifying a mix that fits for you.

However large and diverse mixes of 10-14 cover crops are found more often in situations after small grains or whenever a situation allows for a summer planted cover crop. Again, we need to apply the concept of what your goal is and under which conditions you are growing the cover crop to determine whether a blend of species is suitable for you. Supercharging your biology and feeding the multitude of bacteria, fungi, and other organisms below the soil with a diverse range of root exudates from 14 species is a great goal, but in general, this is not practical in most high productivity corn and soybean acres in Illinois. If this is your goal, you will probably need to rethink your cropping system in most cases to include small grains and livestock grazing. However, simple blends of 2 to 3 species representing multiple families can bring a lot of benefit and can be grown effectively. Generally, most farmers will start with a winter hardy grass, then add some brassicas and finally work at integrating legumes over time as they become more experienced.

In the next article we will look at the most used cover crop in the Illinois, Cereal Rye.


Jim Isermann

Jim Isermann is the Soil Health Specialist with the Illinois Sustainable Ag Partnership and a farmer in Central Illinois. Jim coordinates ISAP's Soil Health Leadership Program, an 18-month intensive training program that provides in-depth education on soil biology, cover crops, tillage practices, and nutrient management. To get in touch with Jim, send him an email at soilhealthspecialist@ilsustainableag.org