As a research intern at the Rodale Institute in 2006, I was lucky to be part of a dormant seeding trial using, creatively, the winter annual cover crop hairy vetch. Dave Wilson, my supervisor at the time and encyclopedia for all things agronomic, got the idea for dormant seeding from hearing stories of farmers in North Dakota who used the practice to get a head start on planting spring wheat, since conditions for driving a tractor in a northern spring can be quite tricky, often excessively wet and muddy. Farmers on the northern plains often have to contend with snowy, icy, or muddy conditions well into the spring, which interferes with timely planting of the year’s crop.
Not too early, not too late
Successful dormant seeding requires that they get the seeds in the ground before the ground freezes and winter-long snow cover arrives, but not so early that warm weather coaxes a tender sprout from the ground to face the violent elements of winter before it has a chance to put on adequate growth and energy reserves for survival. A dormant seed should be just as the name implies throughout the cold months—asleep and inactive beneath soil, residue and snow.
Our trial was quite unusual, because we took this practice and applied it to a winter annual, in effect shifting and condensing the vetch’s lifecycle to fit our rotation.
Fall temperatures during the experiment year were warmer than usual, so many time-strapped farmers were calling Dave in October to ask if they could still plant hairy vetch. The answer was yes and no. Despite the deceptive warmth, growing hairy vetch as it was destined to grow—as a winter annual—would have been risky business. Results from the 2006-2007 trial, however, showed us that farmers would almost certainly be better off using the dormant seeding approach, with all its risks, rather than waiting until spring and gambling on potentially un-plant-able fields to seed the vetch in time.
Warm mid- to late-fall weather can support hairy vetch emergence and early growth, but these hospitable temperatures can plunge without warning, leaving vulnerable seedlings with too little biomass and insufficient energy supplies left to withstand brutal weather. If farmers miss the critical window for planting hairy vetch (typically 40 to 50 days before the first killing frost, or mid-August to mid-September here in Berks County, Pennsylvania), the data shows that dormant seeding may be their next best bet.
When used as a winter-annual cover crop and planted during this window in early fall, hairy vetch puts on preliminary growth, goes dormant in seedling form over winter, and blooms the following spring. Farmers often plow it under as a green manure for nitrogen or roll it for the weed-suppressing mulch—a good application for the thick tangle of leguminous tendrils, some reaching 6 or 7 feet in length.
Vetch’s optimum germination temperature range is 15° to 23° C (59° to 73.4° F). After emergence, the seedlings begin to grow before the frost, form a crown (indicated by several secondary shoots from the base of the main stem), and then use stored root carbohydrate reserves to survive dormancy over the winter. The ideal temperatures for early root growth range from 20° to 25° C (68° to 77° F). Growth correlates with growing degree days (units of average daily temperature compared to a base at which no growth takes place) which are calculated for hairy vetch using a base temperature of 4° C (39.2° F).
A later planting date means increased probability of temperatures dipping below these ranges, which means less likelihood that plants will grow enough to conduct adequate energy reserves, through photosynthesis, into their root storage systems.
Seeding for delayed germination
The idea behind dormant seeding is to plant when it’s cold enough to inhibit the seed’s germination so that it lies dormant over winter and is ready to germinate when the ground first begins to thaw—but is still too wet to drive through. As temperatures gradually warm and days lengthen, seeds begin to absorb moisture, and the new seedlings grow rapidly as growing degree day units increase.
Following a dormant seeding, the seed’s metabolic activity will be low, preventing it from germinating, if temperatures remain below 35° F. Once the seed’s temperature reaches 38° F to 40 °F, with sufficient moisture, germination begins. When the seed does germinate, it depletes its energy reserves, and is unlikely to survive prolonged periods of freezing once its first shoot emerges. It is therefore vital that dormant seeding be followed by a consistently cold winter. Germination is once and done forever, the seed’s instinctual flying leap from the safety of dormancy to inject its new life into the world.
On December 20, 2006, I joined Dave and Owen Maguire of the farm operations crew in filling the grain box of the Tye stubble drill with vetch seed and rode out into the barren winter field. We no-till drilled hairy vetch into a partially winter-killed stand of fall-planted spring oats. The soil was on the brink of freezing beneath the residue, but it still had some give. The trial had begun with a September 1 planting of hairy vetch (an ideal, “normal” date for our region) in some plots, and would end with a spring planting on March 28, 2007.
We increased the seeding rates for the dormant- and spring-seeded vetch, from the fall rate of 27 lbs/acre to 33.5 lbs/acre. This increased rate helps to compensate for the more extreme conditions that the seedlings are likely to weather during emergence and the earliest and most vulnerable stages of growth.
The goal was to follow the vetch cover crop with a no-till corn crop, ideally planted into the rolled vetch mulch. The corn would need not only an ample amount of nitrogen from the vetch but also plenty of dense biomass for weed suppression. Biomass comparisons of the three planting dates on June 1, 2007, revealed that fall-planted fared the best, yielding 7016 lbs/acre of dry matter, followed by the December dormant seeded vetch, at 5983 lbs/acre, and spring seeded with 4255 lbs/acre.
It turns out that dormant seeding is a feasible Plan B. Although it will not provide winter ground cover (we made up for this by planting into residue), it provides a sound alternative to spring planting. However, neither dormant seeding nor spring seeding is likely to yield enough biomass for an effective weed-suppressing mulch in a no-till system. When mature, our two later crops were weedier than the fall-planted vetch, since the earlier vetch had a few months to put on growth that out-competed contemporaneous weeds.
These options are best plowed down for nitrogen for the following crop, and may require additional compost or manure for fertility.
With a little creativity, hairy vetch can be worked into almost any rotation, and it’s well worth the effort for the soil enrichment, fertility and weed suppression it provides. Dormant seeding is indeed a risk, and like any other planting event it takes careful planning and attention to weather patterns. But it is probably better than nothing for the farmer who missed their ideal fall seeding window. For northern farmers who need a more winter-hardy vetch, look for a northern seed tag origin. Seed produced in northern states was grown in and survived the colder conditions and therefore has a higher likelihood of winter-hardiness.