Cover Crop Termination for Organic Growers

By Collin Thompson, Johnny's Farm Operations Manager

One of the often-overlooked aspects of a good cover cropping program is termination. It is easy to get excited about the abundant biomass, weed suppressing effects, and erosion control provided by a properly established cover crop. However, these precise benefits can also pose challenges if not anticipated properly. For example, a mature crop of winter rye standing 6’ tall will produce incredible biomass and do an amazing job of outcompeting weeds and holding the soil in place. However, this same crop will become a source of immense frustration if the plan is to incorporate that mature residue into the soil with tillage. Instead, we would be better off adjusting our timing or method of termination, matching the tools and techniques to the stage of growth. If managed properly, our excitement about the abundant biomass can continue as it becomes organic matter that feeds the soil, and in turn, our crops.

Methods of Cover Crop Termination

There are several tools and methods that can be used to terminate cover crops, many of which can work on various scales of production. The tool itself may look different (a tractor-powered flail mower vs. a push mower or weedwhacker), but the mechanism can be similar. The method of termination that is selected will largely depend on what tools are available, the size of the planting, and the stage of growth of the cover crop.

In choosing a termination method, it is important to consider the growth stage of the cover crop—some methods work best when the crop has transitioned out of the vegetative stage (early foliar growth) to the reproductive phase (when the plants are flowering).

Mowing

Mowing down Spring Green Manure Mix on the Johnny's Research Farm. Mowing on smaller-scale farms and gardens might be done with a push mower or weedwhacker.

When timed properly, mowing can be a very efficient, low-impact method of termination. In most cases, the cover crop will need to transition out of the vegetative stage and into the reproductive phase for mowing to be effective. Even then, some cover crops can withstand repeated mowing, remaining largely unaffected.

Understanding a specific crop’s reproductive cycle (annual, biennial, perennial) can help determine how effective mowing will be as a termination strategy. For example, most annuals (i.e. grains, some broadleaf species) can be effectively terminated when mowed during the reproductive phase. However, perennial and biennial crops (i.e. some clovers, alfalfa), will continue to grow through multiple mowings. This can be advantageous if your goal is to have a resilient living mulch, but more challenging if you are wanting to transition to a different crop.

Without any subsequent tillage, mowing will leave crop residue (debris) on the soil’s surface. This can have a desirable mulching effect but can be challenging if we plan to transition to a fine-seeded crop (i.e. carrots or lettuce). To expedite residue breakdown, aim to chop the cover crop as finely as possible. To do so, select a tool like a flail mower, which will create smaller fragments than a weedwhacker or rotary mower will. Alternately, if your goal is to maximize the mulching effect of the cover crop residue, using a scythe, sicklebar mower, or weedwhacker will leave the cover crop largely intact, slowing its decomposition.

Tillage

Tillage can include a wide diversity of tools and techniques, but the shared goal is to incorporate the cover crop residue into the soil. More aggressive tillage will more thoroughly chop and bury cover crop material, resulting in faster decomposition. However, this tends to require bigger or more powerful equipment and can have negative effects on soil life if used improperly.

Just as with mowing, matching the tillage tool to the growth stage of the cover crop is critical to yield the desired results. A rototiller or disc harrow may be effective at chopping and burying a young, vegetative wheat crop. However, that same wheat crop will likely plug and bind these tools if it is flowering.

Generally, tillage tends to be more effective during the vegetative stage for most cover crops. Additionally, the more aggressive the tool, the fewer passes will be required to get a successful termination. There are exceptions to these rules, so it is important to become familiar with individual crop characteristics and trial different systems to find what works best in your context.

Smothering/Tarping

Silage tarps are a good option for smothering a cover crop.

Using tarps to smother growing cover crops can be an effective no-till method of termination. Tarps prevent light from reaching the crop, preventing photosynthesis, thereby starving it of the sugars needed to support growth.

Because this method relies on depleting the reserves of a crop until it can no longer survive, it tends to take longer than other methods. Depending on the crop, it can take weeks to get an effective kill, which may be a challenge if the space is needed for another crop. However, when timed appropriately, this is a low-impact method of termination that can be very effective.

Just like with mowing, tarping tends to leave residue on the soil’s surface. When left for extended periods, tarps can promote faster decomposition by retaining moisture and warmth, particularly with lush, vegetative cover crops.

Perhaps one of the largest challenges with tarping is managing the physical mass of the cover crop. A tall grain may need to be mowed or crimped beforehand to allow the tarps to lay flat. Additionally, tarps need to be well secured (typically with sandbags, concrete blocks, or rocks), especially in windy areas, and storage of the tarps when not in use can be challenging if space is limited. That said, tarps are a great option for smaller plantings and no-till systems, where they will effectively terminate a wide range of cover crops without disturbing the soil.

Crimping

Crimping a cover crop by hand with a simple knock-down bar.

Plants rely on vascular pathways to transport water and nutrients throughout their tissues. When these are severed or blocked, the plant can no longer do so, resulting in plant death. Crimping uses a tool with blunt ridges to flatten a crop and crush the vascular tissues, resulting in a terminated crop laid on the soil surface, providing mulch that can be used to protect the soil and prevent weed growth. Crimping is generally most effective during the reproductive phase. At this point, plants are redirecting energy to the development of flowers and seeds and cannot recover from crimping as easily. During the vegetative phase, many crops can send out new shoots or leaves to overcome the stress of crimping.

