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Summer seeding advantages over spring seeding include:

• Considerably less weed pressure; thus optimum timing to seed legume-grass mixtures, which have limited herbicide options, and species with limited seedling vigor.
• Spreads out the planting workload.
• Can harvest a crop (eg. small grain) followed by a full forage production year after seeding.
• Allows perennial forages to get a jump on the growing season and produce at or near established stand production the following year. 
• Protects against wind and water erosion from fall through spring.
• Can allow time to prepare a good, clean seedbed in summer.

Alfalfa seedlings need 6 to 8 weeks of growth before the first killing frost. Thus, in northern Minnesota, the recommended window for conventionally tilled legume summer seeding is between about July 20 and August 1. In southern Minnesota, seeding between August 1 and 15 is recommended. These dates apply to all legumes, reed canarygrass, and probably tall fescue, too. Other common perennial grasses such as smooth bromegrass, timothy, and orchardgrass can be seeded in early August in northern MN, and until late August in southern MN.

It’s summer, not fall, seeding. The importance of summer seeding date was demonstrated nicely in research in central Pennsylvania (Table 1). Each day planting was delayed after August 1, total DM yield the year after seeding declined 160 lb/ac for alfalfa, 110 lb/ac for red clover, 80 lb/ac for birdsfoot trefoil, and 120 lb/ac for reed canarygrass. After mid-August, orchardgrass yield declined 90 lb/ac for each day planting was delayed. Conversely, perennial ryegrass yields the year after seeding increased 1.0 ton/ac by delaying seeding from early to late August. However, planting perennial ryegrass after late August resulted in 130 lb/ac decreases per day seeding was delayed. Perennial ryegrass seeded in early August and/or >6” tall in fall should be mowed or grazed to improve its persistent since shading reduces its tiller survival. 

Table 1. Summer-fall seeding date influenced total season yields (ton DM/ac) of six forage species the year after seeding in central Pennsylvania. Number in parentheses is seedling height in late November of the seeding year. Averaged over 2 establishment years (Hall, 1995.)

Thus, the terms “fall seeding” and even “late summer seeding” can be misleading, implying seeding date windows with less chance for success. Summer seeding is a more reliable time and term.

Virginia work in the 1950s demonstrated the striking growth rate differences among forage seedlings, and how growth rates differ between spring and summer seedings (Figure 1). Growth rates were scored relative to alfalfa’s summer seedling growth rate (set at 100). The figure clearly shows the vigor of summer-seeded alfalfa compared to other perennial forages. This demonstrates that with a firm and moist seedbed, summer-seeded alfalfa has the potential to establish rapidly. Based on these data, the Virginia researchers also suggest that when grasses are mixed with alfalfa, relatively heavier grass rates should be used with summer seeding when the grasses are relatively less competitive seedlings than alfalfa.

Maximize seed/soil contact. Some classic Minnesota research from the 1950s compared various combinations of rolling and harrowing before and after drilled vs. broadcast summer seeding of a mixture of alfalfa, red clover, and timothy. They concluded that best stands were obtained with rolling, drilling, and then rolling again. Rolling before seeding prevents drilling too deeply. They found that rain shortly after seeding favored broadcast seeding, while drought favored drilled seeding.

Firmness and good seed-to-soil contact are essential. Minimize the amount of tillage before summer seeding to avoid unnecessary soil moisture loss. A loose, fluffy seedbed severely reduces germination success. Multiple passes with a roller seeder may be required to get adequate firmness. If the land is idle, it should be prepared in early summer for better moisture conservation. It’s best not to use a companion crop when summer seeding. However, a very low rate can be used on sites with high erosion potential IF moisture is adequate.

Summer seeding depth should be about ½”, and somewhat deeper on sandy soil. Figures 2 and 3 show the influence of seeding depth in clay and sandy soils in Wisconsin. Note that in clay soil, greatest stands were achieved with a ½” seeding depth. Orchardgrass was particularly sensitive to being seeded too deeply. In sand, all species established equally well at ½- or 1”-seeding depths. 

The no-till advantage. Perhaps the best way to provide a firm seedbed and favorable soil moisture is by no-till seeding. Four years of no-till research on a clay soil in Virginia demonstrated the merits of the no-till approach. They consistently achieved successful establishment and winter survival by no-tilling 15 lb/ac of alfalfa 1” deep up to 3 weeks later than broadcast seeding with a cultipacker seeder in a prepared seedbed. They attributed the no-till advantage to: 1) consistently more rapid emergence of no-tilled seedlings - broadcast seedlings often required 3 or more weeks to emerge when moisture was limiting, 2) greater root development under no-till due to less temperature extremes and in particular higher minimum soil temperatures in the root zone later into the fall, and 3) less heaving losses with no-till.

In Minnesota, the potential to no-till seed up to 3 weeks later than seeding into a tilled seedbed may be a stretch; a 10-14 day longer/later window for no-till seeding may be a safer bet here. So for alfalfa (and other legumes, reed canarygrass, and tall fescue), in northern MN, a no-till seeding could probably be done as late as August 15 instead of August 1; and as late as Sept. 1 in southern MN.  Smooth bromegrass, orchardgrass, and timothy could be no-tilled even later. In a sandy loam at Mandan, ND, both August 15 and September 4 no-till seeding of 8 lb/ac of smooth bromegrass ½” deep into spring wheat stubble resulted in successful establishment in 3 of 3 years tried. Dr. George Rehm and co-workers assessed different methods and dates for establishing forage legumes on sandy soil, and found no-till seeding in early August to be very successful (Table 2).

Table 2. Average annual yields (ton DM/ac) for two years after the establishment year for three legumes as influenced by planting date and method on a sandy loam soil at Staples, MN. (Averaged over 2 establishment years; Rehm and co-workers, 1998).

So, if timing and seedbed management are right, summer can be a great time to get dense, productive perennial forage stands established. In addition, no-till drilling helps ensure the most favorable moisture and temperature environment for rapid germination and seedling development of summer-seeded perennial forages.

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