Crimson clover (Trifolium incarnatum) is a popular cool-season annual used widely in agriculture as a cover crop and for forage. Crimson clover definitely spreads, which is a necessary function of its lifecycle. Unlike perennial plants, crimson clover must successfully spread its seeds to reproduce and establish a new stand the following season. This natural tendency to self-seed is foundational to its use in farming systems.
The Annual Cycle and Self-Seeding
Crimson clover’s lifecycle dictates its spreading habit because it is an annual plant that completes its entire cycle from germination to seed production. To return to the same location, the plant must produce a new generation of seeds before it dies. Reproduction typically occurs in late spring or early summer, when the characteristic bright red, cone-shaped flowers mature.
Once the flowers are pollinated, the plant produces a large quantity of seeds, each encased in a small, hard pod. The natural process of self-seeding occurs when these mature seed heads dry out and shatter, dropping the seeds directly onto the soil surface below the parent plant. This localized dispersal ensures the stand is renewed in the immediate area.
Some commercial varieties, known as “hard-seeded” types, contain a high percentage of seeds with a coat that is resistant to immediate germination. This trait ensures that a portion of the seed bank remains dormant throughout the hot summer months, only germinating later in the fall when cooler temperatures and sufficient moisture return. This mechanism of delayed germination is a biological strategy for sustaining a localized, self-seeding stand year after year.
How Crimson Clover Seeds Disperse
While self-seeding accounts for localized spread, crimson clover seeds can travel much farther through various external forces. Dry seed heads are easily detached and moved by gravity, wind, and water, carrying the seeds away from the initial planting area. These seeds are larger than many clover varieties (approximately 120,000 to 150,000 seeds per pound), which affects wind dispersal but makes them easily moved by other means.
Water movement is a significant vector for long-distance spread, particularly in agricultural fields. Because the seeds are often dropped onto the soil surface, they are easily picked up and carried by surface runoff during heavy rain or by irrigation water. This can result in seeds being transported far down a field or watershed, establishing new growth in unexpected areas.
Animal vectoring is a common mechanism, especially since crimson clover is used as forage for livestock. Seeds can pass through the digestive tracts of grazing animals (like cattle, sheep, or deer), a process known as endozoochory. These seeds are then deposited in manure, which provides an ideal environment for germination. Seeds can also stick to the fur or feathers of wildlife and livestock, allowing for dispersal as the animals move.
Human and mechanical transport contributes substantially to the unintentional spread of the clover. Seeds can be carried in contaminated hay or silage used for feeding animals. Farm equipment, such as tractors and mowers, can pick up mature seed heads and soil particles, transporting them to other fields or properties. Boots and clothing can also carry seeds from one area to another, establishing new plants far from the original source.
Controlling and Managing Spreading
Managing the spread of crimson clover requires specific actions based on whether the goal is to encourage or prevent self-seeding. If the intent is to prevent the clover from returning, the most effective method is to interrupt the annual cycle before the seeds mature. This is achieved by mowing or terminating the plants when they are in the early bloom stage, but before the flowers have fully set seed.
For mechanical termination, mowing the stand closely just as the earliest flowers are fading, but before the majority of the seeds are hard, will drastically reduce viable seed production. Another approach is to use tillage, such as light disking or moldboard plowing, which buries the surface seeds deep enough to prevent them from germinating. This also works to incorporate the plant material into the soil as green manure.
Conversely, if the goal is to maintain a self-seeding cover crop, management focuses on ensuring successful seed set. This involves removing any grazing livestock six to eight weeks before the expected flowering time to allow the plant to fully mature its seed heads. For reseeding varieties, allowing the seeds to drop naturally and ensuring good soil-to-seed contact, sometimes facilitated by light grazing or a roller, secures the stand for the following season.