Frost Seeding Time is Here

Sjoerd Willem Duiker, Professor of Soil Management and Applied Soil Physics, Penn State University
Zachary Larson, Field and Forage Crops Educator, Penn State University
David Hartman, Extension Educator – Livestock, Penn State University
Dave Wilson, Former Extension Educator – Agronomy, Penn State University
(Previously published online with PennState Extension: March 2, 2022)

Frost seeding is an economical method to establish legume cover crops into small grain stands or to fill in run-down pastures.

Frost seeding is an economical way to establish cover crops in the winter in standing wheat or barley or to supplement a thin forage stand. Though not as fool-proof as drilling, it is a reasonably successful practice.

Now is the time to perform this practice as the soil is going through freeze-thaw cycles. This causes a ‘honey-combing’ of the soil surface which helps to improve seed-to-soil contact. Frost seeding works well on loamy and clay soils that hold water but is not suited for use on sandy or shaley soils that dry out quickly. The best time to perform frost seeding is early in the morning when the soil is frozen, and a thaw is expected during the day. This reduces the chance for soil compaction while providing the desired soil heaving that improves seed-to-soil contact.

The best species for frost seeding generally are small seeded, germinate quickly, and grow well in cool conditions. Red, white, and sweet clover are the most successful species, while birdsfoot trefoil can also be used for pasture renovation despite slower germination and early growth. And though yellow sweet clover can cause animal health problems due to coumarin content (a blood thinner), it is not likely to cause livestock health issues if it is only a small percentage of a pasture. When seeding legumes, be sure to inoculate them with the appropriate rhizobium so the symbiosis will take place to fix N.  In pastures, some non-fluffy grass species such as annual or perennial ryegrass may also be frost seeded. Do not mix grass and legume seed for broadcast application as the legume seeds will throw farther than the grass seeds due to their greater density, which leads to non-uniform seed distribution.

Make every attempt to guarantee uniform coverage by knowing the width of spread and spacing between passes. Recommended species and seeding rates for the two scenarios discussed here are given in Table 1. Seeding rates into small grains are higher because no repeat application is possible, while with pasture renovation frost seeding complements an already established stand and can be repeated next year if not successful. Heavier seeding rates for pasture renovation would be used in thinner stands. It is common to mix clovers for pasture renovation. Red and ladino white clover make a good combination, where you use twice the seeding rate of red clover as white clover (e.g. 2 lbs./Ac. red clover + 1 lbs./Ac. white clover up to 6 lbs./Ac. red clover + 3 lbs./Ac. white clover).

Table 1. Species and seeding rates (pure stands, lbs./Ac.) for frost seeding into winter small grain or for pasture renovation.

Species In small grain Pasture renovation
Red clover 10-15 4-8
Yellow blossom sweet clover 15-20 5-10
White, ladino clover NR 2-3
Birdsfoot trefoil NR 4-6
Perennial or annual ryegrass NR 4-6

NR = not recommended.

Frost seeding will likely be most successful in pastures with bare spots or those that are overgrazed. Besides relying on the freeze-thaw action at seeding, you can also use grazing animals to tramp in the seed shortly after broadcasting in late winter. This practice may be especially helpful for improving seed-to-soil contact if a thick thatch layer that would compromise frost seeding success is present. However, don’t turn out animals in wet conditions and cause soil compaction. If you miss the best ‘window’ for frost-seeding, clover seed will remain viable in the soil and much of it will likely grow when the conditions are right. If you notice your stand is not adequate in summer, you can selectively no-till legumes and/or grasses in late summer to fill in thin spots to resolve any lingering issues.