Native Seed Germination

Ever wonder why such a small fraction of our indigenous flora is available for sale through nurseries, even the good native plant nurseries?  There are three primary factors.

1) Lack of demand, 2) lack of seed, and 3) lack of germination expertise.  Demand is picking up as restoration practitioners, architects, and designers gain experience and want to try new things.  Each state in the northeast now seems to have one native nursery that is serious about local ecotype seed production (e.g. ArcheWild for PA/MD/northern NJ).  But lack of germination experience seems to be pervasive in the native nursery trade and the leading cause of lack of species availability.

Most of the large, popular native nurseries get around the lack of germination expertise by using cuttings, particularly those nurseries that sell many native cultivars and that have no seed production capacity (you know who they are).  Learning how to decipher and then unlock the code for seed germination requires diligent research and access to fresh seed.  ArcheWild has been testing various methods of germination at its plug production research site in Quakertown for the last few years.  Here is what we’ve learned ourselves and/or have learned from others.

ArcheWild – premature seed germination ArcheWild – premature seed germination

All seeds have some type of germination inhibitor lest the seed germinates while it is still attached to the plant.  The picture above shows what happens when something unusual happens and the seeds germinate prematurely.  The germination inhibitor must be destroyed or the seed cannot germinate.  For all seeds, there is a two step process for destroying the inhibitor.  The first is purely time and the second is some special sequence of environmental factors.  Most native seed use some type of chemical fuse, or timer, that must burn out before the germination inhibitor can be destroyed by environmental factors.  The amount of time that it takes for this fuse to burn seems to be around 30-60 days or so.  We surmise that this is to allow for the seed to be dispersed to its desired germination location (wind, birds, ants, etc.) and to help prevent premature germination.  Only after this fuse burns out can the process of destroying the germination inhibitor begin.

The term stratification is used extensively in the native seed industry; this is an inadequate term for describing the process of breaking down the germination inhibitor.  A Penn State professor suggested that a better word is conditioning.  The conditioning process for some species is super-simple, for others it is complex and lengthy, and for a small few it is nearly impossible to replicate in a nursery.  The species that are super-simple are familiar to everyone because they are so easy and include the asters, grasses, goldenrods, and other ‘weedy’ natives.  The species that require complex and lengthy conditioning process are almost unavailable in the nursery trade such as orange-fruited horse gentian, carpenter’s square, and maple-leaved viburnum.  The impossible species include those that require a symbiotic partner of some kind such as pink lady’s slipper and false yellow foxglove.

The next phase in the maturation of the native plant nursery industry will embody two major shifts.  One is towards open-pollinated, local-ecotype (OPEL) genetics and the other, closely related, is the availability of species that have complex and lengthy conditioning requirements.

Baptisia australis seeds. Baptisia australis seeds.

These different conditioning requirements present challenges to the seed producer, the nurseryman, the restoration professional, and to engineers/landscape architects. The seed producer should attempt to replicate how a seed would naturally be conditioned while it is held in storage.  Keeping a New England aster seed in a burlap bag in a warehouse is perfect, but this practice would kill False Solomon’s Seal seed, for example.  Discarding seed stored dry for more than one year would help prevent disappointing results.  The nurseryman can develop production techniques that mimic natural conditioning processes to enable themselves to grow a wider range of OPLE plants.  The restoration professional and engineer/landscape architect should modify their seeding specifications to include at least two seeding events; once in the fall for those species requiring dark, cold, moist conditioning and once in early spring for species requiring only light and warm, moist conditions.  This would accelerate planting development, reduce seed loss, and reduce the amount of seed purchased.

The environmental factors that condition a native seed for germination are pretty easy to figure out.  Light, temperature, moisture, and time.  The conditioning process for a seed is often discernible by considering how the seed is held on the plant before it is dispersed.  Species in the Asteraceae family have the ability to hold their seeds, with pappus attached, late into the winter and drift off to lay on open soil.  This observation leads to the good guess that seeds prefer a lengthy period of being fairly dry before germinating in light during warm spring rains.  Seeds of species in the Scrophulariaceae family are squarish grains generally held in capsules that remain mostly closed through the winter.  These seeds generally fall out during the first heavy snow and nestle in soil pockets. This observation leads to the good guess that these seeds prefer a cool, moist, and dark period before germinating during warm spring rains.  Rarely will a seed remain viable if held high and dry for more than a year so some type of timely conditioning is generally required. Exceptions include those species that require high temperatures or smoke.  And there are rare cases where a seed must be subjected to nearly seven years of conditioning in the soil before the germination inhibitor breaks down.

Here’s a challenge for a plant-loving industrial designer or engineer.  Design a seed conditioning apparatus that allows the seed producer, nurseryman, or professional to precisely control the conditioning process for a batch of seeds.  ArcheWild would be your first customer!

Guest Post by Mark Brownlee of ArcheWild Nursery a wholesale-only native plant nursery.  -- Izel Plants sells an extensive list of ArcheWild native plants.--

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