Friday’s seeds: Comptonia

If you are looking for a shrub with a distinct foliage spring to late fall, think about adding a sweet one – Comptonia peregrina, sweet fern.
Although this native shrub is sometimes commercially available, it has remained underused despite its many qualities. It may be one of those cases when you have to see it first to understand its beauty.

The Sweet fern is a deciduous shrub with a mounded habit and deep green, shiny, finely serrate narrow leaves. The foliage it is very aromatic when crushed (think Myrica). In the fall it changes to superb shades of russet to golden bronze! The flowers are small, inconspicuous and wind-pollinated. The fruit is an ovoid nut enclosed in a burr-like cluster of elongated bracts.

Sweet fern is a pioneer species, growing on dry, sandy, gravelly soils, pine barrens, woods edges, from Northeastern US to Central & Eastern Canada. It is a nitrogen fixing plant that will form rhizomatous colonies. The French common name,  Comptonie voyageuse, alludes to the character.

For all these reasons it is an excellent choice not only for xeric gardens but also for other difficult sites, sandy soils, erosion control, naturalistic plantings, to mention just a few possibilities. Indigenous People have also used it for ceremonial, medicinal and food purposes (foliage and the fruits).

The easiest propagation method for the sweet fern is through cuttings from the rhizomes and division. With the genetic variability in mind, we should also grow it from seeds.

The Friday’s seeds segment; or is it a fruit?

Like in many other cases, for Comptonia peregrina we usually call ‘seeds’ the fruits, which I already mentioned, are ovoid nuts. They fall to the ground when ripe and one study found that they have a very long longevity in the soil: up to 70 years! (Del Tredici). Apparently the germination is difficult, if it is to believe what others are saying.

Let’s keep in mind that we will sow the whole fruits not the seeds per se (like for most nuts). All nuts have hard or thick fruit walls and may require mechanical treatments before sowing.
Also, an inhibitor in the seed coat of Comptonia, possibly abscisic acid, it is believed to be responsible for the dormancy.

All in all, these point to a combination of physical and physiological dormancy.

The highest percentage of germination was found in a study with the following treatments: scarification (using the HWT) + GA3 500 ppm.
There is agreement that “most buried seeds still require exposure to strong daily temperature fluctuations before they germinate”.

We can translate all the above as: scarify the seeds (sandpaper), sow in the fall and keep the pots outdoors over the winter. Let the nature work for you (in a cold climate it works better than GA3 treatments).

I just scarified a few nuts to test the germination on a small sample. I’ll share a trick for doing it right. Use 120 grit sandpaper and start slowly. Crack open a nut (it happened naturally in this case) to see how thick the fruit wall is. We can also observe the position of the seed inside the nut (green arrows) and scarify just the end where the seed is attached. 

Keep the scarified nuts in moist paper for 1-2 days (or more) to see if the scarification was successful, and sow them in a pot after that. In a cold climate, there is no problem with providing a good cold/moist stratification outdoors over the winter.