UPDATE ON PEONY SEED GERMINATION

Don Hollingsworth
American Peony Society Bulletin No. 244. December, 1982. (pp 17-20).
The production of peony seedlings can give some of the most rewarding experiences which can be
found in pleasure gardening. The basic work of peony species hybridization has already been done and
the fertility barriers within the hybrid progeny are rapidly falling. Yet, the discovery of what new
breakthroughs can be found by mixing the species has only just begun. We are greatly encouraged in
seeing and hearing about the desirable new peonies which are coming from the work of the few people
who have been actively raising hybrid seedlings during the recent years.
Attempts to grow peonies from seed have been sometimes disappointing until a certain amount of
favorable experience has been obtained. Even then it is often frustrating. According to Western
translators of an ancient Japanese writer, "the seeds should be sown immediately after they are ripe. . .
(while still moist from the pod). . . when sown immediately after, no vital power will be lost. If, on the
contrary, the seeds are dried previous to sowing, hardly one in a hundred will germinate, and often not
even one." (See Bulletin No. 220, Dec. 1976.) Well, it turns out that, after a little experience in planting
peony seeds, one usually concludes the consequences are not quite so absolute. Yet, as with most rules
handed down by our elders, one does find a relationship in the results of current experience. When the
relationship is not clear-cut, one is challenged to sort out what there is in the rule which is meaningful.
Modern study of peony germination has benefited immensely from advances in the understanding of
plant growth and development in general and of the internal controls of growth processes which enable
plants to function in concert with their natural environment. These are the adaptations which have
enabled the species to survive in nature. As we select plants for domestic use and raise them through the
generations, we tend to break down those precise adaptations which work against our purposes.
However, in perennial ornamental plants, some of the characteristics are highly fixed and we have been
able to meet our purposes without making much change in processes such as the control of seed
germination. We can get along very well by simply learning to understand what are the inherent
environmental requirements for the particular species or strain we are working with.
We can now understand that drying of peony seeds does not very much affect their inherent vitality.
(This never did make any sense in relation to seeing peony seeds naturally dry in the pods of standing
stems over winter.) The effect of drying is to delay the internal development processes of the seed. The
prevalent method in the general handling of domestic crop seeds which are to be replanted is to collect
them and hold them dry until the chosen planting time. The seeds most commonly used tend to
germinate quite soon upon being planted. While some commonly grown seeds used in horticulture and
agriculture take a little longer, the information necessary to understand this is widely available, so few
handlers have any trouble and then not for long.
Most peony species, like several of the woody genera used in ornamental horticulture, have a
relatively complex series of developmental phases during the process of seed germination. In some
species this type of germination control assures that seedlings after seed maturation in nature do not
emerge for growth until the second spring. In at least some peonies (species, strains, individual plants) it
is found that by keeping the seeds moist and planting as soon as possible, some seedlings can be
obtained in the first spring. Since we generally find "sooner" more appealing than "later," we tend to
focus on how to improve the results of first year germination.
It can now be seen that a trap which peony seed handlers have often fallen into is that of holding
seeds in dry storage until harvest is complete, then going ahead with attempts to get them germinated
for the following spring. Drying delays development. When we harvest seeds and let them dry, then put
them back into moist environment while the environment remains warm in early autumn, we have
delayed their development, but not to the same extent as when nature allows them to remain dry until
cold weather. What this short delay does is to increase the probability that germination will go past the
point-of-no-return, but will not have enough time to get through all phases of development before the soil
becomes warm in the spring.
There are four phases of development in peony seed germination with respect to different
temperature requirements and these roughly correspond with the seasons of the year.
First is post-maturation growth of the embryo. When the peony seed is mature, the embryo is very
small—perhaps two millimeters long. When the seed is allowed to dry, there is apparently no further
development. This is the ultimate resting condition and is the best condition for storage of the seeds to
retain viability. When put into a warm, moist environment (growing season temperature) this resting
stage becomes broken, perhaps not immediately, and growth of the embryo into the endosperm cavity
takes place. This development runs its course and as long as the soil remains warm nothing further that
is obvious happens.
Secondly, when the soil temperature cools down in autumn to a level which agrees with the
genetically controlled requirements of a particular seed, root germination will occur – 55-70 °F,
depending on species. This is true germination, technically. The development process is now past the
point of being held indefinitely in a quiescent state. The root will elongate and the cotyledons will
elongate so that the bud area is extended outside the seed. The bud, however, is dormant. It requires a
long period of chilling in order to grow. If kept around 60 °F or above (perhaps lower), the buds will
never grow.
Phase three is the dormancy reduction period. Contrary to some belief, it takes place at
temperatures above freezing. Research with fruit tree species indicates that the most favorable
temperature for those species is 46 °F. In fruit tree research, the chill requirement is expressed in terms
of numbers of chill units. A chill unit is one hour at 46° or the equivalent. Chilling will go forward at
temperatures above 32° and up to 59 °F, but it takes increasingly more time to equal one chill unit as the
temperature gets further away from 46°. In peonies this is the phase where many late planted seeds fail.
They get through root germination, but fail to receive sufficient chill units before the soil becomes too
warm in spring.
Phase four is leafy shoot production. I have never deliberately separated phase four from three, for
most peony shoots will grow at temperatures under which chilling can still go forward. However, I had
some seeds of a Mediterranean area species once which, when germinated indoors-using a refrigerator
set at about 36° for dormancy reduction-took almost seven months before there was enough evidence of
growth in the little shoots that I was willing to try getting them to grow in outdoor temperatures
(August). If I had given these germinated seeds a higher chilling temperature, the shoots may have
shown advancement much sooner. However, the question would remain unanswered whether it was due
to more rapid chilling or to a more favorable temperature for seeing shoot extension. Because of the
overlap of temperatures at which phases three and four go forward, it is important to have indoor
germinated plants into the light for leafy shoot production. Otherwise there will be an extreme extension
of etiolated shoot stem (like potato sprouts in the dark).
Back to the question of how seedlings are lost by allowing seeds to dry after harvest, we can
confidently say the following. It is not the drying that hurts, it is the developmental delay. This delay is
only crucial if one then terminates the delay by putting the seeds into warm moist environment too late
in the growing season. If held until the following spring for planting, good results should be expected in
the second spring. "Too late" is the time after which the seed will complete phase one, because then
rooting is sure to occur, but after which it cannot make complete reduction of bud dormancy before the
soil becomes too warm in spring for any more reduction to take place. (We can only place this time in the
calendar on a probability basis.) If the seedling does not produce a leafy shoot, it cannot produce and
store food to support the second growth cycle which will be initiated with the onset of cool temperatures
the following autumn.
One other eventuality should be noted. What about the seeds which lay for a couple of years and
then germinate? I tentatively see this as a delay in post-maturation of the embryo. In seeds of hybrid
peony kept in warm, moist medium continuously for as long as three years, there would be a gradual
dropping out—death and decay, sometimes a germination—suggesting that they will eventually
deteriorate, possibly due to the deterioration of their barriers to invasion by organisms in the favorable
warm moist conditions. I mentioned hybrid seeds, although this isn't limited to them. I do expect,
however, that in the process of hybridization there is more chance to lose some factor critical to control of
some detail or another of the life cycle processes. These can be thought of as the casualties of the project.
So long as some desirable plants which also retain breeding fitness are obtained, the casualties are soon
forgotten. (If I wasn't doing indoor germination, they possibly would never have been noticed in the first
place.)
For a discussion of details of seed germination methods, see the articles in the Peony Society book, The
Best of 75 Years, in the hybridizing section.