|
Posted January 12, 2007: Dormant seeding
is a practice that perhaps few people in Pennsylvania (and
states south) have heard of, which may be why our first reaction
is commonly that it’s completely off the wall. The practice
is more widespread among conventional farmers in places like
North Dakota (and throughout the upper Midwest), where farmers
have to get creative in order to maximize their returns from
the short growing season. It is in many ways ideally suited
to colder climates, but there is no reason why it cannot be
tweaked and adapted for inclusion in the cover crop experiments
here at The Rodale Institute.
Basically, dormant seeding involves planting just before
the soil freezes, with the hope that the soil temperatures
stay cold enough throughout the winter that the seed does
not germinate until the weather gets consistently warm enough
in the spring to foster germination and steady growth. Obviously,
this is a huge gamble, and an especially precarious one in
the very late fall and early spring.
The farmer has to be willing to bet that soil temperatures
will be consistently cold enough from the planting date on
throughout the winter to inhibit germination, and then that
they’ll be consistently warm enough following the first
major ground thaw to support the rapid growth that many crops
put on in the early spring. In fact, in the case of a winter
annual cover crop like hairy vetch, the farmer will rely heavily
on this rapid growth to compensate for the early fall growth
he has missed, so many growing conditions have to line up
pretty nicely to ensure success.
Dormant seeding is in these respects like any cultural practice,
involving a lot of risk and delicate timing, but North Dakota
is a great place to try it. Its consistently cold temperatures
and fairly dependable winter snow cover keep the ground cold
and the cold sealed in. Snow-insulated soil is less subject
to dramatic variances in temperature, which would either initiate
germination and then promptly kill the seedling, or force
the buried seed up out of the ground through the heaving action
of freeze and thaw cycles. The ideal conditions would be to
have continuous snow cover throughout the winter, with soil
temperatures never reaching above 35°F (this is about
the threshold below which the seed’s metabolic activity
dips to a minimum, and where little moisture is available
for it to imbibe). The top two inches of soil should be near
or below 32°F. Dormant seeding must take place in the
window of time between the onset of freezing temperatures
and the first snow.
In North Dakota, where the practice is used mostly on canola
and spring wheat, dormant seeding may actually be used to
diversify the flowering dates, much the same way that intercropping
is used to deal with pests. If the crop is broken down into
different stages of maturity, then the entire crop would not
be wiped out by hot, dry spells or other harsh conditions
(drought tolerance varies at different stages of development).
It also sometimes helps to get the crop established before
the springtime emergence of predators like the flea beetle.
Again, these benefits have to be carefully weighed against
the risks and guesses involved.
Whether in the Midwest or in Pennsylvania, dormant seeding
works better if it can be done into residue. Ground covered
with stubble or other crop/weed residue is much better insulated
and can magnify the soil’s already buffering effects
against the variances in ambient temperature. A more consistent
stand results in the no-till or minimum-till systems because
the protective layer of residue that provides insulation and
shelter is left undisturbed.
The real challenge in adapting dormant seeding for a cover
crop is that it has traditionally been used not as a last
resort for a winter annual, but as a head-start for a spring
crop such as spring wheat. Farmers in the upper Midwest typically
take this chance with dormant seeding not out of desperation,
but to save time and spread out labor on spring planting and
to beat unpredictable spring weather conditions.
At The Rodale Institute, we are currently experimenting with
dormant seeding hairy vetch, a winter annual legume cover
crop, and now whether dormant seeding can be adapted to it.
We have typically planted it 40 to 50 days before the first
killing frost (mid-August to mid-September in Kutztown, Pennsylvania),
which gives it time to establish root and plant growth before
winter dormancy. It then blooms in the spring and can be rolled
or plowed under. If the farmer misses the critical window
of time for a fall planting of hairy vetch, the hypothesis
is that he could fall back on dormant seeding.
In our dormant seeding trials, hairy vetch was no-till drilled
into four treatments: 1) dead berseem clover/weed residue;
2) dead annual sweet clover/weed residue; 3) heavy residue
of dead (August-planted) spring oats; and 4) a clean-till
field with an established cover crop of cereal rye. These
December 20 plantings will be compared with an August 1 planting,
a September 1 planting and a spring 2007 planting. The amount
of biomass put on in the fall has not shown to be a good predictor
of spring biomass or of flowering date but will tell us much
more about the optimum planting date to minimize winter kill.
Since dormant seeding for a winter annual is pretty much
unheard of, this trial may also answer some major questions
about the need for vernalization of a winter annual. Vernalization
is a hormonal response to temperature; winter annuals generally
require a cold spell in order to enter a reproductive stage
and flower. Dormant-seeded hairy vetch may be able to get
some of this cold exposure during its early growth in the
spring, but the plant itself will not undergo a stage of dormancy.
If it can survive these abnormal conditions, the next question
is whether its growth in the spring will be quick enough to
catch up with its fall-planted counterparts so that it may
be rolled in time for a no-till corn planting, or at least
to have enough biomass to be plowed under for nitrogen.
Trying to dormant seed winter annuals in a warmer climate
(a technique originally designed for spring grains in a much
colder climate) is one manifestation of learning how to adapt
similar methods to very different environments, and to tailor
agronomic principles to the geography of our locale. Rather
than a one-size-fits-all approach, i.e. chemical application,
organic farmers are forced to build a consciousness about
their land’s individuality and perhaps learn more about
their own soil conditions.
Our climate is much less predictable than a more northern
climate (in general but especially this winter), and the adaptation
of dormant seeding in Pennsylvania requires a lot of soil
scrutiny in terms of temperature, composition, drainage, etc.
The important thing is to build diversity of technique and
to have as many “rescue” routes for a farmer to
fall back on as possible.
So far, dormant seeding a hairy vetch cover crop is pure
speculation but may be perfected over several winters of trial
and error.  |
