As temperatures begin to increase, pastures and hayfields are entering a period of warm days and cold nights. These conditions result in soils going through freeze-thaw cycles, which make for a perfect time for frost seeding.
Frost seeding involves strategically broadcasting or overseeding legumes into pastures, hayfields or small grains. It’s a potentially cost-effective solution to revitalize a diminishing stand or introduce new legumes. However, the success of frost seeding hinges on assessment of the existing stand, proper timing and soil conditions.
A common misconception about frost seeding is that spreading the seed on top of snow works best. The goal of frost seeding, however, is to get the seed into the soil. Freeze-thaw cycles then provide shallow coverage of the seed, which helps shield it from early spring rains. Frost seeding is the easiest method to add new forage legumes or grasses to pastures and is likely the least expensive method as well.
Assessing the stand
Iowa State University Extension forage specialist Shelby Gruss and ISU Extension field agronomist Rebecca Vittetoe offer helpful observations and guidelines for frost seeding.
A thick thatch or dense stand of existing plants can be a challenge, hindering optimal seed-to-soil contact and fostering competition for the seedlings. To mitigate this, it’s recommended to closely harvest the hay or overgraze the forage in the fall or winter. That will open the canopy in preparation for frost seeding the following spring.
Ideally, visible bare spots are a good indicator of a pasture or hay stand where frost seeding is more likely to be successful.
Timing and soil conditions
Two important factors to consider for successful frost seeding are timing and soil conditions. Typically, frost seeding is done between mid-February and early March, coinciding with the freeze-thaw cycles.
This year, in Iowa, the last week of February started to exhibit the cycle with cold nights and warm days, indicating potential opportunity for frost seeding. However, checking future weather forecasts is crucial, as many parts of the state can be experiencing warmer temperatures.
You need to check the lows and highs for the freeze-thaw interaction. The rhythmic expansion and contraction of soil during freezing and thawing creates a honeycombing effect. That effect facilitates seed incorporation into the soil profile, ensuring good seed-to-soil contact. Looking for this honeycombing is a good indicator that the freeze-thaw cycle is occurring and can help determine when to frost seed.
Additionally, attention is required to avoid creating soil compaction. It’s necessary to do early-morning operations in fields while the soil remains frozen to prevent compaction when soil is wet. Under some conditions, the use of livestock can help incorporate the seed, ensuring seed-to-soil contact through their hoof action. But doing this — having livestock walk on a field when the field is too wet — can lead to the plugging up of the pasture.
Other considerations
Seed selection plays a crucial role in frost-seeding success. Red clover and birdsfoot trefoil are great options, as they establish fairly well with frost seeding and provide excellent forage quality. Other legumes, such as white clover and alfalfa, also can be considered depending on the pasture's needs and soil conditions; however, success is more variable with these species. Typically, grasses are not good frost-seeding options.
The fate of the seeded stand hinges significantly on spring weather and soil moisture levels. Adequate moisture is vital for establishing a healthy stand, and soil moisture is necessary for the freeze-thaw cycle to occur effectively.
In many parts of Iowa, soil moisture or proper temperatures may not be appropriate for an effective freeze-thaw cycle for frost seeding. Thus, while the potential benefits of frost seeding are high, keep in mind that timing, soil condition and moisture play important roles in success.
Evaluation of your stand, the future weather forecast and soil moisture conditions help determine whether frost seeding is an appropriate option this spring. More helpful information is available on the ISU Extension and Outreach website.