By Dr. John E. Baylor
Director of Market Development
Beachley-Hardy Seed Company
Dr. John E. Baylor
Director of
Market Development
Beachley-Hardy Seed Co.
Recent questions on hard seeds
in legumes has prompted me to do
a column on the subject. Frankly,
the whole matter of hard seeds is
complex. Hopefully, however, the
following may help all of us to
better understand what hard seeds
are, and their role in agriculture.
Seeds that do not absorb water
when placed in wet or moist sur
roundings, because of an imper
meable seed coat, are referred to
as hard seeds. In Held crop seeds,
the largest number of hard seeds
are found in small seeded legumes
such as alfalfa, red clover, birds
foot trefoil, and crownvetch. Hard
seeds in general are viable or live
seeds, and normally over time, do
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germinate. However, because they
do not take up water immediately
when exposed to moisture, they
are considered dormant or hard.
What Causes
Hard Seeds?
Technically, “hardseededness”
is credited to an impermeable lay
er either on or just under the sur
face of the seed, and while hard
seededness is a generally charac
teristic of most small seeded
legumes, there are differences
among species and among variet
ies within species. Among the
clovers, for example, ranked from
high to low, species differ in their
degree of resistance to becoming
permeable as follows; white, ladi
no, alsike, red, crimson. Thus,
hard seeds of red clover would
become permeable and germinate
sooner than hard seeds of white,
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ladino, or alsike. And, while spe
cific data are not available, I
would expect hard seeds of red
clover to also germinate sooner
than those of alfalfa.
Speaking of alfalfa, we know
that some varieties regularly deve
lop more hard seeds than other
varieties. On the other hand, the
same variety grown in different
fields may show wide differences
in hard-seed content.
Environmental conditions
under which the seed is grown are
a major factor determining the
percentage of hard seeds and the
degree of hardseededness, with
temperature during and immedi
ately following maturity playing a
major role. For example, hard
seed percentages of 40 to 50 per
cent are common in alfalfa seed
lots grown in Washington and
other northwestern stales, while
alfalfa seed lots of the same varie
ty grown in California seldom
have more than a 20 to 30 percent
hard-seed content. The imperme
able layer is formed late in the
seed development process. Thus,
if growth is stopped abruptly, the
impermeable layer may have lim
ited time to develop fully. Like
wise, drying of the seed, both in
the field and in storage, seems to
play an important part in the deve
lopment of hardseededness.
The temperature and humidity
of seed storage facilities also have
a pronounced effect on hard
seededness. High relative humidi
ty during seed storage reduces,
and in some legumes, prevents
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formation of hardseededness that
occurs in storage at low humidity.
In the past, as much as 20 to 30
percent hard seed in legumes was
thought to be advantageous in the
event of poor or unfavorable
weather conditions following
planting, and this level is still con
sidered okay under some
conditions.
However, with alfalfa, at least
most seedsmen prefer no more
than 10 percent hard seed. Recent
studies do indicate that with alfal
fa late germinating seeds cannot
compete well with established
seedlings and new plants make
only a minor contribution to
improving stands. This would not
be as true with red clover. Regard
less, many authorities still feel that
for most legume species, hard
seeds can be, and are, very benefi
cial for filling in where old plants
disappear.
The three most frequently used
commercial methods to lower
hardseededness are storage,
blending, and scarification. As
suggested earlier, normal storage
from harvest to planting time will
reduce hard seed percentages,
although the exact results are
unpredictable. Blending seed lots
of the same variety produced in
different areas is a common
method of providing seed to the
consumer with fewer hard seeds.
If necessary, the number of hard
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seeds can be reduced by mechani
cal scarification. This occurs to
some degree in the normal process
of harvesting and cleaning seed.
Beyond this, great care must be
used to avoid over-scarification,
and the danger of reducing germi
nation and increasing the percen
tage of abnormal seedlings.
A high percentage of die hard
seed coats also are made perme
able by the normal freezing and
thawing action of the soil during
the winter following sowing, as
well as by the action of soil organ
isms and soil acids.
Many other methods to
decrease hardseededness have
been investigated by researchers,
and for species where hard seeds
are a special problem, special
techniques to reduce hard seeds
can be and have been used by the
seed industry. However, because
of the risks involved, we do not
recommend scarification on the
farm.
Hard seeds in small seeded
legumes are common, but the
mechanisms involved are com
plex. There are many factors that
influence the percentage of hard
seeds and the degree of hardness,
and while excessive amounts of
hard seeds, especially in alfalfa,
may be a concern, we still feel that
hard seeds can be, and are, very
beneficial for filling in where old
plants disappear.
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