Specialized roller crimpers typically feature a drum with flat steel ridges in a chevron pattern that is rolled over the crop. The pattern of the ridges allows for a smooth rolling action, as the concentrated downward pressure on the ridges crushes the stems of the plant. Ideally, the pressure is sufficient to crush the stem in multiple locations, but not enough to sever the stem. This will allow for slow decomposition of the residue and maximum mulching effect.

Using a roller crimper to terminate a cover crop.

On a smaller scale, growers can use other common items such as t-posts or two 2x4 boards screwed together in a T-shape to crimp a crop. Typically, a rope is secured at both ends of the post or board as a handle and the post or boards are laid flat on the ground. One or two people step on the post, flattening the crop and crimping the stems. It is then lifted, moved forward 8-12” and repeated.

Winter Kill

Many frost-sensitive cover crops can be naturally killed in areas that have cold winters. When planted in the summer or fall, these crops will grow until the cold weather sets in and they can no longer survive. The primary advantage of this approach is that it requires little effort from the grower. Many cover crops will be easily incorporated in the spring, leaving minimal residue. However, crops with excessive biomass or woody stems (such as sudangrass or sunflowers) may require additional mowing or tillage to handle the residue prior to planting the next crop.

To determine whether a crop is likely to winter kill in a specific region, refer to the USDA Hardiness Zone map and the Johnny’s Farm Seed Comparison Chart, which includes minimum hardiness zones for individual cover crops.

Stacking Techniques

Oftentimes, the most effective way to terminate a cover crop is to use multiple techniques in sequence. This can be done strategically to not only ensure an effective kill, but also to leave the field or garden bed in the most desirable condition for the next crop.

Common combinations include:

• Mowing prior to tillage: this chops up the cover crop residue to make it easier to incorporate into the soil
• Crimping prior to tarping: this flattens the crop and make it easier to cover with tarps
• Mowing prior to/after winterkill: the mowing will chop up the standing cover crop before/after the cold temperatures kill it, allowing for easier incorporation in the spring

Termination Efficacy by Cover Crop

Here is a chart designed to help you determine the most effective termination method (or combination of methods) for a given cover crop. Note that the chart differentiates between termination methods used at the vegetative phase (early foliar growth) vs. the reproductive phase (when the plants are flowering).

Recommended Termination Methods by Cover Crop
Key	1: Not effective; very difficult to manage due to crop size or persistence.		2: Limited effectiveness; difficult to manage due to crop size or persistence.		3: Somewhat effective; manageable with the right equipment and timing.		4: Effective; easy to manage.
	Tillage (vegetative)	Tillage (flowering)	Mowing (vegetative)	Mowing (flowering)	Crimping (flowering)	Tarping (vegetative)	Tarping (flowering)	Freeze
Alfalfa, Summer	3	1	1	1	1	4	4	1
Barley	3	1	1	4	4	3	2	3
Buckwheat	4	4	3	3	3	3	3	4
Clover, Crimson	3	2	1	2	2	4	4	2
Clover, Mammoth Red	2	2	1	1	1	4	3	1
Clover, Medium Red	2	2	1	1	1	4	4	1
Clover, New Zealand White	2	2	1	1	1	4	4	1
Clover, Sweet	2	1	1	2	1	4	2	1
Manure Mix, Fall Green	2	1	1	1	1	3	2	1
Manure Mix, Spring Green*	3	1	1	2	2	4	2	3
Millet, Pearl	2	1	1	4	4	2	1	4
Mustard	4	3	2	4	2	4	2	3
Oats, Common	3	1	1	4	3	4	2	4
Oats, Hulless	3	1	1	4	3	4	2	4
Peas and Oats Mix	3	3	1	4	2	4	3	4
Peas, Field	3	3	3	4	2	4	3	4
Radish, Oilseed	3	3	2	4	1	4	3	4
Rye, Winter	2	1	1	4	4	4	2	1
Ryegrass	2	2	1	3	1	4	3	2
Sudangrass	1	1	1	3	1	2	1	4
Sunflower	2	1	2	4	1	2	1	4
Sunn Hemp	2	1	2	3	1	2	2	4
Teff	2	2	1	1	1	3	3	4
Vetch, Hairy	1	1	1	3	4	4	2	1
Wheat, Spring	3	1	1	4	4	4	2	3
Key	1: Not effective; very difficult to manage due to crop size or persistence.		2: Limited effectiveness; difficult to manage due to crop size or persistence.		3: Somewhat effective; manageable with the right equipment and timing.		4: Effective; easy to manage.
*vetch will survive frost, but peas and oats will not

Effective termination is a critical step in successfully capturing the many benefits that cover crops can offer. By matching the appropriate technique to a crop’s stage of growth, available tools, and individual goals, termination can be seamless and simple. With careful timing, thoughtful management, and a willingness to experiment, cover crops can be the backbone of a sustainable farming system.